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| 224 | PDF</a> |
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| 225 | </div> |
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| 226 | <h1>Map/Reduce Tutorial</h1> |
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| 227 | <div id="minitoc-area"> |
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| 228 | <ul class="minitoc"> |
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| 229 | <li> |
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| 230 | <a href="#Purpose">Purpose</a> |
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| 231 | </li> |
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| 232 | <li> |
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| 233 | <a href="#Pre-requisites">Pre-requisites</a> |
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| 234 | </li> |
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| 235 | <li> |
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| 236 | <a href="#Overview">Overview</a> |
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| 237 | </li> |
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| 238 | <li> |
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| 239 | <a href="#Inputs+and+Outputs">Inputs and Outputs</a> |
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| 240 | </li> |
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| 241 | <li> |
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| 242 | <a href="#Example%3A+WordCount+v1.0">Example: WordCount v1.0</a> |
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| 243 | <ul class="minitoc"> |
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| 244 | <li> |
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| 245 | <a href="#Source+Code">Source Code</a> |
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| 246 | </li> |
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| 247 | <li> |
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| 248 | <a href="#Usage">Usage</a> |
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| 249 | </li> |
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| 250 | <li> |
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| 251 | <a href="#Walk-through">Walk-through</a> |
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| 252 | </li> |
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| 253 | </ul> |
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| 254 | </li> |
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| 255 | <li> |
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| 256 | <a href="#Map%2FReduce+-+User+Interfaces">Map/Reduce - User Interfaces</a> |
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| 257 | <ul class="minitoc"> |
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| 258 | <li> |
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| 259 | <a href="#Payload">Payload</a> |
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| 260 | <ul class="minitoc"> |
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| 261 | <li> |
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| 262 | <a href="#Mapper">Mapper</a> |
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| 263 | </li> |
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| 264 | <li> |
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| 265 | <a href="#Reducer">Reducer</a> |
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| 266 | </li> |
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| 267 | <li> |
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| 268 | <a href="#Partitioner">Partitioner</a> |
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| 269 | </li> |
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| 270 | <li> |
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| 271 | <a href="#Reporter">Reporter</a> |
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| 272 | </li> |
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| 273 | <li> |
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| 274 | <a href="#OutputCollector">OutputCollector</a> |
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| 275 | </li> |
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| 276 | </ul> |
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| 277 | </li> |
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| 278 | <li> |
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| 279 | <a href="#Job+Configuration">Job Configuration</a> |
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| 280 | </li> |
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| 281 | <li> |
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| 282 | <a href="#Task+Execution+%26+Environment">Task Execution & Environment</a> |
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| 283 | <ul class="minitoc"> |
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| 284 | <li> |
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| 285 | <a href="#Memory+management"> Memory management</a> |
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| 286 | </li> |
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| 287 | <li> |
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| 288 | <a href="#Map+Parameters">Map Parameters</a> |
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| 289 | </li> |
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| 290 | <li> |
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| 291 | <a href="#Shuffle%2FReduce+Parameters">Shuffle/Reduce Parameters</a> |
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| 292 | </li> |
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| 293 | <li> |
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| 294 | <a href="#Directory+Structure"> Directory Structure </a> |
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| 295 | </li> |
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| 296 | <li> |
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| 297 | <a href="#Task+JVM+Reuse">Task JVM Reuse</a> |
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| 298 | </li> |
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| 299 | </ul> |
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| 300 | </li> |
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| 301 | <li> |
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| 302 | <a href="#Job+Submission+and+Monitoring">Job Submission and Monitoring</a> |
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| 303 | <ul class="minitoc"> |
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| 304 | <li> |
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| 305 | <a href="#Job+Control">Job Control</a> |
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| 306 | </li> |
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| 307 | </ul> |
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| 308 | </li> |
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| 309 | <li> |
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| 310 | <a href="#Job+Input">Job Input</a> |
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| 311 | <ul class="minitoc"> |
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| 312 | <li> |
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| 313 | <a href="#InputSplit">InputSplit</a> |
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| 314 | </li> |
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| 315 | <li> |
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| 316 | <a href="#RecordReader">RecordReader</a> |
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| 317 | </li> |
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| 318 | </ul> |
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| 319 | </li> |
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| 320 | <li> |
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| 321 | <a href="#Job+Output">Job Output</a> |
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| 322 | <ul class="minitoc"> |
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| 323 | <li> |
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| 324 | <a href="#OutputCommitter">OutputCommitter</a> |
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| 325 | </li> |
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| 326 | <li> |
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| 327 | <a href="#Task+Side-Effect+Files">Task Side-Effect Files</a> |
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| 328 | </li> |
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| 329 | <li> |
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| 330 | <a href="#RecordWriter">RecordWriter</a> |
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| 331 | </li> |
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| 332 | </ul> |
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| 333 | </li> |
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| 334 | <li> |
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| 335 | <a href="#Other+Useful+Features">Other Useful Features</a> |
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| 336 | <ul class="minitoc"> |
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| 337 | <li> |
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| 338 | <a href="#Submitting+Jobs+to+Queues">Submitting Jobs to Queues</a> |
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| 339 | </li> |
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| 340 | <li> |
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| 341 | <a href="#Counters">Counters</a> |
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| 342 | </li> |
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| 343 | <li> |
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| 344 | <a href="#DistributedCache">DistributedCache</a> |
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| 345 | </li> |
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| 346 | <li> |
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| 347 | <a href="#Tool">Tool</a> |
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| 348 | </li> |
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| 349 | <li> |
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| 350 | <a href="#IsolationRunner">IsolationRunner</a> |
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| 351 | </li> |
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| 352 | <li> |
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| 353 | <a href="#Profiling">Profiling</a> |
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| 354 | </li> |
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| 355 | <li> |
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| 356 | <a href="#Debugging">Debugging</a> |
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| 357 | </li> |
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| 358 | <li> |
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| 359 | <a href="#JobControl">JobControl</a> |
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| 360 | </li> |
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| 361 | <li> |
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| 362 | <a href="#Data+Compression">Data Compression</a> |
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| 363 | </li> |
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| 364 | <li> |
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| 365 | <a href="#Skipping+Bad+Records">Skipping Bad Records</a> |
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| 366 | </li> |
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| 367 | </ul> |
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| 368 | </li> |
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| 369 | </ul> |
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| 370 | </li> |
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| 371 | <li> |
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| 372 | <a href="#Example%3A+WordCount+v2.0">Example: WordCount v2.0</a> |
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| 373 | <ul class="minitoc"> |
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| 374 | <li> |
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| 375 | <a href="#Source+Code-N10FD0">Source Code</a> |
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| 376 | </li> |
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| 377 | <li> |
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| 378 | <a href="#Sample+Runs">Sample Runs</a> |
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| 379 | </li> |
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| 380 | <li> |
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| 381 | <a href="#Highlights">Highlights</a> |
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| 382 | </li> |
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| 383 | </ul> |
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| 384 | </li> |
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| 385 | </ul> |
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| 386 | </div> |
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| 387 | |
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| 388 | |
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| 389 | <a name="N1000D"></a><a name="Purpose"></a> |
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| 390 | <h2 class="h3">Purpose</h2> |
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| 391 | <div class="section"> |
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| 392 | <p>This document comprehensively describes all user-facing facets of the |
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| 393 | Hadoop Map/Reduce framework and serves as a tutorial. |
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| 394 | </p> |
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| 395 | </div> |
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| 396 | |
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| 397 | |
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| 398 | <a name="N10017"></a><a name="Pre-requisites"></a> |
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| 399 | <h2 class="h3">Pre-requisites</h2> |
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| 400 | <div class="section"> |
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| 401 | <p>Ensure that Hadoop is installed, configured and is running. More |
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| 402 | details:</p> |
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| 403 | <ul> |
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| 404 | |
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| 405 | <li> |
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| 406 | |
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| 407 | <a href="quickstart.html">Hadoop Quick Start</a> for first-time users. |
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| 408 | </li> |
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| 409 | |
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| 410 | <li> |
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| 411 | |
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| 412 | <a href="cluster_setup.html">Hadoop Cluster Setup</a> for large, |
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| 413 | distributed clusters. |
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| 414 | </li> |
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| 415 | |
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| 416 | </ul> |
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| 417 | </div> |
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| 418 | |
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| 419 | |
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| 420 | <a name="N10032"></a><a name="Overview"></a> |
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| 421 | <h2 class="h3">Overview</h2> |
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| 422 | <div class="section"> |
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| 423 | <p>Hadoop Map/Reduce is a software framework for easily writing |
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| 424 | applications which process vast amounts of data (multi-terabyte data-sets) |
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| 425 | in-parallel on large clusters (thousands of nodes) of commodity |
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| 426 | hardware in a reliable, fault-tolerant manner.</p> |
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| 427 | <p>A Map/Reduce <em>job</em> usually splits the input data-set into |
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| 428 | independent chunks which are processed by the <em>map tasks</em> in a |
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| 429 | completely parallel manner. The framework sorts the outputs of the maps, |
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| 430 | which are then input to the <em>reduce tasks</em>. Typically both the |
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| 431 | input and the output of the job are stored in a file-system. The framework |
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| 432 | takes care of scheduling tasks, monitoring them and re-executes the failed |
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| 433 | tasks.</p> |
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| 434 | <p>Typically the compute nodes and the storage nodes are the same, that is, |
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| 435 | the Map/Reduce framework and the Hadoop Distributed File System (see <a href="hdfs_design.html">HDFS Architecture </a>) |
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| 436 | are running on the same set of nodes. This configuration |
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| 437 | allows the framework to effectively schedule tasks on the nodes where data |
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| 438 | is already present, resulting in very high aggregate bandwidth across the |
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| 439 | cluster.</p> |
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| 440 | <p>The Map/Reduce framework consists of a single master |
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| 441 | <span class="codefrag">JobTracker</span> and one slave <span class="codefrag">TaskTracker</span> per |
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| 442 | cluster-node. The master is responsible for scheduling the jobs' component |
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| 443 | tasks on the slaves, monitoring them and re-executing the failed tasks. The |
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| 444 | slaves execute the tasks as directed by the master.</p> |
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| 445 | <p>Minimally, applications specify the input/output locations and supply |
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| 446 | <em>map</em> and <em>reduce</em> functions via implementations of |
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| 447 | appropriate interfaces and/or abstract-classes. These, and other job |
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| 448 | parameters, comprise the <em>job configuration</em>. The Hadoop |
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| 449 | <em>job client</em> then submits the job (jar/executable etc.) and |
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| 450 | configuration to the <span class="codefrag">JobTracker</span> which then assumes the |
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| 451 | responsibility of distributing the software/configuration to the slaves, |
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| 452 | scheduling tasks and monitoring them, providing status and diagnostic |
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| 453 | information to the job-client.</p> |
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| 454 | <p>Although the Hadoop framework is implemented in Java<sup>TM</sup>, |
---|
| 455 | Map/Reduce applications need not be written in Java.</p> |
---|
| 456 | <ul> |
---|
| 457 | |
---|
| 458 | <li> |
---|
| 459 | |
---|
| 460 | <a href="api/org/apache/hadoop/streaming/package-summary.html"> |
---|
| 461 | Hadoop Streaming</a> is a utility which allows users to create and run |
---|
| 462 | jobs with any executables (e.g. shell utilities) as the mapper and/or |
---|
| 463 | the reducer. |
---|
| 464 | </li> |
---|
| 465 | |
---|
| 466 | <li> |
---|
| 467 | |
---|
| 468 | <a href="api/org/apache/hadoop/mapred/pipes/package-summary.html"> |
---|
| 469 | Hadoop Pipes</a> is a <a href="http://www.swig.org/">SWIG</a>- |
---|
| 470 | compatible <em>C++ API</em> to implement Map/Reduce applications (non |
---|
| 471 | JNI<sup>TM</sup> based). |
---|
| 472 | </li> |
---|
| 473 | |
---|
| 474 | </ul> |
---|
| 475 | </div> |
---|
| 476 | |
---|
| 477 | |
---|
| 478 | <a name="N1008B"></a><a name="Inputs+and+Outputs"></a> |
---|
| 479 | <h2 class="h3">Inputs and Outputs</h2> |
---|
| 480 | <div class="section"> |
---|
| 481 | <p>The Map/Reduce framework operates exclusively on |
---|
| 482 | <span class="codefrag"><key, value></span> pairs, that is, the framework views the |
---|
| 483 | input to the job as a set of <span class="codefrag"><key, value></span> pairs and |
---|
| 484 | produces a set of <span class="codefrag"><key, value></span> pairs as the output of |
---|
| 485 | the job, conceivably of different types.</p> |
---|
| 486 | <p>The <span class="codefrag">key</span> and <span class="codefrag">value</span> classes have to be |
---|
| 487 | serializable by the framework and hence need to implement the |
---|
| 488 | <a href="api/org/apache/hadoop/io/Writable.html">Writable</a> |
---|
| 489 | interface. Additionally, the <span class="codefrag">key</span> classes have to implement the |
---|
| 490 | <a href="api/org/apache/hadoop/io/WritableComparable.html"> |
---|
| 491 | WritableComparable</a> interface to facilitate sorting by the framework. |
---|
| 492 | </p> |
---|
| 493 | <p>Input and Output types of a Map/Reduce job:</p> |
---|
| 494 | <p> |
---|
| 495 | (input) <span class="codefrag"><k1, v1></span> |
---|
| 496 | -> |
---|
| 497 | <strong>map</strong> |
---|
| 498 | -> |
---|
| 499 | <span class="codefrag"><k2, v2></span> |
---|
| 500 | -> |
---|
| 501 | <strong>combine</strong> |
---|
| 502 | -> |
---|
| 503 | <span class="codefrag"><k2, v2></span> |
---|
| 504 | -> |
---|
| 505 | <strong>reduce</strong> |
---|
| 506 | -> |
---|
| 507 | <span class="codefrag"><k3, v3></span> (output) |
---|
| 508 | </p> |
---|
| 509 | </div> |
---|
| 510 | |
---|
| 511 | |
---|
| 512 | <a name="N100CD"></a><a name="Example%3A+WordCount+v1.0"></a> |
---|
| 513 | <h2 class="h3">Example: WordCount v1.0</h2> |
---|
| 514 | <div class="section"> |
---|
| 515 | <p>Before we jump into the details, lets walk through an example Map/Reduce |
---|
| 516 | application to get a flavour for how they work.</p> |
---|
| 517 | <p> |
---|
| 518 | <span class="codefrag">WordCount</span> is a simple application that counts the number of |
---|
| 519 | occurences of each word in a given input set.</p> |
---|
| 520 | <p>This works with a local-standalone, pseudo-distributed or fully-distributed |
---|
| 521 | Hadoop installation(see <a href="quickstart.html"> Hadoop Quick Start</a>).</p> |
---|
| 522 | <a name="N100E2"></a><a name="Source+Code"></a> |
---|
| 523 | <h3 class="h4">Source Code</h3> |
---|
| 524 | <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
---|
| 525 | |
---|
| 526 | <tr> |
---|
| 527 | |
---|
| 528 | <th colspan="1" rowspan="1"></th> |
---|
| 529 | <th colspan="1" rowspan="1">WordCount.java</th> |
---|
| 530 | |
---|
| 531 | </tr> |
---|
| 532 | |
---|
| 533 | <tr> |
---|
| 534 | |
---|
| 535 | <td colspan="1" rowspan="1">1.</td> |
---|
| 536 | <td colspan="1" rowspan="1"> |
---|
| 537 | <span class="codefrag">package org.myorg;</span> |
---|
| 538 | </td> |
---|
| 539 | |
---|
| 540 | </tr> |
---|
| 541 | |
---|
| 542 | <tr> |
---|
| 543 | |
---|
| 544 | <td colspan="1" rowspan="1">2.</td> |
---|
| 545 | <td colspan="1" rowspan="1"></td> |
---|
| 546 | |
---|
| 547 | </tr> |
---|
| 548 | |
---|
| 549 | <tr> |
---|
| 550 | |
---|
| 551 | <td colspan="1" rowspan="1">3.</td> |
---|
| 552 | <td colspan="1" rowspan="1"> |
---|
| 553 | <span class="codefrag">import java.io.IOException;</span> |
---|
| 554 | </td> |
---|
| 555 | |
---|
| 556 | </tr> |
---|
| 557 | |
---|
| 558 | <tr> |
---|
| 559 | |
---|
| 560 | <td colspan="1" rowspan="1">4.</td> |
---|
| 561 | <td colspan="1" rowspan="1"> |
---|
| 562 | <span class="codefrag">import java.util.*;</span> |
---|
| 563 | </td> |
---|
| 564 | |
---|
| 565 | </tr> |
---|
| 566 | |
---|
| 567 | <tr> |
---|
| 568 | |
---|
| 569 | <td colspan="1" rowspan="1">5.</td> |
---|
| 570 | <td colspan="1" rowspan="1"></td> |
---|
| 571 | |
---|
| 572 | </tr> |
---|
| 573 | |
---|
| 574 | <tr> |
---|
| 575 | |
---|
| 576 | <td colspan="1" rowspan="1">6.</td> |
---|
| 577 | <td colspan="1" rowspan="1"> |
---|
| 578 | <span class="codefrag">import org.apache.hadoop.fs.Path;</span> |
---|
| 579 | </td> |
---|
| 580 | |
---|
| 581 | </tr> |
---|
| 582 | |
---|
| 583 | <tr> |
---|
| 584 | |
---|
| 585 | <td colspan="1" rowspan="1">7.</td> |
---|
| 586 | <td colspan="1" rowspan="1"> |
---|
| 587 | <span class="codefrag">import org.apache.hadoop.conf.*;</span> |
---|
| 588 | </td> |
---|
| 589 | |
---|
| 590 | </tr> |
---|
| 591 | |
---|
| 592 | <tr> |
---|
| 593 | |
---|
| 594 | <td colspan="1" rowspan="1">8.</td> |
---|
| 595 | <td colspan="1" rowspan="1"> |
---|
| 596 | <span class="codefrag">import org.apache.hadoop.io.*;</span> |
---|
| 597 | </td> |
---|
| 598 | |
---|
| 599 | </tr> |
---|
| 600 | |
---|
| 601 | <tr> |
---|
| 602 | |
---|
| 603 | <td colspan="1" rowspan="1">9.</td> |
---|
| 604 | <td colspan="1" rowspan="1"> |
---|
| 605 | <span class="codefrag">import org.apache.hadoop.mapred.*;</span> |
---|
| 606 | </td> |
---|
| 607 | |
---|
| 608 | </tr> |
---|
| 609 | |
---|
| 610 | <tr> |
---|
| 611 | |
---|
| 612 | <td colspan="1" rowspan="1">10.</td> |
---|
| 613 | <td colspan="1" rowspan="1"> |
---|
| 614 | <span class="codefrag">import org.apache.hadoop.util.*;</span> |
---|
| 615 | </td> |
---|
| 616 | |
---|
| 617 | </tr> |
---|
| 618 | |
---|
| 619 | <tr> |
---|
| 620 | |
---|
| 621 | <td colspan="1" rowspan="1">11.</td> |
---|
| 622 | <td colspan="1" rowspan="1"></td> |
---|
| 623 | |
---|
| 624 | </tr> |
---|
| 625 | |
---|
| 626 | <tr> |
---|
| 627 | |
---|
| 628 | <td colspan="1" rowspan="1">12.</td> |
---|
| 629 | <td colspan="1" rowspan="1"> |
---|
| 630 | <span class="codefrag">public class WordCount {</span> |
---|
| 631 | </td> |
---|
| 632 | |
---|
| 633 | </tr> |
---|
| 634 | |
---|
| 635 | <tr> |
---|
| 636 | |
---|
| 637 | <td colspan="1" rowspan="1">13.</td> |
---|
| 638 | <td colspan="1" rowspan="1"></td> |
---|
| 639 | |
---|
| 640 | </tr> |
---|
| 641 | |
---|
| 642 | <tr> |
---|
| 643 | |
---|
| 644 | <td colspan="1" rowspan="1">14.</td> |
---|
| 645 | <td colspan="1" rowspan="1"> |
---|
| 646 | |
---|
| 647 | <span class="codefrag"> |
---|
| 648 | public static class Map extends MapReduceBase |
---|
| 649 | implements Mapper<LongWritable, Text, Text, IntWritable> { |
---|
| 650 | </span> |
---|
| 651 | </td> |
---|
| 652 | |
---|
| 653 | </tr> |
---|
| 654 | |
---|
| 655 | <tr> |
---|
| 656 | |
---|
| 657 | <td colspan="1" rowspan="1">15.</td> |
---|
| 658 | <td colspan="1" rowspan="1"> |
---|
| 659 | |
---|
| 660 | <span class="codefrag"> |
---|
| 661 | private final static IntWritable one = new IntWritable(1); |
---|
| 662 | </span> |
---|
| 663 | </td> |
---|
| 664 | |
---|
| 665 | </tr> |
---|
| 666 | |
---|
| 667 | <tr> |
---|
| 668 | |
---|
| 669 | <td colspan="1" rowspan="1">16.</td> |
---|
| 670 | <td colspan="1" rowspan="1"> |
---|
| 671 | |
---|
| 672 | <span class="codefrag">private Text word = new Text();</span> |
---|
| 673 | </td> |
---|
| 674 | |
---|
| 675 | </tr> |
---|
| 676 | |
---|
| 677 | <tr> |
---|
| 678 | |
---|
| 679 | <td colspan="1" rowspan="1">17.</td> |
---|
| 680 | <td colspan="1" rowspan="1"></td> |
---|
| 681 | |
---|
| 682 | </tr> |
---|
| 683 | |
---|
| 684 | <tr> |
---|
| 685 | |
---|
| 686 | <td colspan="1" rowspan="1">18.</td> |
---|
| 687 | <td colspan="1" rowspan="1"> |
---|
| 688 | |
---|
| 689 | <span class="codefrag"> |
---|
| 690 | public void map(LongWritable key, Text value, |
---|
| 691 | OutputCollector<Text, IntWritable> output, |
---|
| 692 | Reporter reporter) throws IOException { |
---|
| 693 | </span> |
---|
| 694 | </td> |
---|
| 695 | |
---|
| 696 | </tr> |
---|
| 697 | |
---|
| 698 | <tr> |
---|
| 699 | |
---|
| 700 | <td colspan="1" rowspan="1">19.</td> |
---|
| 701 | <td colspan="1" rowspan="1"> |
---|
| 702 | |
---|
| 703 | <span class="codefrag">String line = value.toString();</span> |
---|
| 704 | </td> |
---|
| 705 | |
---|
| 706 | </tr> |
---|
| 707 | |
---|
| 708 | <tr> |
---|
| 709 | |
---|
| 710 | <td colspan="1" rowspan="1">20.</td> |
---|
| 711 | <td colspan="1" rowspan="1"> |
---|
| 712 | |
---|
| 713 | <span class="codefrag">StringTokenizer tokenizer = new StringTokenizer(line);</span> |
---|
| 714 | </td> |
---|
| 715 | |
---|
| 716 | </tr> |
---|
| 717 | |
---|
| 718 | <tr> |
---|
| 719 | |
---|
| 720 | <td colspan="1" rowspan="1">21.</td> |
---|
| 721 | <td colspan="1" rowspan="1"> |
---|
| 722 | |
---|
| 723 | <span class="codefrag">while (tokenizer.hasMoreTokens()) {</span> |
---|
| 724 | </td> |
---|
| 725 | |
---|
| 726 | </tr> |
---|
| 727 | |
---|
| 728 | <tr> |
---|
| 729 | |
---|
| 730 | <td colspan="1" rowspan="1">22.</td> |
---|
| 731 | <td colspan="1" rowspan="1"> |
---|
| 732 | |
---|
| 733 | <span class="codefrag">word.set(tokenizer.nextToken());</span> |
---|
| 734 | </td> |
---|
| 735 | |
---|
| 736 | </tr> |
---|
| 737 | |
---|
| 738 | <tr> |
---|
| 739 | |
---|
| 740 | <td colspan="1" rowspan="1">23.</td> |
---|
| 741 | <td colspan="1" rowspan="1"> |
---|
| 742 | |
---|
| 743 | <span class="codefrag">output.collect(word, one);</span> |
---|
| 744 | </td> |
---|
| 745 | |
---|
| 746 | </tr> |
---|
| 747 | |
---|
| 748 | <tr> |
---|
| 749 | |
---|
| 750 | <td colspan="1" rowspan="1">24.</td> |
---|
| 751 | <td colspan="1" rowspan="1"> |
---|
| 752 | |
---|
| 753 | <span class="codefrag">}</span> |
---|
| 754 | </td> |
---|
| 755 | |
---|
| 756 | </tr> |
---|
| 757 | |
---|
| 758 | <tr> |
---|
| 759 | |
---|
| 760 | <td colspan="1" rowspan="1">25.</td> |
---|
| 761 | <td colspan="1" rowspan="1"> |
---|
| 762 | |
---|
| 763 | <span class="codefrag">}</span> |
---|
| 764 | </td> |
---|
| 765 | |
---|
| 766 | </tr> |
---|
| 767 | |
---|
| 768 | <tr> |
---|
| 769 | |
---|
| 770 | <td colspan="1" rowspan="1">26.</td> |
---|
| 771 | <td colspan="1" rowspan="1"> |
---|
| 772 | |
---|
| 773 | <span class="codefrag">}</span> |
---|
| 774 | </td> |
---|
| 775 | |
---|
| 776 | </tr> |
---|
| 777 | |
---|
| 778 | <tr> |
---|
| 779 | |
---|
| 780 | <td colspan="1" rowspan="1">27.</td> |
---|
| 781 | <td colspan="1" rowspan="1"></td> |
---|
| 782 | |
---|
| 783 | </tr> |
---|
| 784 | |
---|
| 785 | <tr> |
---|
| 786 | |
---|
| 787 | <td colspan="1" rowspan="1">28.</td> |
---|
| 788 | <td colspan="1" rowspan="1"> |
---|
| 789 | |
---|
| 790 | <span class="codefrag"> |
---|
| 791 | public static class Reduce extends MapReduceBase implements |
---|
| 792 | Reducer<Text, IntWritable, Text, IntWritable> { |
---|
| 793 | </span> |
---|
| 794 | </td> |
---|
| 795 | |
---|
| 796 | </tr> |
---|
| 797 | |
---|
| 798 | <tr> |
---|
| 799 | |
---|
| 800 | <td colspan="1" rowspan="1">29.</td> |
---|
| 801 | <td colspan="1" rowspan="1"> |
---|
| 802 | |
---|
| 803 | <span class="codefrag"> |
---|
| 804 | public void reduce(Text key, Iterator<IntWritable> values, |
---|
| 805 | OutputCollector<Text, IntWritable> output, |
---|
| 806 | Reporter reporter) throws IOException { |
---|
| 807 | </span> |
---|
| 808 | </td> |
---|
| 809 | |
---|
| 810 | </tr> |
---|
| 811 | |
---|
| 812 | <tr> |
---|
| 813 | |
---|
| 814 | <td colspan="1" rowspan="1">30.</td> |
---|
| 815 | <td colspan="1" rowspan="1"> |
---|
| 816 | |
---|
| 817 | <span class="codefrag">int sum = 0;</span> |
---|
| 818 | </td> |
---|
| 819 | |
---|
| 820 | </tr> |
---|
| 821 | |
---|
| 822 | <tr> |
---|
| 823 | |
---|
| 824 | <td colspan="1" rowspan="1">31.</td> |
---|
| 825 | <td colspan="1" rowspan="1"> |
---|
| 826 | |
---|
| 827 | <span class="codefrag">while (values.hasNext()) {</span> |
---|
| 828 | </td> |
---|
| 829 | |
---|
| 830 | </tr> |
---|
| 831 | |
---|
| 832 | <tr> |
---|
| 833 | |
---|
| 834 | <td colspan="1" rowspan="1">32.</td> |
---|
| 835 | <td colspan="1" rowspan="1"> |
---|
| 836 | |
---|
| 837 | <span class="codefrag">sum += values.next().get();</span> |
---|
| 838 | </td> |
---|
| 839 | |
---|
| 840 | </tr> |
---|
| 841 | |
---|
| 842 | <tr> |
---|
| 843 | |
---|
| 844 | <td colspan="1" rowspan="1">33.</td> |
---|
| 845 | <td colspan="1" rowspan="1"> |
---|
| 846 | |
---|
| 847 | <span class="codefrag">}</span> |
---|
| 848 | </td> |
---|
| 849 | |
---|
| 850 | </tr> |
---|
| 851 | |
---|
| 852 | <tr> |
---|
| 853 | |
---|
| 854 | <td colspan="1" rowspan="1">34.</td> |
---|
| 855 | <td colspan="1" rowspan="1"> |
---|
| 856 | |
---|
| 857 | <span class="codefrag">output.collect(key, new IntWritable(sum));</span> |
---|
| 858 | </td> |
---|
| 859 | |
---|
| 860 | </tr> |
---|
| 861 | |
---|
| 862 | <tr> |
---|
| 863 | |
---|
| 864 | <td colspan="1" rowspan="1">35.</td> |
---|
| 865 | <td colspan="1" rowspan="1"> |
---|
| 866 | |
---|
| 867 | <span class="codefrag">}</span> |
---|
| 868 | </td> |
---|
| 869 | |
---|
| 870 | </tr> |
---|
| 871 | |
---|
| 872 | <tr> |
---|
| 873 | |
---|
| 874 | <td colspan="1" rowspan="1">36.</td> |
---|
| 875 | <td colspan="1" rowspan="1"> |
---|
| 876 | |
---|
| 877 | <span class="codefrag">}</span> |
---|
| 878 | </td> |
---|
| 879 | |
---|
| 880 | </tr> |
---|
| 881 | |
---|
| 882 | <tr> |
---|
| 883 | |
---|
| 884 | <td colspan="1" rowspan="1">37.</td> |
---|
| 885 | <td colspan="1" rowspan="1"></td> |
---|
| 886 | |
---|
| 887 | </tr> |
---|
| 888 | |
---|
| 889 | <tr> |
---|
| 890 | |
---|
| 891 | <td colspan="1" rowspan="1">38.</td> |
---|
| 892 | <td colspan="1" rowspan="1"> |
---|
| 893 | |
---|
| 894 | <span class="codefrag"> |
---|
| 895 | public static void main(String[] args) throws Exception { |
---|
| 896 | </span> |
---|
| 897 | </td> |
---|
| 898 | |
---|
| 899 | </tr> |
---|
| 900 | |
---|
| 901 | <tr> |
---|
| 902 | |
---|
| 903 | <td colspan="1" rowspan="1">39.</td> |
---|
| 904 | <td colspan="1" rowspan="1"> |
---|
| 905 | |
---|
| 906 | <span class="codefrag"> |
---|
| 907 | JobConf conf = new JobConf(WordCount.class); |
---|
| 908 | </span> |
---|
| 909 | </td> |
---|
| 910 | |
---|
| 911 | </tr> |
---|
| 912 | |
---|
| 913 | <tr> |
---|
| 914 | |
---|
| 915 | <td colspan="1" rowspan="1">40.</td> |
---|
| 916 | <td colspan="1" rowspan="1"> |
---|
| 917 | |
---|
| 918 | <span class="codefrag">conf.setJobName("wordcount");</span> |
---|
| 919 | </td> |
---|
| 920 | |
---|
| 921 | </tr> |
---|
| 922 | |
---|
| 923 | <tr> |
---|
| 924 | |
---|
| 925 | <td colspan="1" rowspan="1">41.</td> |
---|
| 926 | <td colspan="1" rowspan="1"></td> |
---|
| 927 | |
---|
| 928 | </tr> |
---|
| 929 | |
---|
| 930 | <tr> |
---|
| 931 | |
---|
| 932 | <td colspan="1" rowspan="1">42.</td> |
---|
| 933 | <td colspan="1" rowspan="1"> |
---|
| 934 | |
---|
| 935 | <span class="codefrag">conf.setOutputKeyClass(Text.class);</span> |
---|
| 936 | </td> |
---|
| 937 | |
---|
| 938 | </tr> |
---|
| 939 | |
---|
| 940 | <tr> |
---|
| 941 | |
---|
| 942 | <td colspan="1" rowspan="1">43.</td> |
---|
| 943 | <td colspan="1" rowspan="1"> |
---|
| 944 | |
---|
| 945 | <span class="codefrag">conf.setOutputValueClass(IntWritable.class);</span> |
---|
| 946 | </td> |
---|
| 947 | |
---|
| 948 | </tr> |
---|
| 949 | |
---|
| 950 | <tr> |
---|
| 951 | |
---|
| 952 | <td colspan="1" rowspan="1">44.</td> |
---|
| 953 | <td colspan="1" rowspan="1"></td> |
---|
| 954 | |
---|
| 955 | </tr> |
---|
| 956 | |
---|
| 957 | <tr> |
---|
| 958 | |
---|
| 959 | <td colspan="1" rowspan="1">45.</td> |
---|
| 960 | <td colspan="1" rowspan="1"> |
---|
| 961 | |
---|
| 962 | <span class="codefrag">conf.setMapperClass(Map.class);</span> |
---|
| 963 | </td> |
---|
| 964 | |
---|
| 965 | </tr> |
---|
| 966 | |
---|
| 967 | <tr> |
---|
| 968 | |
---|
| 969 | <td colspan="1" rowspan="1">46.</td> |
---|
| 970 | <td colspan="1" rowspan="1"> |
---|
| 971 | |
---|
| 972 | <span class="codefrag">conf.setCombinerClass(Reduce.class);</span> |
---|
| 973 | </td> |
---|
| 974 | |
---|
| 975 | </tr> |
---|
| 976 | |
---|
| 977 | <tr> |
---|
| 978 | |
---|
| 979 | <td colspan="1" rowspan="1">47.</td> |
---|
| 980 | <td colspan="1" rowspan="1"> |
---|
| 981 | |
---|
| 982 | <span class="codefrag">conf.setReducerClass(Reduce.class);</span> |
---|
| 983 | </td> |
---|
| 984 | |
---|
| 985 | </tr> |
---|
| 986 | |
---|
| 987 | <tr> |
---|
| 988 | |
---|
| 989 | <td colspan="1" rowspan="1">48.</td> |
---|
| 990 | <td colspan="1" rowspan="1"></td> |
---|
| 991 | |
---|
| 992 | </tr> |
---|
| 993 | |
---|
| 994 | <tr> |
---|
| 995 | |
---|
| 996 | <td colspan="1" rowspan="1">49.</td> |
---|
| 997 | <td colspan="1" rowspan="1"> |
---|
| 998 | |
---|
| 999 | <span class="codefrag">conf.setInputFormat(TextInputFormat.class);</span> |
---|
| 1000 | </td> |
---|
| 1001 | |
---|
| 1002 | </tr> |
---|
| 1003 | |
---|
| 1004 | <tr> |
---|
| 1005 | |
---|
| 1006 | <td colspan="1" rowspan="1">50.</td> |
---|
| 1007 | <td colspan="1" rowspan="1"> |
---|
| 1008 | |
---|
| 1009 | <span class="codefrag">conf.setOutputFormat(TextOutputFormat.class);</span> |
---|
| 1010 | </td> |
---|
| 1011 | |
---|
| 1012 | </tr> |
---|
| 1013 | |
---|
| 1014 | <tr> |
---|
| 1015 | |
---|
| 1016 | <td colspan="1" rowspan="1">51.</td> |
---|
| 1017 | <td colspan="1" rowspan="1"></td> |
---|
| 1018 | |
---|
| 1019 | </tr> |
---|
| 1020 | |
---|
| 1021 | <tr> |
---|
| 1022 | |
---|
| 1023 | <td colspan="1" rowspan="1">52.</td> |
---|
| 1024 | <td colspan="1" rowspan="1"> |
---|
| 1025 | |
---|
| 1026 | <span class="codefrag">FileInputFormat.setInputPaths(conf, new Path(args[0]));</span> |
---|
| 1027 | </td> |
---|
| 1028 | |
---|
| 1029 | </tr> |
---|
| 1030 | |
---|
| 1031 | <tr> |
---|
| 1032 | |
---|
| 1033 | <td colspan="1" rowspan="1">53.</td> |
---|
| 1034 | <td colspan="1" rowspan="1"> |
---|
| 1035 | |
---|
| 1036 | <span class="codefrag">FileOutputFormat.setOutputPath(conf, new Path(args[1]));</span> |
---|
| 1037 | </td> |
---|
| 1038 | |
---|
| 1039 | </tr> |
---|
| 1040 | |
---|
| 1041 | <tr> |
---|
| 1042 | |
---|
| 1043 | <td colspan="1" rowspan="1">54.</td> |
---|
| 1044 | <td colspan="1" rowspan="1"></td> |
---|
| 1045 | |
---|
| 1046 | </tr> |
---|
| 1047 | |
---|
| 1048 | <tr> |
---|
| 1049 | |
---|
| 1050 | <td colspan="1" rowspan="1">55.</td> |
---|
| 1051 | <td colspan="1" rowspan="1"> |
---|
| 1052 | |
---|
| 1053 | <span class="codefrag">JobClient.runJob(conf);</span> |
---|
| 1054 | </td> |
---|
| 1055 | |
---|
| 1056 | </tr> |
---|
| 1057 | |
---|
| 1058 | <tr> |
---|
| 1059 | |
---|
| 1060 | <td colspan="1" rowspan="1">57.</td> |
---|
| 1061 | <td colspan="1" rowspan="1"> |
---|
| 1062 | |
---|
| 1063 | <span class="codefrag">}</span> |
---|
| 1064 | </td> |
---|
| 1065 | |
---|
| 1066 | </tr> |
---|
| 1067 | |
---|
| 1068 | <tr> |
---|
| 1069 | |
---|
| 1070 | <td colspan="1" rowspan="1">58.</td> |
---|
| 1071 | <td colspan="1" rowspan="1"> |
---|
| 1072 | <span class="codefrag">}</span> |
---|
| 1073 | </td> |
---|
| 1074 | |
---|
| 1075 | </tr> |
---|
| 1076 | |
---|
| 1077 | <tr> |
---|
| 1078 | |
---|
| 1079 | <td colspan="1" rowspan="1">59.</td> |
---|
| 1080 | <td colspan="1" rowspan="1"></td> |
---|
| 1081 | |
---|
| 1082 | </tr> |
---|
| 1083 | |
---|
| 1084 | </table> |
---|
| 1085 | <a name="N10464"></a><a name="Usage"></a> |
---|
| 1086 | <h3 class="h4">Usage</h3> |
---|
| 1087 | <p>Assuming <span class="codefrag">HADOOP_HOME</span> is the root of the installation and |
---|
| 1088 | <span class="codefrag">HADOOP_VERSION</span> is the Hadoop version installed, compile |
---|
| 1089 | <span class="codefrag">WordCount.java</span> and create a jar:</p> |
---|
| 1090 | <p> |
---|
| 1091 | |
---|
| 1092 | <span class="codefrag">$ mkdir wordcount_classes</span> |
---|
| 1093 | <br> |
---|
| 1094 | |
---|
| 1095 | <span class="codefrag"> |
---|
| 1096 | $ javac -classpath ${HADOOP_HOME}/hadoop-${HADOOP_VERSION}-core.jar |
---|
| 1097 | -d wordcount_classes WordCount.java |
---|
| 1098 | </span> |
---|
| 1099 | <br> |
---|
| 1100 | |
---|
| 1101 | <span class="codefrag">$ jar -cvf /usr/joe/wordcount.jar -C wordcount_classes/ .</span> |
---|
| 1102 | |
---|
| 1103 | </p> |
---|
| 1104 | <p>Assuming that:</p> |
---|
| 1105 | <ul> |
---|
| 1106 | |
---|
| 1107 | <li> |
---|
| 1108 | |
---|
| 1109 | <span class="codefrag">/usr/joe/wordcount/input</span> - input directory in HDFS |
---|
| 1110 | </li> |
---|
| 1111 | |
---|
| 1112 | <li> |
---|
| 1113 | |
---|
| 1114 | <span class="codefrag">/usr/joe/wordcount/output</span> - output directory in HDFS |
---|
| 1115 | </li> |
---|
| 1116 | |
---|
| 1117 | </ul> |
---|
| 1118 | <p>Sample text-files as input:</p> |
---|
| 1119 | <p> |
---|
| 1120 | |
---|
| 1121 | <span class="codefrag">$ bin/hadoop dfs -ls /usr/joe/wordcount/input/</span> |
---|
| 1122 | <br> |
---|
| 1123 | |
---|
| 1124 | <span class="codefrag">/usr/joe/wordcount/input/file01</span> |
---|
| 1125 | <br> |
---|
| 1126 | |
---|
| 1127 | <span class="codefrag">/usr/joe/wordcount/input/file02</span> |
---|
| 1128 | <br> |
---|
| 1129 | |
---|
| 1130 | <br> |
---|
| 1131 | |
---|
| 1132 | <span class="codefrag">$ bin/hadoop dfs -cat /usr/joe/wordcount/input/file01</span> |
---|
| 1133 | <br> |
---|
| 1134 | |
---|
| 1135 | <span class="codefrag">Hello World Bye World</span> |
---|
| 1136 | <br> |
---|
| 1137 | |
---|
| 1138 | <br> |
---|
| 1139 | |
---|
| 1140 | <span class="codefrag">$ bin/hadoop dfs -cat /usr/joe/wordcount/input/file02</span> |
---|
| 1141 | <br> |
---|
| 1142 | |
---|
| 1143 | <span class="codefrag">Hello Hadoop Goodbye Hadoop</span> |
---|
| 1144 | |
---|
| 1145 | </p> |
---|
| 1146 | <p>Run the application:</p> |
---|
| 1147 | <p> |
---|
| 1148 | |
---|
| 1149 | <span class="codefrag"> |
---|
| 1150 | $ bin/hadoop jar /usr/joe/wordcount.jar org.myorg.WordCount |
---|
| 1151 | /usr/joe/wordcount/input /usr/joe/wordcount/output |
---|
| 1152 | </span> |
---|
| 1153 | |
---|
| 1154 | </p> |
---|
| 1155 | <p>Output:</p> |
---|
| 1156 | <p> |
---|
| 1157 | |
---|
| 1158 | <span class="codefrag"> |
---|
| 1159 | $ bin/hadoop dfs -cat /usr/joe/wordcount/output/part-00000 |
---|
| 1160 | </span> |
---|
| 1161 | |
---|
| 1162 | <br> |
---|
| 1163 | |
---|
| 1164 | <span class="codefrag">Bye 1</span> |
---|
| 1165 | <br> |
---|
| 1166 | |
---|
| 1167 | <span class="codefrag">Goodbye 1</span> |
---|
| 1168 | <br> |
---|
| 1169 | |
---|
| 1170 | <span class="codefrag">Hadoop 2</span> |
---|
| 1171 | <br> |
---|
| 1172 | |
---|
| 1173 | <span class="codefrag">Hello 2</span> |
---|
| 1174 | <br> |
---|
| 1175 | |
---|
| 1176 | <span class="codefrag">World 2</span> |
---|
| 1177 | <br> |
---|
| 1178 | |
---|
| 1179 | </p> |
---|
| 1180 | <p> Applications can specify a comma separated list of paths which |
---|
| 1181 | would be present in the current working directory of the task |
---|
| 1182 | using the option <span class="codefrag">-files</span>. The <span class="codefrag">-libjars</span> |
---|
| 1183 | option allows applications to add jars to the classpaths of the maps |
---|
| 1184 | and reduces. The <span class="codefrag">-archives</span> allows them to pass archives |
---|
| 1185 | as arguments that are unzipped/unjarred and a link with name of the |
---|
| 1186 | jar/zip are created in the current working directory of tasks. More |
---|
| 1187 | details about the command line options are available at |
---|
| 1188 | <a href="commands_manual.html"> Hadoop Command Guide.</a> |
---|
| 1189 | </p> |
---|
| 1190 | <p>Running <span class="codefrag">wordcount</span> example with |
---|
| 1191 | <span class="codefrag">-libjars</span> and <span class="codefrag">-files</span>:<br> |
---|
| 1192 | |
---|
| 1193 | <span class="codefrag"> hadoop jar hadoop-examples.jar wordcount -files cachefile.txt |
---|
| 1194 | -libjars mylib.jar input output </span> |
---|
| 1195 | |
---|
| 1196 | </p> |
---|
| 1197 | <a name="N10504"></a><a name="Walk-through"></a> |
---|
| 1198 | <h3 class="h4">Walk-through</h3> |
---|
| 1199 | <p>The <span class="codefrag">WordCount</span> application is quite straight-forward.</p> |
---|
| 1200 | <p>The <span class="codefrag">Mapper</span> implementation (lines 14-26), via the |
---|
| 1201 | <span class="codefrag">map</span> method (lines 18-25), processes one line at a time, |
---|
| 1202 | as provided by the specified <span class="codefrag">TextInputFormat</span> (line 49). |
---|
| 1203 | It then splits the line into tokens separated by whitespaces, via the |
---|
| 1204 | <span class="codefrag">StringTokenizer</span>, and emits a key-value pair of |
---|
| 1205 | <span class="codefrag">< <word>, 1></span>.</p> |
---|
| 1206 | <p> |
---|
| 1207 | For the given sample input the first map emits:<br> |
---|
| 1208 | |
---|
| 1209 | <span class="codefrag">< Hello, 1></span> |
---|
| 1210 | <br> |
---|
| 1211 | |
---|
| 1212 | <span class="codefrag">< World, 1></span> |
---|
| 1213 | <br> |
---|
| 1214 | |
---|
| 1215 | <span class="codefrag">< Bye, 1></span> |
---|
| 1216 | <br> |
---|
| 1217 | |
---|
| 1218 | <span class="codefrag">< World, 1></span> |
---|
| 1219 | <br> |
---|
| 1220 | |
---|
| 1221 | </p> |
---|
| 1222 | <p> |
---|
| 1223 | The second map emits:<br> |
---|
| 1224 | |
---|
| 1225 | <span class="codefrag">< Hello, 1></span> |
---|
| 1226 | <br> |
---|
| 1227 | |
---|
| 1228 | <span class="codefrag">< Hadoop, 1></span> |
---|
| 1229 | <br> |
---|
| 1230 | |
---|
| 1231 | <span class="codefrag">< Goodbye, 1></span> |
---|
| 1232 | <br> |
---|
| 1233 | |
---|
| 1234 | <span class="codefrag">< Hadoop, 1></span> |
---|
| 1235 | <br> |
---|
| 1236 | |
---|
| 1237 | </p> |
---|
| 1238 | <p>We'll learn more about the number of maps spawned for a given job, and |
---|
| 1239 | how to control them in a fine-grained manner, a bit later in the |
---|
| 1240 | tutorial.</p> |
---|
| 1241 | <p> |
---|
| 1242 | <span class="codefrag">WordCount</span> also specifies a <span class="codefrag">combiner</span> (line |
---|
| 1243 | 46). Hence, the output of each map is passed through the local combiner |
---|
| 1244 | (which is same as the <span class="codefrag">Reducer</span> as per the job |
---|
| 1245 | configuration) for local aggregation, after being sorted on the |
---|
| 1246 | <em>key</em>s.</p> |
---|
| 1247 | <p> |
---|
| 1248 | The output of the first map:<br> |
---|
| 1249 | |
---|
| 1250 | <span class="codefrag">< Bye, 1></span> |
---|
| 1251 | <br> |
---|
| 1252 | |
---|
| 1253 | <span class="codefrag">< Hello, 1></span> |
---|
| 1254 | <br> |
---|
| 1255 | |
---|
| 1256 | <span class="codefrag">< World, 2></span> |
---|
| 1257 | <br> |
---|
| 1258 | |
---|
| 1259 | </p> |
---|
| 1260 | <p> |
---|
| 1261 | The output of the second map:<br> |
---|
| 1262 | |
---|
| 1263 | <span class="codefrag">< Goodbye, 1></span> |
---|
| 1264 | <br> |
---|
| 1265 | |
---|
| 1266 | <span class="codefrag">< Hadoop, 2></span> |
---|
| 1267 | <br> |
---|
| 1268 | |
---|
| 1269 | <span class="codefrag">< Hello, 1></span> |
---|
| 1270 | <br> |
---|
| 1271 | |
---|
| 1272 | </p> |
---|
| 1273 | <p>The <span class="codefrag">Reducer</span> implementation (lines 28-36), via the |
---|
| 1274 | <span class="codefrag">reduce</span> method (lines 29-35) just sums up the values, |
---|
| 1275 | which are the occurence counts for each key (i.e. words in this example). |
---|
| 1276 | </p> |
---|
| 1277 | <p> |
---|
| 1278 | Thus the output of the job is:<br> |
---|
| 1279 | |
---|
| 1280 | <span class="codefrag">< Bye, 1></span> |
---|
| 1281 | <br> |
---|
| 1282 | |
---|
| 1283 | <span class="codefrag">< Goodbye, 1></span> |
---|
| 1284 | <br> |
---|
| 1285 | |
---|
| 1286 | <span class="codefrag">< Hadoop, 2></span> |
---|
| 1287 | <br> |
---|
| 1288 | |
---|
| 1289 | <span class="codefrag">< Hello, 2></span> |
---|
| 1290 | <br> |
---|
| 1291 | |
---|
| 1292 | <span class="codefrag">< World, 2></span> |
---|
| 1293 | <br> |
---|
| 1294 | |
---|
| 1295 | </p> |
---|
| 1296 | <p>The <span class="codefrag">run</span> method specifies various facets of the job, such |
---|
| 1297 | as the input/output paths (passed via the command line), key/value |
---|
| 1298 | types, input/output formats etc., in the <span class="codefrag">JobConf</span>. |
---|
| 1299 | It then calls the <span class="codefrag">JobClient.runJob</span> (line 55) to submit the |
---|
| 1300 | and monitor its progress.</p> |
---|
| 1301 | <p>We'll learn more about <span class="codefrag">JobConf</span>, <span class="codefrag">JobClient</span>, |
---|
| 1302 | <span class="codefrag">Tool</span> and other interfaces and classes a bit later in the |
---|
| 1303 | tutorial.</p> |
---|
| 1304 | </div> |
---|
| 1305 | |
---|
| 1306 | |
---|
| 1307 | <a name="N105BB"></a><a name="Map%2FReduce+-+User+Interfaces"></a> |
---|
| 1308 | <h2 class="h3">Map/Reduce - User Interfaces</h2> |
---|
| 1309 | <div class="section"> |
---|
| 1310 | <p>This section provides a reasonable amount of detail on every user-facing |
---|
| 1311 | aspect of the Map/Reduce framwork. This should help users implement, |
---|
| 1312 | configure and tune their jobs in a fine-grained manner. However, please |
---|
| 1313 | note that the javadoc for each class/interface remains the most |
---|
| 1314 | comprehensive documentation available; this is only meant to be a tutorial. |
---|
| 1315 | </p> |
---|
| 1316 | <p>Let us first take the <span class="codefrag">Mapper</span> and <span class="codefrag">Reducer</span> |
---|
| 1317 | interfaces. Applications typically implement them to provide the |
---|
| 1318 | <span class="codefrag">map</span> and <span class="codefrag">reduce</span> methods.</p> |
---|
| 1319 | <p>We will then discuss other core interfaces including |
---|
| 1320 | <span class="codefrag">JobConf</span>, <span class="codefrag">JobClient</span>, <span class="codefrag">Partitioner</span>, |
---|
| 1321 | <span class="codefrag">OutputCollector</span>, <span class="codefrag">Reporter</span>, |
---|
| 1322 | <span class="codefrag">InputFormat</span>, <span class="codefrag">OutputFormat</span>, |
---|
| 1323 | <span class="codefrag">OutputCommitter</span> and others.</p> |
---|
| 1324 | <p>Finally, we will wrap up by discussing some useful features of the |
---|
| 1325 | framework such as the <span class="codefrag">DistributedCache</span>, |
---|
| 1326 | <span class="codefrag">IsolationRunner</span> etc.</p> |
---|
| 1327 | <a name="N105F7"></a><a name="Payload"></a> |
---|
| 1328 | <h3 class="h4">Payload</h3> |
---|
| 1329 | <p>Applications typically implement the <span class="codefrag">Mapper</span> and |
---|
| 1330 | <span class="codefrag">Reducer</span> interfaces to provide the <span class="codefrag">map</span> and |
---|
| 1331 | <span class="codefrag">reduce</span> methods. These form the core of the job.</p> |
---|
| 1332 | <a name="N1060C"></a><a name="Mapper"></a> |
---|
| 1333 | <h4>Mapper</h4> |
---|
| 1334 | <p> |
---|
| 1335 | <a href="api/org/apache/hadoop/mapred/Mapper.html"> |
---|
| 1336 | Mapper</a> maps input key/value pairs to a set of intermediate |
---|
| 1337 | key/value pairs.</p> |
---|
| 1338 | <p>Maps are the individual tasks that transform input records into |
---|
| 1339 | intermediate records. The transformed intermediate records do not need |
---|
| 1340 | to be of the same type as the input records. A given input pair may |
---|
| 1341 | map to zero or many output pairs.</p> |
---|
| 1342 | <p>The Hadoop Map/Reduce framework spawns one map task for each |
---|
| 1343 | <span class="codefrag">InputSplit</span> generated by the <span class="codefrag">InputFormat</span> for |
---|
| 1344 | the job.</p> |
---|
| 1345 | <p>Overall, <span class="codefrag">Mapper</span> implementations are passed the |
---|
| 1346 | <span class="codefrag">JobConf</span> for the job via the |
---|
| 1347 | <a href="api/org/apache/hadoop/mapred/JobConfigurable.html#configure(org.apache.hadoop.mapred.JobConf)"> |
---|
| 1348 | JobConfigurable.configure(JobConf)</a> method and override it to |
---|
| 1349 | initialize themselves. The framework then calls |
---|
| 1350 | <a href="api/org/apache/hadoop/mapred/Mapper.html#map(K1, V1, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"> |
---|
| 1351 | map(WritableComparable, Writable, OutputCollector, Reporter)</a> for |
---|
| 1352 | each key/value pair in the <span class="codefrag">InputSplit</span> for that task. |
---|
| 1353 | Applications can then override the |
---|
| 1354 | <a href="api/org/apache/hadoop/io/Closeable.html#close()"> |
---|
| 1355 | Closeable.close()</a> method to perform any required cleanup.</p> |
---|
| 1356 | <p>Output pairs do not need to be of the same types as input pairs. A |
---|
| 1357 | given input pair may map to zero or many output pairs. Output pairs |
---|
| 1358 | are collected with calls to |
---|
| 1359 | <a href="api/org/apache/hadoop/mapred/OutputCollector.html#collect(K, V)"> |
---|
| 1360 | OutputCollector.collect(WritableComparable,Writable)</a>.</p> |
---|
| 1361 | <p>Applications can use the <span class="codefrag">Reporter</span> to report |
---|
| 1362 | progress, set application-level status messages and update |
---|
| 1363 | <span class="codefrag">Counters</span>, or just indicate that they are alive.</p> |
---|
| 1364 | <p>All intermediate values associated with a given output key are |
---|
| 1365 | subsequently grouped by the framework, and passed to the |
---|
| 1366 | <span class="codefrag">Reducer</span>(s) to determine the final output. Users can |
---|
| 1367 | control the grouping by specifying a <span class="codefrag">Comparator</span> via |
---|
| 1368 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setOutputKeyComparatorClass(java.lang.Class)"> |
---|
| 1369 | JobConf.setOutputKeyComparatorClass(Class)</a>.</p> |
---|
| 1370 | <p>The <span class="codefrag">Mapper</span> outputs are sorted and then |
---|
| 1371 | partitioned per <span class="codefrag">Reducer</span>. The total number of partitions is |
---|
| 1372 | the same as the number of reduce tasks for the job. Users can control |
---|
| 1373 | which keys (and hence records) go to which <span class="codefrag">Reducer</span> by |
---|
| 1374 | implementing a custom <span class="codefrag">Partitioner</span>.</p> |
---|
| 1375 | <p>Users can optionally specify a <span class="codefrag">combiner</span>, via |
---|
| 1376 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setCombinerClass(java.lang.Class)"> |
---|
| 1377 | JobConf.setCombinerClass(Class)</a>, to perform local aggregation of |
---|
| 1378 | the intermediate outputs, which helps to cut down the amount of data |
---|
| 1379 | transferred from the <span class="codefrag">Mapper</span> to the <span class="codefrag">Reducer</span>. |
---|
| 1380 | </p> |
---|
| 1381 | <p>The intermediate, sorted outputs are always stored in a simple |
---|
| 1382 | (key-len, key, value-len, value) format. |
---|
| 1383 | Applications can control if, and how, the |
---|
| 1384 | intermediate outputs are to be compressed and the |
---|
| 1385 | <a href="api/org/apache/hadoop/io/compress/CompressionCodec.html"> |
---|
| 1386 | CompressionCodec</a> to be used via the <span class="codefrag">JobConf</span>. |
---|
| 1387 | </p> |
---|
| 1388 | <a name="N10682"></a><a name="How+Many+Maps%3F"></a> |
---|
| 1389 | <h5>How Many Maps?</h5> |
---|
| 1390 | <p>The number of maps is usually driven by the total size of the |
---|
| 1391 | inputs, that is, the total number of blocks of the input files.</p> |
---|
| 1392 | <p>The right level of parallelism for maps seems to be around 10-100 |
---|
| 1393 | maps per-node, although it has been set up to 300 maps for very |
---|
| 1394 | cpu-light map tasks. Task setup takes awhile, so it is best if the |
---|
| 1395 | maps take at least a minute to execute.</p> |
---|
| 1396 | <p>Thus, if you expect 10TB of input data and have a blocksize of |
---|
| 1397 | <span class="codefrag">128MB</span>, you'll end up with 82,000 maps, unless |
---|
| 1398 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setNumMapTasks(int)"> |
---|
| 1399 | setNumMapTasks(int)</a> (which only provides a hint to the framework) |
---|
| 1400 | is used to set it even higher.</p> |
---|
| 1401 | <a name="N1069A"></a><a name="Reducer"></a> |
---|
| 1402 | <h4>Reducer</h4> |
---|
| 1403 | <p> |
---|
| 1404 | <a href="api/org/apache/hadoop/mapred/Reducer.html"> |
---|
| 1405 | Reducer</a> reduces a set of intermediate values which share a key to |
---|
| 1406 | a smaller set of values.</p> |
---|
| 1407 | <p>The number of reduces for the job is set by the user |
---|
| 1408 | via <a href="api/org/apache/hadoop/mapred/JobConf.html#setNumReduceTasks(int)"> |
---|
| 1409 | JobConf.setNumReduceTasks(int)</a>.</p> |
---|
| 1410 | <p>Overall, <span class="codefrag">Reducer</span> implementations are passed the |
---|
| 1411 | <span class="codefrag">JobConf</span> for the job via the |
---|
| 1412 | <a href="api/org/apache/hadoop/mapred/JobConfigurable.html#configure(org.apache.hadoop.mapred.JobConf)"> |
---|
| 1413 | JobConfigurable.configure(JobConf)</a> method and can override it to |
---|
| 1414 | initialize themselves. The framework then calls |
---|
| 1415 | <a href="api/org/apache/hadoop/mapred/Reducer.html#reduce(K2, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"> |
---|
| 1416 | reduce(WritableComparable, Iterator, OutputCollector, Reporter)</a> |
---|
| 1417 | method for each <span class="codefrag"><key, (list of values)></span> |
---|
| 1418 | pair in the grouped inputs. Applications can then override the |
---|
| 1419 | <a href="api/org/apache/hadoop/io/Closeable.html#close()"> |
---|
| 1420 | Closeable.close()</a> method to perform any required cleanup.</p> |
---|
| 1421 | <p> |
---|
| 1422 | <span class="codefrag">Reducer</span> has 3 primary phases: shuffle, sort and reduce. |
---|
| 1423 | </p> |
---|
| 1424 | <a name="N106CA"></a><a name="Shuffle"></a> |
---|
| 1425 | <h5>Shuffle</h5> |
---|
| 1426 | <p>Input to the <span class="codefrag">Reducer</span> is the sorted output of the |
---|
| 1427 | mappers. In this phase the framework fetches the relevant partition |
---|
| 1428 | of the output of all the mappers, via HTTP.</p> |
---|
| 1429 | <a name="N106D7"></a><a name="Sort"></a> |
---|
| 1430 | <h5>Sort</h5> |
---|
| 1431 | <p>The framework groups <span class="codefrag">Reducer</span> inputs by keys (since |
---|
| 1432 | different mappers may have output the same key) in this stage.</p> |
---|
| 1433 | <p>The shuffle and sort phases occur simultaneously; while |
---|
| 1434 | map-outputs are being fetched they are merged.</p> |
---|
| 1435 | <a name="N106E6"></a><a name="Secondary+Sort"></a> |
---|
| 1436 | <h5>Secondary Sort</h5> |
---|
| 1437 | <p>If equivalence rules for grouping the intermediate keys are |
---|
| 1438 | required to be different from those for grouping keys before |
---|
| 1439 | reduction, then one may specify a <span class="codefrag">Comparator</span> via |
---|
| 1440 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setOutputValueGroupingComparator(java.lang.Class)"> |
---|
| 1441 | JobConf.setOutputValueGroupingComparator(Class)</a>. Since |
---|
| 1442 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setOutputKeyComparatorClass(java.lang.Class)"> |
---|
| 1443 | JobConf.setOutputKeyComparatorClass(Class)</a> can be used to |
---|
| 1444 | control how intermediate keys are grouped, these can be used in |
---|
| 1445 | conjunction to simulate <em>secondary sort on values</em>.</p> |
---|
| 1446 | <a name="N106FF"></a><a name="Reduce"></a> |
---|
| 1447 | <h5>Reduce</h5> |
---|
| 1448 | <p>In this phase the |
---|
| 1449 | <a href="api/org/apache/hadoop/mapred/Reducer.html#reduce(K2, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"> |
---|
| 1450 | reduce(WritableComparable, Iterator, OutputCollector, Reporter)</a> |
---|
| 1451 | method is called for each <span class="codefrag"><key, (list of values)></span> |
---|
| 1452 | pair in the grouped inputs.</p> |
---|
| 1453 | <p>The output of the reduce task is typically written to the |
---|
| 1454 | <a href="api/org/apache/hadoop/fs/FileSystem.html"> |
---|
| 1455 | FileSystem</a> via |
---|
| 1456 | <a href="api/org/apache/hadoop/mapred/OutputCollector.html#collect(K, V)"> |
---|
| 1457 | OutputCollector.collect(WritableComparable, Writable)</a>.</p> |
---|
| 1458 | <p>Applications can use the <span class="codefrag">Reporter</span> to report |
---|
| 1459 | progress, set application-level status messages and update |
---|
| 1460 | <span class="codefrag">Counters</span>, or just indicate that they are alive.</p> |
---|
| 1461 | <p>The output of the <span class="codefrag">Reducer</span> is <em>not sorted</em>.</p> |
---|
| 1462 | <a name="N1072D"></a><a name="How+Many+Reduces%3F"></a> |
---|
| 1463 | <h5>How Many Reduces?</h5> |
---|
| 1464 | <p>The right number of reduces seems to be <span class="codefrag">0.95</span> or |
---|
| 1465 | <span class="codefrag">1.75</span> multiplied by (<<em>no. of nodes</em>> * |
---|
| 1466 | <span class="codefrag">mapred.tasktracker.reduce.tasks.maximum</span>).</p> |
---|
| 1467 | <p>With <span class="codefrag">0.95</span> all of the reduces can launch immediately |
---|
| 1468 | and start transfering map outputs as the maps finish. With |
---|
| 1469 | <span class="codefrag">1.75</span> the faster nodes will finish their first round of |
---|
| 1470 | reduces and launch a second wave of reduces doing a much better job |
---|
| 1471 | of load balancing.</p> |
---|
| 1472 | <p>Increasing the number of reduces increases the framework overhead, |
---|
| 1473 | but increases load balancing and lowers the cost of failures.</p> |
---|
| 1474 | <p>The scaling factors above are slightly less than whole numbers to |
---|
| 1475 | reserve a few reduce slots in the framework for speculative-tasks and |
---|
| 1476 | failed tasks.</p> |
---|
| 1477 | <a name="N10752"></a><a name="Reducer+NONE"></a> |
---|
| 1478 | <h5>Reducer NONE</h5> |
---|
| 1479 | <p>It is legal to set the number of reduce-tasks to <em>zero</em> if |
---|
| 1480 | no reduction is desired.</p> |
---|
| 1481 | <p>In this case the outputs of the map-tasks go directly to the |
---|
| 1482 | <span class="codefrag">FileSystem</span>, into the output path set by |
---|
| 1483 | <a href="api/org/apache/hadoop/mapred/FileOutputFormat.html#setOutputPath(org.apache.hadoop.mapred.JobConf,%20org.apache.hadoop.fs.Path)"> |
---|
| 1484 | setOutputPath(Path)</a>. The framework does not sort the |
---|
| 1485 | map-outputs before writing them out to the <span class="codefrag">FileSystem</span>. |
---|
| 1486 | </p> |
---|
| 1487 | <a name="N1076D"></a><a name="Partitioner"></a> |
---|
| 1488 | <h4>Partitioner</h4> |
---|
| 1489 | <p> |
---|
| 1490 | <a href="api/org/apache/hadoop/mapred/Partitioner.html"> |
---|
| 1491 | Partitioner</a> partitions the key space.</p> |
---|
| 1492 | <p>Partitioner controls the partitioning of the keys of the |
---|
| 1493 | intermediate map-outputs. The key (or a subset of the key) is used to |
---|
| 1494 | derive the partition, typically by a <em>hash function</em>. The total |
---|
| 1495 | number of partitions is the same as the number of reduce tasks for the |
---|
| 1496 | job. Hence this controls which of the <span class="codefrag">m</span> reduce tasks the |
---|
| 1497 | intermediate key (and hence the record) is sent to for reduction.</p> |
---|
| 1498 | <p> |
---|
| 1499 | <a href="api/org/apache/hadoop/mapred/lib/HashPartitioner.html"> |
---|
| 1500 | HashPartitioner</a> is the default <span class="codefrag">Partitioner</span>.</p> |
---|
| 1501 | <a name="N1078C"></a><a name="Reporter"></a> |
---|
| 1502 | <h4>Reporter</h4> |
---|
| 1503 | <p> |
---|
| 1504 | <a href="api/org/apache/hadoop/mapred/Reporter.html"> |
---|
| 1505 | Reporter</a> is a facility for Map/Reduce applications to report |
---|
| 1506 | progress, set application-level status messages and update |
---|
| 1507 | <span class="codefrag">Counters</span>.</p> |
---|
| 1508 | <p> |
---|
| 1509 | <span class="codefrag">Mapper</span> and <span class="codefrag">Reducer</span> implementations can use |
---|
| 1510 | the <span class="codefrag">Reporter</span> to report progress or just indicate |
---|
| 1511 | that they are alive. In scenarios where the application takes a |
---|
| 1512 | significant amount of time to process individual key/value pairs, |
---|
| 1513 | this is crucial since the framework might assume that the task has |
---|
| 1514 | timed-out and kill that task. Another way to avoid this is to |
---|
| 1515 | set the configuration parameter <span class="codefrag">mapred.task.timeout</span> to a |
---|
| 1516 | high-enough value (or even set it to <em>zero</em> for no time-outs). |
---|
| 1517 | </p> |
---|
| 1518 | <p>Applications can also update <span class="codefrag">Counters</span> using the |
---|
| 1519 | <span class="codefrag">Reporter</span>.</p> |
---|
| 1520 | <a name="N107B6"></a><a name="OutputCollector"></a> |
---|
| 1521 | <h4>OutputCollector</h4> |
---|
| 1522 | <p> |
---|
| 1523 | <a href="api/org/apache/hadoop/mapred/OutputCollector.html"> |
---|
| 1524 | OutputCollector</a> is a generalization of the facility provided by |
---|
| 1525 | the Map/Reduce framework to collect data output by the |
---|
| 1526 | <span class="codefrag">Mapper</span> or the <span class="codefrag">Reducer</span> (either the |
---|
| 1527 | intermediate outputs or the output of the job).</p> |
---|
| 1528 | <p>Hadoop Map/Reduce comes bundled with a |
---|
| 1529 | <a href="api/org/apache/hadoop/mapred/lib/package-summary.html"> |
---|
| 1530 | library</a> of generally useful mappers, reducers, and partitioners.</p> |
---|
| 1531 | <a name="N107D1"></a><a name="Job+Configuration"></a> |
---|
| 1532 | <h3 class="h4">Job Configuration</h3> |
---|
| 1533 | <p> |
---|
| 1534 | <a href="api/org/apache/hadoop/mapred/JobConf.html"> |
---|
| 1535 | JobConf</a> represents a Map/Reduce job configuration.</p> |
---|
| 1536 | <p> |
---|
| 1537 | <span class="codefrag">JobConf</span> is the primary interface for a user to describe |
---|
| 1538 | a Map/Reduce job to the Hadoop framework for execution. The framework |
---|
| 1539 | tries to faithfully execute the job as described by <span class="codefrag">JobConf</span>, |
---|
| 1540 | however:</p> |
---|
| 1541 | <ul> |
---|
| 1542 | |
---|
| 1543 | <li>f |
---|
| 1544 | Some configuration parameters may have been marked as |
---|
| 1545 | <a href="api/org/apache/hadoop/conf/Configuration.html#FinalParams"> |
---|
| 1546 | final</a> by administrators and hence cannot be altered. |
---|
| 1547 | </li> |
---|
| 1548 | |
---|
| 1549 | <li> |
---|
| 1550 | While some job parameters are straight-forward to set (e.g. |
---|
| 1551 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setNumReduceTasks(int)"> |
---|
| 1552 | setNumReduceTasks(int)</a>), other parameters interact subtly with |
---|
| 1553 | the rest of the framework and/or job configuration and are |
---|
| 1554 | more complex to set (e.g. |
---|
| 1555 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setNumMapTasks(int)"> |
---|
| 1556 | setNumMapTasks(int)</a>). |
---|
| 1557 | </li> |
---|
| 1558 | |
---|
| 1559 | </ul> |
---|
| 1560 | <p> |
---|
| 1561 | <span class="codefrag">JobConf</span> is typically used to specify the |
---|
| 1562 | <span class="codefrag">Mapper</span>, combiner (if any), <span class="codefrag">Partitioner</span>, |
---|
| 1563 | <span class="codefrag">Reducer</span>, <span class="codefrag">InputFormat</span>, |
---|
| 1564 | <span class="codefrag">OutputFormat</span> and <span class="codefrag">OutputCommitter</span> |
---|
| 1565 | implementations. <span class="codefrag">JobConf</span> also |
---|
| 1566 | indicates the set of input files |
---|
| 1567 | (<a href="api/org/apache/hadoop/mapred/FileInputFormat.html#setInputPaths(org.apache.hadoop.mapred.JobConf,%20org.apache.hadoop.fs.Path[])">setInputPaths(JobConf, Path...)</a> |
---|
| 1568 | /<a href="api/org/apache/hadoop/mapred/FileInputFormat.html#addInputPath(org.apache.hadoop.mapred.JobConf,%20org.apache.hadoop.fs.Path)">addInputPath(JobConf, Path)</a>) |
---|
| 1569 | and (<a href="api/org/apache/hadoop/mapred/FileInputFormat.html#setInputPaths(org.apache.hadoop.mapred.JobConf,%20java.lang.String)">setInputPaths(JobConf, String)</a> |
---|
| 1570 | /<a href="api/org/apache/hadoop/mapred/FileInputFormat.html#addInputPath(org.apache.hadoop.mapred.JobConf,%20java.lang.String)">addInputPaths(JobConf, String)</a>) |
---|
| 1571 | and where the output files should be written |
---|
| 1572 | (<a href="api/org/apache/hadoop/mapred/FileOutputFormat.html#setOutputPath(org.apache.hadoop.mapred.JobConf,%20org.apache.hadoop.fs.Path)">setOutputPath(Path)</a>).</p> |
---|
| 1573 | <p>Optionally, <span class="codefrag">JobConf</span> is used to specify other advanced |
---|
| 1574 | facets of the job such as the <span class="codefrag">Comparator</span> to be used, files |
---|
| 1575 | to be put in the <span class="codefrag">DistributedCache</span>, whether intermediate |
---|
| 1576 | and/or job outputs are to be compressed (and how), debugging via |
---|
| 1577 | user-provided scripts |
---|
| 1578 | (<a href="api/org/apache/hadoop/mapred/JobConf.html#setMapDebugScript(java.lang.String)">setMapDebugScript(String)</a>/<a href="api/org/apache/hadoop/mapred/JobConf.html#setReduceDebugScript(java.lang.String)">setReduceDebugScript(String)</a>) |
---|
| 1579 | , whether job tasks can be executed in a <em>speculative</em> manner |
---|
| 1580 | (<a href="api/org/apache/hadoop/mapred/JobConf.html#setMapSpeculativeExecution(boolean)">setMapSpeculativeExecution(boolean)</a>)/(<a href="api/org/apache/hadoop/mapred/JobConf.html#setReduceSpeculativeExecution(boolean)">setReduceSpeculativeExecution(boolean)</a>) |
---|
| 1581 | , maximum number of attempts per task |
---|
| 1582 | (<a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxMapAttempts(int)">setMaxMapAttempts(int)</a>/<a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxReduceAttempts(int)">setMaxReduceAttempts(int)</a>) |
---|
| 1583 | , percentage of tasks failure which can be tolerated by the job |
---|
| 1584 | (<a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxMapTaskFailuresPercent(int)">setMaxMapTaskFailuresPercent(int)</a>/<a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxReduceTaskFailuresPercent(int)">setMaxReduceTaskFailuresPercent(int)</a>) |
---|
| 1585 | etc.</p> |
---|
| 1586 | <p>Of course, users can use |
---|
| 1587 | <a href="api/org/apache/hadoop/conf/Configuration.html#set(java.lang.String, java.lang.String)">set(String, String)</a>/<a href="api/org/apache/hadoop/conf/Configuration.html#get(java.lang.String, java.lang.String)">get(String, String)</a> |
---|
| 1588 | to set/get arbitrary parameters needed by applications. However, use the |
---|
| 1589 | <span class="codefrag">DistributedCache</span> for large amounts of (read-only) data.</p> |
---|
| 1590 | <a name="N10866"></a><a name="Task+Execution+%26+Environment"></a> |
---|
| 1591 | <h3 class="h4">Task Execution & Environment</h3> |
---|
| 1592 | <p>The <span class="codefrag">TaskTracker</span> executes the <span class="codefrag">Mapper</span>/ |
---|
| 1593 | <span class="codefrag">Reducer</span> <em>task</em> as a child process in a separate jvm. |
---|
| 1594 | </p> |
---|
| 1595 | <p>The child-task inherits the environment of the parent |
---|
| 1596 | <span class="codefrag">TaskTracker</span>. The user can specify additional options to the |
---|
| 1597 | child-jvm via the <span class="codefrag">mapred.child.java.opts</span> configuration |
---|
| 1598 | parameter in the <span class="codefrag">JobConf</span> such as non-standard paths for the |
---|
| 1599 | run-time linker to search shared libraries via |
---|
| 1600 | <span class="codefrag">-Djava.library.path=<></span> etc. If the |
---|
| 1601 | <span class="codefrag">mapred.child.java.opts</span> contains the symbol <em>@taskid@</em> |
---|
| 1602 | it is interpolated with value of <span class="codefrag">taskid</span> of the map/reduce |
---|
| 1603 | task.</p> |
---|
| 1604 | <p>Here is an example with multiple arguments and substitutions, |
---|
| 1605 | showing jvm GC logging, and start of a passwordless JVM JMX agent so that |
---|
| 1606 | it can connect with jconsole and the likes to watch child memory, |
---|
| 1607 | threads and get thread dumps. It also sets the maximum heap-size of the |
---|
| 1608 | child jvm to 512MB and adds an additional path to the |
---|
| 1609 | <span class="codefrag">java.library.path</span> of the child-jvm.</p> |
---|
| 1610 | <p> |
---|
| 1611 | |
---|
| 1612 | <span class="codefrag"><property></span> |
---|
| 1613 | <br> |
---|
| 1614 | <span class="codefrag"><name>mapred.child.java.opts</name></span> |
---|
| 1615 | <br> |
---|
| 1616 | <span class="codefrag"><value></span> |
---|
| 1617 | <br> |
---|
| 1618 | <span class="codefrag"> |
---|
| 1619 | -Xmx512M -Djava.library.path=/home/mycompany/lib |
---|
| 1620 | -verbose:gc -Xloggc:/tmp/@taskid@.gc</span> |
---|
| 1621 | <br> |
---|
| 1622 | <span class="codefrag"> |
---|
| 1623 | -Dcom.sun.management.jmxremote.authenticate=false |
---|
| 1624 | -Dcom.sun.management.jmxremote.ssl=false</span> |
---|
| 1625 | <br> |
---|
| 1626 | <span class="codefrag"></value></span> |
---|
| 1627 | <br> |
---|
| 1628 | |
---|
| 1629 | <span class="codefrag"></property></span> |
---|
| 1630 | |
---|
| 1631 | </p> |
---|
| 1632 | <a name="N108B7"></a><a name="Memory+management"></a> |
---|
| 1633 | <h4> Memory management</h4> |
---|
| 1634 | <p>Users/admins can also specify the maximum virtual memory |
---|
| 1635 | of the launched child-task, and any sub-process it launches |
---|
| 1636 | recursively, using <span class="codefrag">mapred.child.ulimit</span>. Note that |
---|
| 1637 | the value set here is a per process limit. |
---|
| 1638 | The value for <span class="codefrag">mapred.child.ulimit</span> should be specified |
---|
| 1639 | in kilo bytes (KB). And also the value must be greater than |
---|
| 1640 | or equal to the -Xmx passed to JavaVM, else the VM might not start. |
---|
| 1641 | </p> |
---|
| 1642 | <p>Note: <span class="codefrag">mapred.child.java.opts</span> are used only for |
---|
| 1643 | configuring the launched child tasks from task tracker. Configuring |
---|
| 1644 | the memory options for daemons is documented in |
---|
| 1645 | <a href="cluster_setup.html#Configuring+the+Environment+of+the+Hadoop+Daemons"> |
---|
| 1646 | cluster_setup.html </a> |
---|
| 1647 | </p> |
---|
| 1648 | <p>The memory available to some parts of the framework is also |
---|
| 1649 | configurable. In map and reduce tasks, performance may be influenced |
---|
| 1650 | by adjusting parameters influencing the concurrency of operations and |
---|
| 1651 | the frequency with which data will hit disk. Monitoring the filesystem |
---|
| 1652 | counters for a job- particularly relative to byte counts from the map |
---|
| 1653 | and into the reduce- is invaluable to the tuning of these |
---|
| 1654 | parameters.</p> |
---|
| 1655 | <p>Users can choose to override default limits of Virtual Memory and RAM |
---|
| 1656 | enforced by the task tracker, if memory management is enabled. |
---|
| 1657 | Users can set the following parameter per job:</p> |
---|
| 1658 | <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
---|
| 1659 | |
---|
| 1660 | <tr> |
---|
| 1661 | <th colspan="1" rowspan="1">Name</th><th colspan="1" rowspan="1">Type</th><th colspan="1" rowspan="1">Description</th> |
---|
| 1662 | </tr> |
---|
| 1663 | |
---|
| 1664 | <tr> |
---|
| 1665 | <td colspan="1" rowspan="1"><span class="codefrag">mapred.task.maxvmem</span></td><td colspan="1" rowspan="1">int</td> |
---|
| 1666 | <td colspan="1" rowspan="1">A number, in bytes, that represents the maximum Virtual Memory |
---|
| 1667 | task-limit for each task of the job. A task will be killed if |
---|
| 1668 | it consumes more Virtual Memory than this number. |
---|
| 1669 | </td> |
---|
| 1670 | </tr> |
---|
| 1671 | |
---|
| 1672 | <tr> |
---|
| 1673 | <td colspan="1" rowspan="1">mapred.task.maxpmem</td><td colspan="1" rowspan="1">int</td> |
---|
| 1674 | <td colspan="1" rowspan="1">A number, in bytes, that represents the maximum RAM task-limit |
---|
| 1675 | for each task of the job. This number can be optionally used by |
---|
| 1676 | Schedulers to prevent over-scheduling of tasks on a node based |
---|
| 1677 | on RAM needs. |
---|
| 1678 | </td> |
---|
| 1679 | </tr> |
---|
| 1680 | |
---|
| 1681 | </table> |
---|
| 1682 | <a name="N10906"></a><a name="Map+Parameters"></a> |
---|
| 1683 | <h4>Map Parameters</h4> |
---|
| 1684 | <p>A record emitted from a map will be serialized into a buffer and |
---|
| 1685 | metadata will be stored into accounting buffers. As described in the |
---|
| 1686 | following options, when either the serialization buffer or the |
---|
| 1687 | metadata exceed a threshold, the contents of the buffers will be |
---|
| 1688 | sorted and written to disk in the background while the map continues |
---|
| 1689 | to output records. If either buffer fills completely while the spill |
---|
| 1690 | is in progress, the map thread will block. When the map is finished, |
---|
| 1691 | any remaining records are written to disk and all on-disk segments |
---|
| 1692 | are merged into a single file. Minimizing the number of spills to |
---|
| 1693 | disk can decrease map time, but a larger buffer also decreases the |
---|
| 1694 | memory available to the mapper.</p> |
---|
| 1695 | <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
---|
| 1696 | |
---|
| 1697 | <tr> |
---|
| 1698 | <th colspan="1" rowspan="1">Name</th><th colspan="1" rowspan="1">Type</th><th colspan="1" rowspan="1">Description</th> |
---|
| 1699 | </tr> |
---|
| 1700 | |
---|
| 1701 | <tr> |
---|
| 1702 | <td colspan="1" rowspan="1">io.sort.mb</td><td colspan="1" rowspan="1">int</td> |
---|
| 1703 | <td colspan="1" rowspan="1">The cumulative size of the serialization and accounting |
---|
| 1704 | buffers storing records emitted from the map, in megabytes. |
---|
| 1705 | </td> |
---|
| 1706 | </tr> |
---|
| 1707 | |
---|
| 1708 | <tr> |
---|
| 1709 | <td colspan="1" rowspan="1">io.sort.record.percent</td><td colspan="1" rowspan="1">float</td> |
---|
| 1710 | <td colspan="1" rowspan="1">The ratio of serialization to accounting space can be |
---|
| 1711 | adjusted. Each serialized record requires 16 bytes of |
---|
| 1712 | accounting information in addition to its serialized size to |
---|
| 1713 | effect the sort. This percentage of space allocated from |
---|
| 1714 | <span class="codefrag">io.sort.mb</span> affects the probability of a spill to |
---|
| 1715 | disk being caused by either exhaustion of the serialization |
---|
| 1716 | buffer or the accounting space. Clearly, for a map outputting |
---|
| 1717 | small records, a higher value than the default will likely |
---|
| 1718 | decrease the number of spills to disk.</td> |
---|
| 1719 | </tr> |
---|
| 1720 | |
---|
| 1721 | <tr> |
---|
| 1722 | <td colspan="1" rowspan="1">io.sort.spill.percent</td><td colspan="1" rowspan="1">float</td> |
---|
| 1723 | <td colspan="1" rowspan="1">This is the threshold for the accounting and serialization |
---|
| 1724 | buffers. When this percentage of either buffer has filled, |
---|
| 1725 | their contents will be spilled to disk in the background. Let |
---|
| 1726 | <span class="codefrag">io.sort.record.percent</span> be <em>r</em>, |
---|
| 1727 | <span class="codefrag">io.sort.mb</span> be <em>x</em>, and this value be |
---|
| 1728 | <em>q</em>. The maximum number of records collected before the |
---|
| 1729 | collection thread will spill is <span class="codefrag">r * x * q * 2^16</span>. |
---|
| 1730 | Note that a higher value may decrease the number of- or even |
---|
| 1731 | eliminate- merges, but will also increase the probability of |
---|
| 1732 | the map task getting blocked. The lowest average map times are |
---|
| 1733 | usually obtained by accurately estimating the size of the map |
---|
| 1734 | output and preventing multiple spills.</td> |
---|
| 1735 | </tr> |
---|
| 1736 | |
---|
| 1737 | </table> |
---|
| 1738 | <p>Other notes</p> |
---|
| 1739 | <ul> |
---|
| 1740 | |
---|
| 1741 | <li>If either spill threshold is exceeded while a spill is in |
---|
| 1742 | progress, collection will continue until the spill is finished. |
---|
| 1743 | For example, if <span class="codefrag">io.sort.buffer.spill.percent</span> is set |
---|
| 1744 | to 0.33, and the remainder of the buffer is filled while the spill |
---|
| 1745 | runs, the next spill will include all the collected records, or |
---|
| 1746 | 0.66 of the buffer, and will not generate additional spills. In |
---|
| 1747 | other words, the thresholds are defining triggers, not |
---|
| 1748 | blocking.</li> |
---|
| 1749 | |
---|
| 1750 | <li>A record larger than the serialization buffer will first |
---|
| 1751 | trigger a spill, then be spilled to a separate file. It is |
---|
| 1752 | undefined whether or not this record will first pass through the |
---|
| 1753 | combiner.</li> |
---|
| 1754 | |
---|
| 1755 | </ul> |
---|
| 1756 | <a name="N10972"></a><a name="Shuffle%2FReduce+Parameters"></a> |
---|
| 1757 | <h4>Shuffle/Reduce Parameters</h4> |
---|
| 1758 | <p>As described previously, each reduce fetches the output assigned |
---|
| 1759 | to it by the Partitioner via HTTP into memory and periodically |
---|
| 1760 | merges these outputs to disk. If intermediate compression of map |
---|
| 1761 | outputs is turned on, each output is decompressed into memory. The |
---|
| 1762 | following options affect the frequency of these merges to disk prior |
---|
| 1763 | to the reduce and the memory allocated to map output during the |
---|
| 1764 | reduce.</p> |
---|
| 1765 | <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
---|
| 1766 | |
---|
| 1767 | <tr> |
---|
| 1768 | <th colspan="1" rowspan="1">Name</th><th colspan="1" rowspan="1">Type</th><th colspan="1" rowspan="1">Description</th> |
---|
| 1769 | </tr> |
---|
| 1770 | |
---|
| 1771 | <tr> |
---|
| 1772 | <td colspan="1" rowspan="1">io.sort.factor</td><td colspan="1" rowspan="1">int</td> |
---|
| 1773 | <td colspan="1" rowspan="1">Specifies the number of segments on disk to be merged at |
---|
| 1774 | the same time. It limits the number of open files and |
---|
| 1775 | compression codecs during the merge. If the number of files |
---|
| 1776 | exceeds this limit, the merge will proceed in several passes. |
---|
| 1777 | Though this limit also applies to the map, most jobs should be |
---|
| 1778 | configured so that hitting this limit is unlikely |
---|
| 1779 | there.</td> |
---|
| 1780 | </tr> |
---|
| 1781 | |
---|
| 1782 | <tr> |
---|
| 1783 | <td colspan="1" rowspan="1">mapred.inmem.merge.threshold</td><td colspan="1" rowspan="1">int</td> |
---|
| 1784 | <td colspan="1" rowspan="1">The number of sorted map outputs fetched into memory |
---|
| 1785 | before being merged to disk. Like the spill thresholds in the |
---|
| 1786 | preceding note, this is not defining a unit of partition, but |
---|
| 1787 | a trigger. In practice, this is usually set very high (1000) |
---|
| 1788 | or disabled (0), since merging in-memory segments is often |
---|
| 1789 | less expensive than merging from disk (see notes following |
---|
| 1790 | this table). This threshold influences only the frequency of |
---|
| 1791 | in-memory merges during the shuffle.</td> |
---|
| 1792 | </tr> |
---|
| 1793 | |
---|
| 1794 | <tr> |
---|
| 1795 | <td colspan="1" rowspan="1">mapred.job.shuffle.merge.percent</td><td colspan="1" rowspan="1">float</td> |
---|
| 1796 | <td colspan="1" rowspan="1">The memory threshold for fetched map outputs before an |
---|
| 1797 | in-memory merge is started, expressed as a percentage of |
---|
| 1798 | memory allocated to storing map outputs in memory. Since map |
---|
| 1799 | outputs that can't fit in memory can be stalled, setting this |
---|
| 1800 | high may decrease parallelism between the fetch and merge. |
---|
| 1801 | Conversely, values as high as 1.0 have been effective for |
---|
| 1802 | reduces whose input can fit entirely in memory. This parameter |
---|
| 1803 | influences only the frequency of in-memory merges during the |
---|
| 1804 | shuffle.</td> |
---|
| 1805 | </tr> |
---|
| 1806 | |
---|
| 1807 | <tr> |
---|
| 1808 | <td colspan="1" rowspan="1">mapred.job.shuffle.input.buffer.percent</td><td colspan="1" rowspan="1">float</td> |
---|
| 1809 | <td colspan="1" rowspan="1">The percentage of memory- relative to the maximum heapsize |
---|
| 1810 | as typically specified in <span class="codefrag">mapred.child.java.opts</span>- |
---|
| 1811 | that can be allocated to storing map outputs during the |
---|
| 1812 | shuffle. Though some memory should be set aside for the |
---|
| 1813 | framework, in general it is advantageous to set this high |
---|
| 1814 | enough to store large and numerous map outputs.</td> |
---|
| 1815 | </tr> |
---|
| 1816 | |
---|
| 1817 | <tr> |
---|
| 1818 | <td colspan="1" rowspan="1">mapred.job.reduce.input.buffer.percent</td><td colspan="1" rowspan="1">float</td> |
---|
| 1819 | <td colspan="1" rowspan="1">The percentage of memory relative to the maximum heapsize |
---|
| 1820 | in which map outputs may be retained during the reduce. When |
---|
| 1821 | the reduce begins, map outputs will be merged to disk until |
---|
| 1822 | those that remain are under the resource limit this defines. |
---|
| 1823 | By default, all map outputs are merged to disk before the |
---|
| 1824 | reduce begins to maximize the memory available to the reduce. |
---|
| 1825 | For less memory-intensive reduces, this should be increased to |
---|
| 1826 | avoid trips to disk.</td> |
---|
| 1827 | </tr> |
---|
| 1828 | |
---|
| 1829 | </table> |
---|
| 1830 | <p>Other notes</p> |
---|
| 1831 | <ul> |
---|
| 1832 | |
---|
| 1833 | <li>If a map output is larger than 25 percent of the memory |
---|
| 1834 | allocated to copying map outputs, it will be written directly to |
---|
| 1835 | disk without first staging through memory.</li> |
---|
| 1836 | |
---|
| 1837 | <li>When running with a combiner, the reasoning about high merge |
---|
| 1838 | thresholds and large buffers may not hold. For merges started |
---|
| 1839 | before all map outputs have been fetched, the combiner is run |
---|
| 1840 | while spilling to disk. In some cases, one can obtain better |
---|
| 1841 | reduce times by spending resources combining map outputs- making |
---|
| 1842 | disk spills small and parallelizing spilling and fetching- rather |
---|
| 1843 | than aggressively increasing buffer sizes.</li> |
---|
| 1844 | |
---|
| 1845 | <li>When merging in-memory map outputs to disk to begin the |
---|
| 1846 | reduce, if an intermediate merge is necessary because there are |
---|
| 1847 | segments to spill and at least <span class="codefrag">io.sort.factor</span> |
---|
| 1848 | segments already on disk, the in-memory map outputs will be part |
---|
| 1849 | of the intermediate merge.</li> |
---|
| 1850 | |
---|
| 1851 | </ul> |
---|
| 1852 | <a name="N109ED"></a><a name="Directory+Structure"></a> |
---|
| 1853 | <h4> Directory Structure </h4> |
---|
| 1854 | <p>The task tracker has local directory, |
---|
| 1855 | <span class="codefrag"> ${mapred.local.dir}/taskTracker/</span> to create localized |
---|
| 1856 | cache and localized job. It can define multiple local directories |
---|
| 1857 | (spanning multiple disks) and then each filename is assigned to a |
---|
| 1858 | semi-random local directory. When the job starts, task tracker |
---|
| 1859 | creates a localized job directory relative to the local directory |
---|
| 1860 | specified in the configuration. Thus the task tracker directory |
---|
| 1861 | structure looks the following: </p> |
---|
| 1862 | <ul> |
---|
| 1863 | |
---|
| 1864 | <li> |
---|
| 1865 | <span class="codefrag">${mapred.local.dir}/taskTracker/archive/</span> : |
---|
| 1866 | The distributed cache. This directory holds the localized distributed |
---|
| 1867 | cache. Thus localized distributed cache is shared among all |
---|
| 1868 | the tasks and jobs </li> |
---|
| 1869 | |
---|
| 1870 | <li> |
---|
| 1871 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/</span> : |
---|
| 1872 | The localized job directory |
---|
| 1873 | <ul> |
---|
| 1874 | |
---|
| 1875 | <li> |
---|
| 1876 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/work/</span> |
---|
| 1877 | : The job-specific shared directory. The tasks can use this space as |
---|
| 1878 | scratch space and share files among them. This directory is exposed |
---|
| 1879 | to the users through the configuration property |
---|
| 1880 | <span class="codefrag">job.local.dir</span>. The directory can accessed through |
---|
| 1881 | api <a href="api/org/apache/hadoop/mapred/JobConf.html#getJobLocalDir()"> |
---|
| 1882 | JobConf.getJobLocalDir()</a>. It is available as System property also. |
---|
| 1883 | So, users (streaming etc.) can call |
---|
| 1884 | <span class="codefrag">System.getProperty("job.local.dir")</span> to access the |
---|
| 1885 | directory.</li> |
---|
| 1886 | |
---|
| 1887 | <li> |
---|
| 1888 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/jars/</span> |
---|
| 1889 | : The jars directory, which has the job jar file and expanded jar. |
---|
| 1890 | The <span class="codefrag">job.jar</span> is the application's jar file that is |
---|
| 1891 | automatically distributed to each machine. It is expanded in jars |
---|
| 1892 | directory before the tasks for the job start. The job.jar location |
---|
| 1893 | is accessible to the application through the api |
---|
| 1894 | <a href="api/org/apache/hadoop/mapred/JobConf.html#getJar()"> |
---|
| 1895 | JobConf.getJar() </a>. To access the unjarred directory, |
---|
| 1896 | JobConf.getJar().getParent() can be called.</li> |
---|
| 1897 | |
---|
| 1898 | <li> |
---|
| 1899 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/job.xml</span> |
---|
| 1900 | : The job.xml file, the generic job configuration, localized for |
---|
| 1901 | the job. </li> |
---|
| 1902 | |
---|
| 1903 | <li> |
---|
| 1904 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/$taskid</span> |
---|
| 1905 | : The task directory for each task attempt. Each task directory |
---|
| 1906 | again has the following structure : |
---|
| 1907 | <ul> |
---|
| 1908 | |
---|
| 1909 | <li> |
---|
| 1910 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/$taskid/job.xml</span> |
---|
| 1911 | : A job.xml file, task localized job configuration, Task localization |
---|
| 1912 | means that properties have been set that are specific to |
---|
| 1913 | this particular task within the job. The properties localized for |
---|
| 1914 | each task are described below.</li> |
---|
| 1915 | |
---|
| 1916 | <li> |
---|
| 1917 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/$taskid/output</span> |
---|
| 1918 | : A directory for intermediate output files. This contains the |
---|
| 1919 | temporary map reduce data generated by the framework |
---|
| 1920 | such as map output files etc. </li> |
---|
| 1921 | |
---|
| 1922 | <li> |
---|
| 1923 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/$taskid/work</span> |
---|
| 1924 | : The curernt working directory of the task. |
---|
| 1925 | With <a href="#Task+JVM+Reuse">jvm reuse</a> enabled for tasks, this |
---|
| 1926 | directory will be the directory on which the jvm has started</li> |
---|
| 1927 | |
---|
| 1928 | <li> |
---|
| 1929 | <span class="codefrag">${mapred.local.dir}/taskTracker/jobcache/$jobid/$taskid/work/tmp</span> |
---|
| 1930 | : The temporary directory for the task. |
---|
| 1931 | (User can specify the property <span class="codefrag">mapred.child.tmp</span> to set |
---|
| 1932 | the value of temporary directory for map and reduce tasks. This |
---|
| 1933 | defaults to <span class="codefrag">./tmp</span>. If the value is not an absolute path, |
---|
| 1934 | it is prepended with task's working directory. Otherwise, it is |
---|
| 1935 | directly assigned. The directory will be created if it doesn't exist. |
---|
| 1936 | Then, the child java tasks are executed with option |
---|
| 1937 | <span class="codefrag">-Djava.io.tmpdir='the absolute path of the tmp dir'</span>. |
---|
| 1938 | Anp pipes and streaming are set with environment variable, |
---|
| 1939 | <span class="codefrag">TMPDIR='the absolute path of the tmp dir'</span>). This |
---|
| 1940 | directory is created, if <span class="codefrag">mapred.child.tmp</span> has the value |
---|
| 1941 | <span class="codefrag">./tmp</span> |
---|
| 1942 | </li> |
---|
| 1943 | |
---|
| 1944 | </ul> |
---|
| 1945 | |
---|
| 1946 | </li> |
---|
| 1947 | |
---|
| 1948 | </ul> |
---|
| 1949 | |
---|
| 1950 | </li> |
---|
| 1951 | |
---|
| 1952 | </ul> |
---|
| 1953 | <a name="N10A5C"></a><a name="Task+JVM+Reuse"></a> |
---|
| 1954 | <h4>Task JVM Reuse</h4> |
---|
| 1955 | <p>Jobs can enable task JVMs to be reused by specifying the job |
---|
| 1956 | configuration <span class="codefrag">mapred.job.reuse.jvm.num.tasks</span>. If the |
---|
| 1957 | value is 1 (the default), then JVMs are not reused |
---|
| 1958 | (i.e. 1 task per JVM). If it is -1, there is no limit to the number |
---|
| 1959 | of tasks a JVM can run (of the same job). One can also specify some |
---|
| 1960 | value greater than 1 using the api |
---|
| 1961 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setNumTasksToExecutePerJvm(int)"> |
---|
| 1962 | JobConf.setNumTasksToExecutePerJvm(int)</a> |
---|
| 1963 | </p> |
---|
| 1964 | <p>The following properties are localized in the job configuration |
---|
| 1965 | for each task's execution: </p> |
---|
| 1966 | <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
---|
| 1967 | |
---|
| 1968 | <tr> |
---|
| 1969 | <th colspan="1" rowspan="1">Name</th><th colspan="1" rowspan="1">Type</th><th colspan="1" rowspan="1">Description</th> |
---|
| 1970 | </tr> |
---|
| 1971 | |
---|
| 1972 | <tr> |
---|
| 1973 | <td colspan="1" rowspan="1">mapred.job.id</td><td colspan="1" rowspan="1">String</td><td colspan="1" rowspan="1">The job id</td> |
---|
| 1974 | </tr> |
---|
| 1975 | |
---|
| 1976 | <tr> |
---|
| 1977 | <td colspan="1" rowspan="1">mapred.jar</td><td colspan="1" rowspan="1">String</td> |
---|
| 1978 | <td colspan="1" rowspan="1">job.jar location in job directory</td> |
---|
| 1979 | </tr> |
---|
| 1980 | |
---|
| 1981 | <tr> |
---|
| 1982 | <td colspan="1" rowspan="1">job.local.dir</td><td colspan="1" rowspan="1"> String</td> |
---|
| 1983 | <td colspan="1" rowspan="1"> The job specific shared scratch space</td> |
---|
| 1984 | </tr> |
---|
| 1985 | |
---|
| 1986 | <tr> |
---|
| 1987 | <td colspan="1" rowspan="1">mapred.tip.id</td><td colspan="1" rowspan="1"> String</td> |
---|
| 1988 | <td colspan="1" rowspan="1"> The task id</td> |
---|
| 1989 | </tr> |
---|
| 1990 | |
---|
| 1991 | <tr> |
---|
| 1992 | <td colspan="1" rowspan="1">mapred.task.id</td><td colspan="1" rowspan="1"> String</td> |
---|
| 1993 | <td colspan="1" rowspan="1"> The task attempt id</td> |
---|
| 1994 | </tr> |
---|
| 1995 | |
---|
| 1996 | <tr> |
---|
| 1997 | <td colspan="1" rowspan="1">mapred.task.is.map</td><td colspan="1" rowspan="1"> boolean </td> |
---|
| 1998 | <td colspan="1" rowspan="1">Is this a map task</td> |
---|
| 1999 | </tr> |
---|
| 2000 | |
---|
| 2001 | <tr> |
---|
| 2002 | <td colspan="1" rowspan="1">mapred.task.partition</td><td colspan="1" rowspan="1"> int </td> |
---|
| 2003 | <td colspan="1" rowspan="1">The id of the task within the job</td> |
---|
| 2004 | </tr> |
---|
| 2005 | |
---|
| 2006 | <tr> |
---|
| 2007 | <td colspan="1" rowspan="1">map.input.file</td><td colspan="1" rowspan="1"> String</td> |
---|
| 2008 | <td colspan="1" rowspan="1"> The filename that the map is reading from</td> |
---|
| 2009 | </tr> |
---|
| 2010 | |
---|
| 2011 | <tr> |
---|
| 2012 | <td colspan="1" rowspan="1">map.input.start</td><td colspan="1" rowspan="1"> long</td> |
---|
| 2013 | <td colspan="1" rowspan="1"> The offset of the start of the map input split</td> |
---|
| 2014 | </tr> |
---|
| 2015 | |
---|
| 2016 | <tr> |
---|
| 2017 | <td colspan="1" rowspan="1">map.input.length </td><td colspan="1" rowspan="1">long </td> |
---|
| 2018 | <td colspan="1" rowspan="1">The number of bytes in the map input split</td> |
---|
| 2019 | </tr> |
---|
| 2020 | |
---|
| 2021 | <tr> |
---|
| 2022 | <td colspan="1" rowspan="1">mapred.work.output.dir</td><td colspan="1" rowspan="1"> String </td> |
---|
| 2023 | <td colspan="1" rowspan="1">The task's temporary output directory</td> |
---|
| 2024 | </tr> |
---|
| 2025 | |
---|
| 2026 | </table> |
---|
| 2027 | <p>The standard output (stdout) and error (stderr) streams of the task |
---|
| 2028 | are read by the TaskTracker and logged to |
---|
| 2029 | <span class="codefrag">${HADOOP_LOG_DIR}/userlogs</span> |
---|
| 2030 | </p> |
---|
| 2031 | <p>The <a href="#DistributedCache">DistributedCache</a> can also be used |
---|
| 2032 | to distribute both jars and native libraries for use in the map |
---|
| 2033 | and/or reduce tasks. The child-jvm always has its |
---|
| 2034 | <em>current working directory</em> added to the |
---|
| 2035 | <span class="codefrag">java.library.path</span> and <span class="codefrag">LD_LIBRARY_PATH</span>. |
---|
| 2036 | And hence the cached libraries can be loaded via |
---|
| 2037 | <a href="http://java.sun.com/javase/6/docs/api/java/lang/System.html#loadLibrary(java.lang.String)"> |
---|
| 2038 | System.loadLibrary</a> or |
---|
| 2039 | <a href="http://java.sun.com/javase/6/docs/api/java/lang/System.html#load(java.lang.String)"> |
---|
| 2040 | System.load</a>. More details on how to load shared libraries through |
---|
| 2041 | distributed cache are documented at |
---|
| 2042 | <a href="native_libraries.html#Loading+native+libraries+through+DistributedCache"> |
---|
| 2043 | native_libraries.html</a> |
---|
| 2044 | </p> |
---|
| 2045 | <a name="N10B45"></a><a name="Job+Submission+and+Monitoring"></a> |
---|
| 2046 | <h3 class="h4">Job Submission and Monitoring</h3> |
---|
| 2047 | <p> |
---|
| 2048 | <a href="api/org/apache/hadoop/mapred/JobClient.html"> |
---|
| 2049 | JobClient</a> is the primary interface by which user-job interacts |
---|
| 2050 | with the <span class="codefrag">JobTracker</span>.</p> |
---|
| 2051 | <p> |
---|
| 2052 | <span class="codefrag">JobClient</span> provides facilities to submit jobs, track their |
---|
| 2053 | progress, access component-tasks' reports and logs, get the Map/Reduce |
---|
| 2054 | cluster's status information and so on.</p> |
---|
| 2055 | <p>The job submission process involves:</p> |
---|
| 2056 | <ol> |
---|
| 2057 | |
---|
| 2058 | <li>Checking the input and output specifications of the job.</li> |
---|
| 2059 | |
---|
| 2060 | <li>Computing the <span class="codefrag">InputSplit</span> values for the job.</li> |
---|
| 2061 | |
---|
| 2062 | <li> |
---|
| 2063 | Setting up the requisite accounting information for the |
---|
| 2064 | <span class="codefrag">DistributedCache</span> of the job, if necessary. |
---|
| 2065 | </li> |
---|
| 2066 | |
---|
| 2067 | <li> |
---|
| 2068 | Copying the job's jar and configuration to the Map/Reduce system |
---|
| 2069 | directory on the <span class="codefrag">FileSystem</span>. |
---|
| 2070 | </li> |
---|
| 2071 | |
---|
| 2072 | <li> |
---|
| 2073 | Submitting the job to the <span class="codefrag">JobTracker</span> and optionally |
---|
| 2074 | monitoring it's status. |
---|
| 2075 | </li> |
---|
| 2076 | |
---|
| 2077 | </ol> |
---|
| 2078 | <p> Job history files are also logged to user specified directory |
---|
| 2079 | <span class="codefrag">hadoop.job.history.user.location</span> |
---|
| 2080 | which defaults to job output directory. The files are stored in |
---|
| 2081 | "_logs/history/" in the specified directory. Hence, by default they |
---|
| 2082 | will be in mapred.output.dir/_logs/history. User can stop |
---|
| 2083 | logging by giving the value <span class="codefrag">none</span> for |
---|
| 2084 | <span class="codefrag">hadoop.job.history.user.location</span> |
---|
| 2085 | </p> |
---|
| 2086 | <p> User can view the history logs summary in specified directory |
---|
| 2087 | using the following command <br> |
---|
| 2088 | |
---|
| 2089 | <span class="codefrag">$ bin/hadoop job -history output-dir</span> |
---|
| 2090 | <br> |
---|
| 2091 | This command will print job details, failed and killed tip |
---|
| 2092 | details. <br> |
---|
| 2093 | More details about the job such as successful tasks and |
---|
| 2094 | task attempts made for each task can be viewed using the |
---|
| 2095 | following command <br> |
---|
| 2096 | |
---|
| 2097 | <span class="codefrag">$ bin/hadoop job -history all output-dir</span> |
---|
| 2098 | <br> |
---|
| 2099 | </p> |
---|
| 2100 | <p> User can use |
---|
| 2101 | <a href="api/org/apache/hadoop/mapred/OutputLogFilter.html">OutputLogFilter</a> |
---|
| 2102 | to filter log files from the output directory listing. </p> |
---|
| 2103 | <p>Normally the user creates the application, describes various facets |
---|
| 2104 | of the job via <span class="codefrag">JobConf</span>, and then uses the |
---|
| 2105 | <span class="codefrag">JobClient</span> to submit the job and monitor its progress.</p> |
---|
| 2106 | <a name="N10BA5"></a><a name="Job+Control"></a> |
---|
| 2107 | <h4>Job Control</h4> |
---|
| 2108 | <p>Users may need to chain Map/Reduce jobs to accomplish complex |
---|
| 2109 | tasks which cannot be done via a single Map/Reduce job. This is fairly |
---|
| 2110 | easy since the output of the job typically goes to distributed |
---|
| 2111 | file-system, and the output, in turn, can be used as the input for the |
---|
| 2112 | next job.</p> |
---|
| 2113 | <p>However, this also means that the onus on ensuring jobs are |
---|
| 2114 | complete (success/failure) lies squarely on the clients. In such |
---|
| 2115 | cases, the various job-control options are:</p> |
---|
| 2116 | <ul> |
---|
| 2117 | |
---|
| 2118 | <li> |
---|
| 2119 | |
---|
| 2120 | <a href="api/org/apache/hadoop/mapred/JobClient.html#runJob(org.apache.hadoop.mapred.JobConf)"> |
---|
| 2121 | runJob(JobConf)</a> : Submits the job and returns only after the |
---|
| 2122 | job has completed. |
---|
| 2123 | </li> |
---|
| 2124 | |
---|
| 2125 | <li> |
---|
| 2126 | |
---|
| 2127 | <a href="api/org/apache/hadoop/mapred/JobClient.html#submitJob(org.apache.hadoop.mapred.JobConf)"> |
---|
| 2128 | submitJob(JobConf)</a> : Only submits the job, then poll the |
---|
| 2129 | returned handle to the |
---|
| 2130 | <a href="api/org/apache/hadoop/mapred/RunningJob.html"> |
---|
| 2131 | RunningJob</a> to query status and make scheduling decisions. |
---|
| 2132 | </li> |
---|
| 2133 | |
---|
| 2134 | <li> |
---|
| 2135 | |
---|
| 2136 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setJobEndNotificationURI(java.lang.String)"> |
---|
| 2137 | JobConf.setJobEndNotificationURI(String)</a> : Sets up a |
---|
| 2138 | notification upon job-completion, thus avoiding polling. |
---|
| 2139 | </li> |
---|
| 2140 | |
---|
| 2141 | </ul> |
---|
| 2142 | <a name="N10BCF"></a><a name="Job+Input"></a> |
---|
| 2143 | <h3 class="h4">Job Input</h3> |
---|
| 2144 | <p> |
---|
| 2145 | <a href="api/org/apache/hadoop/mapred/InputFormat.html"> |
---|
| 2146 | InputFormat</a> describes the input-specification for a Map/Reduce job. |
---|
| 2147 | </p> |
---|
| 2148 | <p>The Map/Reduce framework relies on the <span class="codefrag">InputFormat</span> of |
---|
| 2149 | the job to:</p> |
---|
| 2150 | <ol> |
---|
| 2151 | |
---|
| 2152 | <li>Validate the input-specification of the job.</li> |
---|
| 2153 | |
---|
| 2154 | <li> |
---|
| 2155 | Split-up the input file(s) into logical <span class="codefrag">InputSplit</span> |
---|
| 2156 | instances, each of which is then assigned to an individual |
---|
| 2157 | <span class="codefrag">Mapper</span>. |
---|
| 2158 | </li> |
---|
| 2159 | |
---|
| 2160 | <li> |
---|
| 2161 | Provide the <span class="codefrag">RecordReader</span> implementation used to |
---|
| 2162 | glean input records from the logical <span class="codefrag">InputSplit</span> for |
---|
| 2163 | processing by the <span class="codefrag">Mapper</span>. |
---|
| 2164 | </li> |
---|
| 2165 | |
---|
| 2166 | </ol> |
---|
| 2167 | <p>The default behavior of file-based <span class="codefrag">InputFormat</span> |
---|
| 2168 | implementations, typically sub-classes of |
---|
| 2169 | <a href="api/org/apache/hadoop/mapred/FileInputFormat.html"> |
---|
| 2170 | FileInputFormat</a>, is to split the input into <em>logical</em> |
---|
| 2171 | <span class="codefrag">InputSplit</span> instances based on the total size, in bytes, of |
---|
| 2172 | the input files. However, the <span class="codefrag">FileSystem</span> blocksize of the |
---|
| 2173 | input files is treated as an upper bound for input splits. A lower bound |
---|
| 2174 | on the split size can be set via <span class="codefrag">mapred.min.split.size</span>.</p> |
---|
| 2175 | <p>Clearly, logical splits based on input-size is insufficient for many |
---|
| 2176 | applications since record boundaries must be respected. In such cases, |
---|
| 2177 | the application should implement a <span class="codefrag">RecordReader</span>, who is |
---|
| 2178 | responsible for respecting record-boundaries and presents a |
---|
| 2179 | record-oriented view of the logical <span class="codefrag">InputSplit</span> to the |
---|
| 2180 | individual task.</p> |
---|
| 2181 | <p> |
---|
| 2182 | <a href="api/org/apache/hadoop/mapred/TextInputFormat.html"> |
---|
| 2183 | TextInputFormat</a> is the default <span class="codefrag">InputFormat</span>.</p> |
---|
| 2184 | <p>If <span class="codefrag">TextInputFormat</span> is the <span class="codefrag">InputFormat</span> for a |
---|
| 2185 | given job, the framework detects input-files with the <em>.gz</em> |
---|
| 2186 | extensions and automatically decompresses them using the |
---|
| 2187 | appropriate <span class="codefrag">CompressionCodec</span>. However, it must be noted that |
---|
| 2188 | compressed files with the above extensions cannot be <em>split</em> and |
---|
| 2189 | each compressed file is processed in its entirety by a single mapper.</p> |
---|
| 2190 | <a name="N10C36"></a><a name="InputSplit"></a> |
---|
| 2191 | <h4>InputSplit</h4> |
---|
| 2192 | <p> |
---|
| 2193 | <a href="api/org/apache/hadoop/mapred/InputSplit.html"> |
---|
| 2194 | InputSplit</a> represents the data to be processed by an individual |
---|
| 2195 | <span class="codefrag">Mapper</span>.</p> |
---|
| 2196 | <p>Typically <span class="codefrag">InputSplit</span> presents a byte-oriented view of |
---|
| 2197 | the input, and it is the responsibility of <span class="codefrag">RecordReader</span> |
---|
| 2198 | to process and present a record-oriented view.</p> |
---|
| 2199 | <p> |
---|
| 2200 | <a href="api/org/apache/hadoop/mapred/FileSplit.html"> |
---|
| 2201 | FileSplit</a> is the default <span class="codefrag">InputSplit</span>. It sets |
---|
| 2202 | <span class="codefrag">map.input.file</span> to the path of the input file for the |
---|
| 2203 | logical split.</p> |
---|
| 2204 | <a name="N10C5B"></a><a name="RecordReader"></a> |
---|
| 2205 | <h4>RecordReader</h4> |
---|
| 2206 | <p> |
---|
| 2207 | <a href="api/org/apache/hadoop/mapred/RecordReader.html"> |
---|
| 2208 | RecordReader</a> reads <span class="codefrag"><key, value></span> pairs from an |
---|
| 2209 | <span class="codefrag">InputSplit</span>.</p> |
---|
| 2210 | <p>Typically the <span class="codefrag">RecordReader</span> converts the byte-oriented |
---|
| 2211 | view of the input, provided by the <span class="codefrag">InputSplit</span>, and |
---|
| 2212 | presents a record-oriented to the <span class="codefrag">Mapper</span> implementations |
---|
| 2213 | for processing. <span class="codefrag">RecordReader</span> thus assumes the |
---|
| 2214 | responsibility of processing record boundaries and presents the tasks |
---|
| 2215 | with keys and values.</p> |
---|
| 2216 | <a name="N10C7E"></a><a name="Job+Output"></a> |
---|
| 2217 | <h3 class="h4">Job Output</h3> |
---|
| 2218 | <p> |
---|
| 2219 | <a href="api/org/apache/hadoop/mapred/OutputFormat.html"> |
---|
| 2220 | OutputFormat</a> describes the output-specification for a Map/Reduce |
---|
| 2221 | job.</p> |
---|
| 2222 | <p>The Map/Reduce framework relies on the <span class="codefrag">OutputFormat</span> of |
---|
| 2223 | the job to:</p> |
---|
| 2224 | <ol> |
---|
| 2225 | |
---|
| 2226 | <li> |
---|
| 2227 | Validate the output-specification of the job; for example, check that |
---|
| 2228 | the output directory doesn't already exist. |
---|
| 2229 | </li> |
---|
| 2230 | |
---|
| 2231 | <li> |
---|
| 2232 | Provide the <span class="codefrag">RecordWriter</span> implementation used to |
---|
| 2233 | write the output files of the job. Output files are stored in a |
---|
| 2234 | <span class="codefrag">FileSystem</span>. |
---|
| 2235 | </li> |
---|
| 2236 | |
---|
| 2237 | </ol> |
---|
| 2238 | <p> |
---|
| 2239 | <span class="codefrag">TextOutputFormat</span> is the default |
---|
| 2240 | <span class="codefrag">OutputFormat</span>.</p> |
---|
| 2241 | <a name="N10CA7"></a><a name="OutputCommitter"></a> |
---|
| 2242 | <h4>OutputCommitter</h4> |
---|
| 2243 | <p> |
---|
| 2244 | <a href="api/org/apache/hadoop/mapred/OutputCommitter.html"> |
---|
| 2245 | OutputCommitter</a> describes the commit of task output for a |
---|
| 2246 | Map/Reduce job.</p> |
---|
| 2247 | <p>The Map/Reduce framework relies on the <span class="codefrag">OutputCommitter</span> |
---|
| 2248 | of the job to:</p> |
---|
| 2249 | <ol> |
---|
| 2250 | |
---|
| 2251 | <li> |
---|
| 2252 | Setup the job during initialization. For example, create |
---|
| 2253 | the temporary output directory for the job during the |
---|
| 2254 | initialization of the job. |
---|
| 2255 | Job setup is done by a separate task when the job is |
---|
| 2256 | in PREP state and after initializing tasks. Once the setup task |
---|
| 2257 | completes, the job will be moved to RUNNING state. |
---|
| 2258 | </li> |
---|
| 2259 | |
---|
| 2260 | <li> |
---|
| 2261 | Cleanup the job after the job completion. For example, remove the |
---|
| 2262 | temporary output directory after the job completion. |
---|
| 2263 | Job cleanup is done by a separate task at the end of the job. |
---|
| 2264 | Job is declared SUCCEDED/FAILED/KILLED after the cleanup |
---|
| 2265 | task completes. |
---|
| 2266 | </li> |
---|
| 2267 | |
---|
| 2268 | <li> |
---|
| 2269 | Setup the task temporary output. |
---|
| 2270 | Task setup is done as part of the same task, during task initialization. |
---|
| 2271 | </li> |
---|
| 2272 | |
---|
| 2273 | <li> |
---|
| 2274 | Check whether a task needs a commit. This is to avoid the commit |
---|
| 2275 | procedure if a task does not need commit. |
---|
| 2276 | </li> |
---|
| 2277 | |
---|
| 2278 | <li> |
---|
| 2279 | Commit of the task output. |
---|
| 2280 | Once task is done, the task will commit it's output if required. |
---|
| 2281 | </li> |
---|
| 2282 | |
---|
| 2283 | <li> |
---|
| 2284 | Discard the task commit. |
---|
| 2285 | If the task has been failed/killed, the output will be cleaned-up. |
---|
| 2286 | If task could not cleanup (in exception block), a separate task |
---|
| 2287 | will be launched with same attempt-id to do the cleanup. |
---|
| 2288 | </li> |
---|
| 2289 | |
---|
| 2290 | </ol> |
---|
| 2291 | <p> |
---|
| 2292 | <span class="codefrag">FileOutputCommitter</span> is the default |
---|
| 2293 | <span class="codefrag">OutputCommitter</span>. Job setup/cleanup tasks occupy |
---|
| 2294 | map or reduce slots, whichever is free on the TaskTracker. And |
---|
| 2295 | JobCleanup task, TaskCleanup tasks and JobSetup task have the highest |
---|
| 2296 | priority, and in that order.</p> |
---|
| 2297 | <a name="N10CD7"></a><a name="Task+Side-Effect+Files"></a> |
---|
| 2298 | <h4>Task Side-Effect Files</h4> |
---|
| 2299 | <p>In some applications, component tasks need to create and/or write to |
---|
| 2300 | side-files, which differ from the actual job-output files.</p> |
---|
| 2301 | <p>In such cases there could be issues with two instances of the same |
---|
| 2302 | <span class="codefrag">Mapper</span> or <span class="codefrag">Reducer</span> running simultaneously (for |
---|
| 2303 | example, speculative tasks) trying to open and/or write to the same |
---|
| 2304 | file (path) on the <span class="codefrag">FileSystem</span>. Hence the |
---|
| 2305 | application-writer will have to pick unique names per task-attempt |
---|
| 2306 | (using the attemptid, say <span class="codefrag">attempt_200709221812_0001_m_000000_0</span>), |
---|
| 2307 | not just per task.</p> |
---|
| 2308 | <p>To avoid these issues the Map/Reduce framework, when the |
---|
| 2309 | <span class="codefrag">OutputCommitter</span> is <span class="codefrag">FileOutputCommitter</span>, |
---|
| 2310 | maintains a special |
---|
| 2311 | <span class="codefrag">${mapred.output.dir}/_temporary/_${taskid}</span> sub-directory |
---|
| 2312 | accessible via <span class="codefrag">${mapred.work.output.dir}</span> |
---|
| 2313 | for each task-attempt on the <span class="codefrag">FileSystem</span> where the output |
---|
| 2314 | of the task-attempt is stored. On successful completion of the |
---|
| 2315 | task-attempt, the files in the |
---|
| 2316 | <span class="codefrag">${mapred.output.dir}/_temporary/_${taskid}</span> (only) |
---|
| 2317 | are <em>promoted</em> to <span class="codefrag">${mapred.output.dir}</span>. Of course, |
---|
| 2318 | the framework discards the sub-directory of unsuccessful task-attempts. |
---|
| 2319 | This process is completely transparent to the application.</p> |
---|
| 2320 | <p>The application-writer can take advantage of this feature by |
---|
| 2321 | creating any side-files required in <span class="codefrag">${mapred.work.output.dir}</span> |
---|
| 2322 | during execution of a task via |
---|
| 2323 | <a href="api/org/apache/hadoop/mapred/FileOutputFormat.html#getWorkOutputPath(org.apache.hadoop.mapred.JobConf)"> |
---|
| 2324 | FileOutputFormat.getWorkOutputPath()</a>, and the framework will promote them |
---|
| 2325 | similarly for succesful task-attempts, thus eliminating the need to |
---|
| 2326 | pick unique paths per task-attempt.</p> |
---|
| 2327 | <p>Note: The value of <span class="codefrag">${mapred.work.output.dir}</span> during |
---|
| 2328 | execution of a particular task-attempt is actually |
---|
| 2329 | <span class="codefrag">${mapred.output.dir}/_temporary/_{$taskid}</span>, and this value is |
---|
| 2330 | set by the Map/Reduce framework. So, just create any side-files in the |
---|
| 2331 | path returned by |
---|
| 2332 | <a href="api/org/apache/hadoop/mapred/FileOutputFormat.html#getWorkOutputPath(org.apache.hadoop.mapred.JobConf)"> |
---|
| 2333 | FileOutputFormat.getWorkOutputPath() </a>from map/reduce |
---|
| 2334 | task to take advantage of this feature.</p> |
---|
| 2335 | <p>The entire discussion holds true for maps of jobs with |
---|
| 2336 | reducer=NONE (i.e. 0 reduces) since output of the map, in that case, |
---|
| 2337 | goes directly to HDFS.</p> |
---|
| 2338 | <a name="N10D25"></a><a name="RecordWriter"></a> |
---|
| 2339 | <h4>RecordWriter</h4> |
---|
| 2340 | <p> |
---|
| 2341 | <a href="api/org/apache/hadoop/mapred/RecordWriter.html"> |
---|
| 2342 | RecordWriter</a> writes the output <span class="codefrag"><key, value></span> |
---|
| 2343 | pairs to an output file.</p> |
---|
| 2344 | <p>RecordWriter implementations write the job outputs to the |
---|
| 2345 | <span class="codefrag">FileSystem</span>.</p> |
---|
| 2346 | <a name="N10D3C"></a><a name="Other+Useful+Features"></a> |
---|
| 2347 | <h3 class="h4">Other Useful Features</h3> |
---|
| 2348 | <a name="N10D42"></a><a name="Submitting+Jobs+to+Queues"></a> |
---|
| 2349 | <h4>Submitting Jobs to Queues</h4> |
---|
| 2350 | <p>Users submit jobs to Queues. Queues, as collection of jobs, |
---|
| 2351 | allow the system to provide specific functionality. For example, |
---|
| 2352 | queues use ACLs to control which users |
---|
| 2353 | who can submit jobs to them. Queues are expected to be primarily |
---|
| 2354 | used by Hadoop Schedulers. </p> |
---|
| 2355 | <p>Hadoop comes configured with a single mandatory queue, called |
---|
| 2356 | 'default'. Queue names are defined in the |
---|
| 2357 | <span class="codefrag">mapred.queue.names</span> property of the Hadoop site |
---|
| 2358 | configuration. Some job schedulers, such as the |
---|
| 2359 | <a href="capacity_scheduler.html">Capacity Scheduler</a>, |
---|
| 2360 | support multiple queues.</p> |
---|
| 2361 | <p>A job defines the queue it needs to be submitted to through the |
---|
| 2362 | <span class="codefrag">mapred.job.queue.name</span> property, or through the |
---|
| 2363 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setQueueName(java.lang.String)">setQueueName(String)</a> |
---|
| 2364 | API. Setting the queue name is optional. If a job is submitted |
---|
| 2365 | without an associated queue name, it is submitted to the 'default' |
---|
| 2366 | queue.</p> |
---|
| 2367 | <a name="N10D60"></a><a name="Counters"></a> |
---|
| 2368 | <h4>Counters</h4> |
---|
| 2369 | <p> |
---|
| 2370 | <span class="codefrag">Counters</span> represent global counters, defined either by |
---|
| 2371 | the Map/Reduce framework or applications. Each <span class="codefrag">Counter</span> can |
---|
| 2372 | be of any <span class="codefrag">Enum</span> type. Counters of a particular |
---|
| 2373 | <span class="codefrag">Enum</span> are bunched into groups of type |
---|
| 2374 | <span class="codefrag">Counters.Group</span>.</p> |
---|
| 2375 | <p>Applications can define arbitrary <span class="codefrag">Counters</span> (of type |
---|
| 2376 | <span class="codefrag">Enum</span>) and update them via |
---|
| 2377 | <a href="api/org/apache/hadoop/mapred/Reporter.html#incrCounter(java.lang.Enum, long)"> |
---|
| 2378 | Reporter.incrCounter(Enum, long)</a> or |
---|
| 2379 | <a href="api/org/apache/hadoop/mapred/Reporter.html#incrCounter(java.lang.String, java.lang.String, long amount)"> |
---|
| 2380 | Reporter.incrCounter(String, String, long)</a> |
---|
| 2381 | in the <span class="codefrag">map</span> and/or |
---|
| 2382 | <span class="codefrag">reduce</span> methods. These counters are then globally |
---|
| 2383 | aggregated by the framework.</p> |
---|
| 2384 | <a name="N10D8F"></a><a name="DistributedCache"></a> |
---|
| 2385 | <h4>DistributedCache</h4> |
---|
| 2386 | <p> |
---|
| 2387 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html"> |
---|
| 2388 | DistributedCache</a> distributes application-specific, large, read-only |
---|
| 2389 | files efficiently.</p> |
---|
| 2390 | <p> |
---|
| 2391 | <span class="codefrag">DistributedCache</span> is a facility provided by the |
---|
| 2392 | Map/Reduce framework to cache files (text, archives, jars and so on) |
---|
| 2393 | needed by applications.</p> |
---|
| 2394 | <p>Applications specify the files to be cached via urls (hdfs://) |
---|
| 2395 | in the <span class="codefrag">JobConf</span>. The <span class="codefrag">DistributedCache</span> |
---|
| 2396 | assumes that the files specified via hdfs:// urls are already present |
---|
| 2397 | on the <span class="codefrag">FileSystem</span>.</p> |
---|
| 2398 | <p>The framework will copy the necessary files to the slave node |
---|
| 2399 | before any tasks for the job are executed on that node. Its |
---|
| 2400 | efficiency stems from the fact that the files are only copied once |
---|
| 2401 | per job and the ability to cache archives which are un-archived on |
---|
| 2402 | the slaves.</p> |
---|
| 2403 | <p> |
---|
| 2404 | <span class="codefrag">DistributedCache</span> tracks the modification timestamps of |
---|
| 2405 | the cached files. Clearly the cache files should not be modified by |
---|
| 2406 | the application or externally while the job is executing.</p> |
---|
| 2407 | <p> |
---|
| 2408 | <span class="codefrag">DistributedCache</span> can be used to distribute simple, |
---|
| 2409 | read-only data/text files and more complex types such as archives and |
---|
| 2410 | jars. Archives (zip, tar, tgz and tar.gz files) are |
---|
| 2411 | <em>un-archived</em> at the slave nodes. Files |
---|
| 2412 | have <em>execution permissions</em> set. </p> |
---|
| 2413 | <p>The files/archives can be distributed by setting the property |
---|
| 2414 | <span class="codefrag">mapred.cache.{files|archives}</span>. If more than one |
---|
| 2415 | file/archive has to be distributed, they can be added as comma |
---|
| 2416 | separated paths. The properties can also be set by APIs |
---|
| 2417 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html#addCacheFile(java.net.URI,%20org.apache.hadoop.conf.Configuration)"> |
---|
| 2418 | DistributedCache.addCacheFile(URI,conf)</a>/ |
---|
| 2419 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html#addCacheArchive(java.net.URI,%20org.apache.hadoop.conf.Configuration)"> |
---|
| 2420 | DistributedCache.addCacheArchive(URI,conf)</a> and |
---|
| 2421 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html#setCacheFiles(java.net.URI[],%20org.apache.hadoop.conf.Configuration)"> |
---|
| 2422 | DistributedCache.setCacheFiles(URIs,conf)</a>/ |
---|
| 2423 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html#setCacheArchives(java.net.URI[],%20org.apache.hadoop.conf.Configuration)"> |
---|
| 2424 | DistributedCache.setCacheArchives(URIs,conf)</a> |
---|
| 2425 | where URI is of the form |
---|
| 2426 | <span class="codefrag">hdfs://host:port/absolute-path#link-name</span>. |
---|
| 2427 | In Streaming, the files can be distributed through command line |
---|
| 2428 | option <span class="codefrag">-cacheFile/-cacheArchive</span>.</p> |
---|
| 2429 | <p>Optionally users can also direct the <span class="codefrag">DistributedCache</span> |
---|
| 2430 | to <em>symlink</em> the cached file(s) into the <span class="codefrag">current working |
---|
| 2431 | directory</span> of the task via the |
---|
| 2432 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html#createSymlink(org.apache.hadoop.conf.Configuration)"> |
---|
| 2433 | DistributedCache.createSymlink(Configuration)</a> api. Or by setting |
---|
| 2434 | the configuration property <span class="codefrag">mapred.create.symlink</span> |
---|
| 2435 | as <span class="codefrag">yes</span>. The DistributedCache will use the |
---|
| 2436 | <span class="codefrag">fragment</span> of the URI as the name of the symlink. |
---|
| 2437 | For example, the URI |
---|
| 2438 | <span class="codefrag">hdfs://namenode:port/lib.so.1#lib.so</span> |
---|
| 2439 | will have the symlink name as <span class="codefrag">lib.so</span> in task's cwd |
---|
| 2440 | for the file <span class="codefrag">lib.so.1</span> in distributed cache.</p> |
---|
| 2441 | <p>The <span class="codefrag">DistributedCache</span> can also be used as a |
---|
| 2442 | rudimentary software distribution mechanism for use in the |
---|
| 2443 | map and/or reduce tasks. It can be used to distribute both |
---|
| 2444 | jars and native libraries. The |
---|
| 2445 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html#addArchiveToClassPath(org.apache.hadoop.fs.Path,%20org.apache.hadoop.conf.Configuration)"> |
---|
| 2446 | DistributedCache.addArchiveToClassPath(Path, Configuration)</a> or |
---|
| 2447 | <a href="api/org/apache/hadoop/filecache/DistributedCache.html#addFileToClassPath(org.apache.hadoop.fs.Path,%20org.apache.hadoop.conf.Configuration)"> |
---|
| 2448 | DistributedCache.addFileToClassPath(Path, Configuration)</a> api |
---|
| 2449 | can be used to cache files/jars and also add them to the |
---|
| 2450 | <em>classpath</em> of child-jvm. The same can be done by setting |
---|
| 2451 | the configuration properties |
---|
| 2452 | <span class="codefrag">mapred.job.classpath.{files|archives}</span>. Similarly the |
---|
| 2453 | cached files that are symlinked into the working directory of the |
---|
| 2454 | task can be used to distribute native libraries and load them.</p> |
---|
| 2455 | <a name="N10E12"></a><a name="Tool"></a> |
---|
| 2456 | <h4>Tool</h4> |
---|
| 2457 | <p>The <a href="api/org/apache/hadoop/util/Tool.html">Tool</a> |
---|
| 2458 | interface supports the handling of generic Hadoop command-line options. |
---|
| 2459 | </p> |
---|
| 2460 | <p> |
---|
| 2461 | <span class="codefrag">Tool</span> is the standard for any Map/Reduce tool or |
---|
| 2462 | application. The application should delegate the handling of |
---|
| 2463 | standard command-line options to |
---|
| 2464 | <a href="api/org/apache/hadoop/util/GenericOptionsParser.html"> |
---|
| 2465 | GenericOptionsParser</a> via |
---|
| 2466 | <a href="api/org/apache/hadoop/util/ToolRunner.html#run(org.apache.hadoop.util.Tool, java.lang.String[])"> |
---|
| 2467 | ToolRunner.run(Tool, String[])</a> and only handle its custom |
---|
| 2468 | arguments.</p> |
---|
| 2469 | <p> |
---|
| 2470 | The generic Hadoop command-line options are:<br> |
---|
| 2471 | |
---|
| 2472 | <span class="codefrag"> |
---|
| 2473 | -conf <configuration file> |
---|
| 2474 | </span> |
---|
| 2475 | |
---|
| 2476 | <br> |
---|
| 2477 | |
---|
| 2478 | <span class="codefrag"> |
---|
| 2479 | -D <property=value> |
---|
| 2480 | </span> |
---|
| 2481 | |
---|
| 2482 | <br> |
---|
| 2483 | |
---|
| 2484 | <span class="codefrag"> |
---|
| 2485 | -fs <local|namenode:port> |
---|
| 2486 | </span> |
---|
| 2487 | |
---|
| 2488 | <br> |
---|
| 2489 | |
---|
| 2490 | <span class="codefrag"> |
---|
| 2491 | -jt <local|jobtracker:port> |
---|
| 2492 | </span> |
---|
| 2493 | |
---|
| 2494 | </p> |
---|
| 2495 | <a name="N10E44"></a><a name="IsolationRunner"></a> |
---|
| 2496 | <h4>IsolationRunner</h4> |
---|
| 2497 | <p> |
---|
| 2498 | <a href="api/org/apache/hadoop/mapred/IsolationRunner.html"> |
---|
| 2499 | IsolationRunner</a> is a utility to help debug Map/Reduce programs.</p> |
---|
| 2500 | <p>To use the <span class="codefrag">IsolationRunner</span>, first set |
---|
| 2501 | <span class="codefrag">keep.failed.tasks.files</span> to <span class="codefrag">true</span> |
---|
| 2502 | (also see <span class="codefrag">keep.tasks.files.pattern</span>).</p> |
---|
| 2503 | <p> |
---|
| 2504 | Next, go to the node on which the failed task ran and go to the |
---|
| 2505 | <span class="codefrag">TaskTracker</span>'s local directory and run the |
---|
| 2506 | <span class="codefrag">IsolationRunner</span>:<br> |
---|
| 2507 | |
---|
| 2508 | <span class="codefrag">$ cd <local path>/taskTracker/${taskid}/work</span> |
---|
| 2509 | <br> |
---|
| 2510 | |
---|
| 2511 | <span class="codefrag"> |
---|
| 2512 | $ bin/hadoop org.apache.hadoop.mapred.IsolationRunner ../job.xml |
---|
| 2513 | </span> |
---|
| 2514 | |
---|
| 2515 | </p> |
---|
| 2516 | <p> |
---|
| 2517 | <span class="codefrag">IsolationRunner</span> will run the failed task in a single |
---|
| 2518 | jvm, which can be in the debugger, over precisely the same input.</p> |
---|
| 2519 | <a name="N10E77"></a><a name="Profiling"></a> |
---|
| 2520 | <h4>Profiling</h4> |
---|
| 2521 | <p>Profiling is a utility to get a representative (2 or 3) sample |
---|
| 2522 | of built-in java profiler for a sample of maps and reduces. </p> |
---|
| 2523 | <p>User can specify whether the system should collect profiler |
---|
| 2524 | information for some of the tasks in the job by setting the |
---|
| 2525 | configuration property <span class="codefrag">mapred.task.profile</span>. The |
---|
| 2526 | value can be set using the api |
---|
| 2527 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setProfileEnabled(boolean)"> |
---|
| 2528 | JobConf.setProfileEnabled(boolean)</a>. If the value is set |
---|
| 2529 | <span class="codefrag">true</span>, the task profiling is enabled. The profiler |
---|
| 2530 | information is stored in the user log directory. By default, |
---|
| 2531 | profiling is not enabled for the job. </p> |
---|
| 2532 | <p>Once user configures that profiling is needed, she/he can use |
---|
| 2533 | the configuration property |
---|
| 2534 | <span class="codefrag">mapred.task.profile.{maps|reduces}</span> to set the ranges |
---|
| 2535 | of map/reduce tasks to profile. The value can be set using the api |
---|
| 2536 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setProfileTaskRange(boolean,%20java.lang.String)"> |
---|
| 2537 | JobConf.setProfileTaskRange(boolean,String)</a>. |
---|
| 2538 | By default, the specified range is <span class="codefrag">0-2</span>.</p> |
---|
| 2539 | <p>User can also specify the profiler configuration arguments by |
---|
| 2540 | setting the configuration property |
---|
| 2541 | <span class="codefrag">mapred.task.profile.params</span>. The value can be specified |
---|
| 2542 | using the api |
---|
| 2543 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setProfileParams(java.lang.String)"> |
---|
| 2544 | JobConf.setProfileParams(String)</a>. If the string contains a |
---|
| 2545 | <span class="codefrag">%s</span>, it will be replaced with the name of the profiling |
---|
| 2546 | output file when the task runs. These parameters are passed to the |
---|
| 2547 | task child JVM on the command line. The default value for |
---|
| 2548 | the profiling parameters is |
---|
| 2549 | <span class="codefrag">-agentlib:hprof=cpu=samples,heap=sites,force=n,thread=y,verbose=n,file=%s</span> |
---|
| 2550 | |
---|
| 2551 | </p> |
---|
| 2552 | <a name="N10EAB"></a><a name="Debugging"></a> |
---|
| 2553 | <h4>Debugging</h4> |
---|
| 2554 | <p>The Map/Reduce framework provides a facility to run user-provided |
---|
| 2555 | scripts for debugging. When a map/reduce task fails, a user can run |
---|
| 2556 | a debug script, to process task logs for example. The script is |
---|
| 2557 | given access to the task's stdout and stderr outputs, syslog and |
---|
| 2558 | jobconf. The output from the debug script's stdout and stderr is |
---|
| 2559 | displayed on the console diagnostics and also as part of the |
---|
| 2560 | job UI. </p> |
---|
| 2561 | <p> In the following sections we discuss how to submit a debug script |
---|
| 2562 | with a job. The script file needs to be distributed and submitted to |
---|
| 2563 | the framework.</p> |
---|
| 2564 | <a name="N10EB7"></a><a name="How+to+distribute+the+script+file%3A"></a> |
---|
| 2565 | <h5> How to distribute the script file: </h5> |
---|
| 2566 | <p> |
---|
| 2567 | The user needs to use |
---|
| 2568 | <a href="mapred_tutorial.html#DistributedCache">DistributedCache</a> |
---|
| 2569 | to <em>distribute</em> and <em>symlink</em> the script file.</p> |
---|
| 2570 | <a name="N10ECB"></a><a name="How+to+submit+the+script%3A"></a> |
---|
| 2571 | <h5> How to submit the script: </h5> |
---|
| 2572 | <p> A quick way to submit the debug script is to set values for the |
---|
| 2573 | properties <span class="codefrag">mapred.map.task.debug.script</span> and |
---|
| 2574 | <span class="codefrag">mapred.reduce.task.debug.script</span>, for debugging map and |
---|
| 2575 | reduce tasks respectively. These properties can also be set by using APIs |
---|
| 2576 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setMapDebugScript(java.lang.String)"> |
---|
| 2577 | JobConf.setMapDebugScript(String) </a> and |
---|
| 2578 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setReduceDebugScript(java.lang.String)"> |
---|
| 2579 | JobConf.setReduceDebugScript(String) </a>. In streaming mode, a debug |
---|
| 2580 | script can be submitted with the command-line options |
---|
| 2581 | <span class="codefrag">-mapdebug</span> and <span class="codefrag">-reducedebug</span>, for debugging |
---|
| 2582 | map and reduce tasks respectively.</p> |
---|
| 2583 | <p>The arguments to the script are the task's stdout, stderr, |
---|
| 2584 | syslog and jobconf files. The debug command, run on the node where |
---|
| 2585 | the map/reduce task failed, is: <br> |
---|
| 2586 | |
---|
| 2587 | <span class="codefrag"> $script $stdout $stderr $syslog $jobconf </span> |
---|
| 2588 | </p> |
---|
| 2589 | <p> Pipes programs have the c++ program name as a fifth argument |
---|
| 2590 | for the command. Thus for the pipes programs the command is <br> |
---|
| 2591 | |
---|
| 2592 | <span class="codefrag">$script $stdout $stderr $syslog $jobconf $program </span> |
---|
| 2593 | |
---|
| 2594 | </p> |
---|
| 2595 | <a name="N10EF9"></a><a name="Default+Behavior%3A"></a> |
---|
| 2596 | <h5> Default Behavior: </h5> |
---|
| 2597 | <p> For pipes, a default script is run to process core dumps under |
---|
| 2598 | gdb, prints stack trace and gives info about running threads. </p> |
---|
| 2599 | <a name="N10F04"></a><a name="JobControl"></a> |
---|
| 2600 | <h4>JobControl</h4> |
---|
| 2601 | <p> |
---|
| 2602 | <a href="api/org/apache/hadoop/mapred/jobcontrol/package-summary.html"> |
---|
| 2603 | JobControl</a> is a utility which encapsulates a set of Map/Reduce jobs |
---|
| 2604 | and their dependencies.</p> |
---|
| 2605 | <a name="N10F11"></a><a name="Data+Compression"></a> |
---|
| 2606 | <h4>Data Compression</h4> |
---|
| 2607 | <p>Hadoop Map/Reduce provides facilities for the application-writer to |
---|
| 2608 | specify compression for both intermediate map-outputs and the |
---|
| 2609 | job-outputs i.e. output of the reduces. It also comes bundled with |
---|
| 2610 | <a href="api/org/apache/hadoop/io/compress/CompressionCodec.html"> |
---|
| 2611 | CompressionCodec</a> implementation for the |
---|
| 2612 | <a href="http://www.zlib.net/">zlib</a> compression |
---|
| 2613 | algorithm. The <a href="http://www.gzip.org/">gzip</a> file format is also |
---|
| 2614 | supported.</p> |
---|
| 2615 | <p>Hadoop also provides native implementations of the above compression |
---|
| 2616 | codecs for reasons of both performance (zlib) and non-availability of |
---|
| 2617 | Java libraries. More details on their usage and availability are |
---|
| 2618 | available <a href="native_libraries.html">here</a>.</p> |
---|
| 2619 | <a name="N10F2D"></a><a name="Intermediate+Outputs"></a> |
---|
| 2620 | <h5>Intermediate Outputs</h5> |
---|
| 2621 | <p>Applications can control compression of intermediate map-outputs |
---|
| 2622 | via the |
---|
| 2623 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setCompressMapOutput(boolean)"> |
---|
| 2624 | JobConf.setCompressMapOutput(boolean)</a> api and the |
---|
| 2625 | <span class="codefrag">CompressionCodec</span> to be used via the |
---|
| 2626 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setMapOutputCompressorClass(java.lang.Class)"> |
---|
| 2627 | JobConf.setMapOutputCompressorClass(Class)</a> api.</p> |
---|
| 2628 | <a name="N10F42"></a><a name="Job+Outputs"></a> |
---|
| 2629 | <h5>Job Outputs</h5> |
---|
| 2630 | <p>Applications can control compression of job-outputs via the |
---|
| 2631 | <a href="api/org/apache/hadoop/mapred/FileOutputFormat.html#setCompressOutput(org.apache.hadoop.mapred.JobConf,%20boolean)"> |
---|
| 2632 | FileOutputFormat.setCompressOutput(JobConf, boolean)</a> api and the |
---|
| 2633 | <span class="codefrag">CompressionCodec</span> to be used can be specified via the |
---|
| 2634 | <a href="api/org/apache/hadoop/mapred/FileOutputFormat.html#setOutputCompressorClass(org.apache.hadoop.mapred.JobConf,%20java.lang.Class)"> |
---|
| 2635 | FileOutputFormat.setOutputCompressorClass(JobConf, Class)</a> api.</p> |
---|
| 2636 | <p>If the job outputs are to be stored in the |
---|
| 2637 | <a href="api/org/apache/hadoop/mapred/SequenceFileOutputFormat.html"> |
---|
| 2638 | SequenceFileOutputFormat</a>, the required |
---|
| 2639 | <span class="codefrag">SequenceFile.CompressionType</span> (i.e. <span class="codefrag">RECORD</span> / |
---|
| 2640 | <span class="codefrag">BLOCK</span> - defaults to <span class="codefrag">RECORD</span>) can be |
---|
| 2641 | specified via the |
---|
| 2642 | <a href="api/org/apache/hadoop/mapred/SequenceFileOutputFormat.html#setOutputCompressionType(org.apache.hadoop.mapred.JobConf,%20org.apache.hadoop.io.SequenceFile.CompressionType)"> |
---|
| 2643 | SequenceFileOutputFormat.setOutputCompressionType(JobConf, |
---|
| 2644 | SequenceFile.CompressionType)</a> api.</p> |
---|
| 2645 | <a name="N10F6F"></a><a name="Skipping+Bad+Records"></a> |
---|
| 2646 | <h4>Skipping Bad Records</h4> |
---|
| 2647 | <p>Hadoop provides an option where a certain set of bad input |
---|
| 2648 | records can be skipped when processing map inputs. Applications |
---|
| 2649 | can control this feature through the |
---|
| 2650 | <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html"> |
---|
| 2651 | SkipBadRecords</a> class.</p> |
---|
| 2652 | <p>This feature can be used when map tasks crash deterministically |
---|
| 2653 | on certain input. This usually happens due to bugs in the |
---|
| 2654 | map function. Usually, the user would have to fix these bugs. |
---|
| 2655 | This is, however, not possible sometimes. The bug may be in third |
---|
| 2656 | party libraries, for example, for which the source code is not |
---|
| 2657 | available. In such cases, the task never completes successfully even |
---|
| 2658 | after multiple attempts, and the job fails. With this feature, only |
---|
| 2659 | a small portion of data surrounding the |
---|
| 2660 | bad records is lost, which may be acceptable for some applications |
---|
| 2661 | (those performing statistical analysis on very large data, for |
---|
| 2662 | example). </p> |
---|
| 2663 | <p>By default this feature is disabled. For enabling it, |
---|
| 2664 | refer to <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setMapperMaxSkipRecords(org.apache.hadoop.conf.Configuration, long)"> |
---|
| 2665 | SkipBadRecords.setMapperMaxSkipRecords(Configuration, long)</a> and |
---|
| 2666 | <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setReducerMaxSkipGroups(org.apache.hadoop.conf.Configuration, long)"> |
---|
| 2667 | SkipBadRecords.setReducerMaxSkipGroups(Configuration, long)</a>. |
---|
| 2668 | </p> |
---|
| 2669 | <p>With this feature enabled, the framework gets into 'skipping |
---|
| 2670 | mode' after a certain number of map failures. For more details, |
---|
| 2671 | see <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setAttemptsToStartSkipping(org.apache.hadoop.conf.Configuration, int)"> |
---|
| 2672 | SkipBadRecords.setAttemptsToStartSkipping(Configuration, int)</a>. |
---|
| 2673 | In 'skipping mode', map tasks maintain the range of records being |
---|
| 2674 | processed. To do this, the framework relies on the processed record |
---|
| 2675 | counter. See <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#COUNTER_MAP_PROCESSED_RECORDS"> |
---|
| 2676 | SkipBadRecords.COUNTER_MAP_PROCESSED_RECORDS</a> and |
---|
| 2677 | <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#COUNTER_REDUCE_PROCESSED_GROUPS"> |
---|
| 2678 | SkipBadRecords.COUNTER_REDUCE_PROCESSED_GROUPS</a>. |
---|
| 2679 | This counter enables the framework to know how many records have |
---|
| 2680 | been processed successfully, and hence, what record range caused |
---|
| 2681 | a task to crash. On further attempts, this range of records is |
---|
| 2682 | skipped.</p> |
---|
| 2683 | <p>The number of records skipped depends on how frequently the |
---|
| 2684 | processed record counter is incremented by the application. |
---|
| 2685 | It is recommended that this counter be incremented after every |
---|
| 2686 | record is processed. This may not be possible in some applications |
---|
| 2687 | that typically batch their processing. In such cases, the framework |
---|
| 2688 | may skip additional records surrounding the bad record. Users can |
---|
| 2689 | control the number of skipped records through |
---|
| 2690 | <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setMapperMaxSkipRecords(org.apache.hadoop.conf.Configuration, long)"> |
---|
| 2691 | SkipBadRecords.setMapperMaxSkipRecords(Configuration, long)</a> and |
---|
| 2692 | <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setReducerMaxSkipGroups(org.apache.hadoop.conf.Configuration, long)"> |
---|
| 2693 | SkipBadRecords.setReducerMaxSkipGroups(Configuration, long)</a>. |
---|
| 2694 | The framework tries to narrow the range of skipped records using a |
---|
| 2695 | binary search-like approach. The skipped range is divided into two |
---|
| 2696 | halves and only one half gets executed. On subsequent |
---|
| 2697 | failures, the framework figures out which half contains |
---|
| 2698 | bad records. A task will be re-executed till the |
---|
| 2699 | acceptable skipped value is met or all task attempts are exhausted. |
---|
| 2700 | To increase the number of task attempts, use |
---|
| 2701 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxMapAttempts(int)"> |
---|
| 2702 | JobConf.setMaxMapAttempts(int)</a> and |
---|
| 2703 | <a href="api/org/apache/hadoop/mapred/JobConf.html#setMaxReduceAttempts(int)"> |
---|
| 2704 | JobConf.setMaxReduceAttempts(int)</a>. |
---|
| 2705 | </p> |
---|
| 2706 | <p>Skipped records are written to HDFS in the sequence file |
---|
| 2707 | format, for later analysis. The location can be changed through |
---|
| 2708 | <a href="api/org/apache/hadoop/mapred/SkipBadRecords.html#setSkipOutputPath(org.apache.hadoop.mapred.JobConf, org.apache.hadoop.fs.Path)"> |
---|
| 2709 | SkipBadRecords.setSkipOutputPath(JobConf, Path)</a>. |
---|
| 2710 | </p> |
---|
| 2711 | </div> |
---|
| 2712 | |
---|
| 2713 | |
---|
| 2714 | <a name="N10FB6"></a><a name="Example%3A+WordCount+v2.0"></a> |
---|
| 2715 | <h2 class="h3">Example: WordCount v2.0</h2> |
---|
| 2716 | <div class="section"> |
---|
| 2717 | <p>Here is a more complete <span class="codefrag">WordCount</span> which uses many of the |
---|
| 2718 | features provided by the Map/Reduce framework we discussed so far.</p> |
---|
| 2719 | <p>This needs the HDFS to be up and running, especially for the |
---|
| 2720 | <span class="codefrag">DistributedCache</span>-related features. Hence it only works with a |
---|
| 2721 | <a href="quickstart.html#SingleNodeSetup">pseudo-distributed</a> or |
---|
| 2722 | <a href="quickstart.html#Fully-Distributed+Operation">fully-distributed</a> |
---|
| 2723 | Hadoop installation.</p> |
---|
| 2724 | <a name="N10FD0"></a><a name="Source+Code-N10FD0"></a> |
---|
| 2725 | <h3 class="h4">Source Code</h3> |
---|
| 2726 | <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
---|
| 2727 | |
---|
| 2728 | <tr> |
---|
| 2729 | |
---|
| 2730 | <th colspan="1" rowspan="1"></th> |
---|
| 2731 | <th colspan="1" rowspan="1">WordCount.java</th> |
---|
| 2732 | |
---|
| 2733 | </tr> |
---|
| 2734 | |
---|
| 2735 | <tr> |
---|
| 2736 | |
---|
| 2737 | <td colspan="1" rowspan="1">1.</td> |
---|
| 2738 | <td colspan="1" rowspan="1"> |
---|
| 2739 | <span class="codefrag">package org.myorg;</span> |
---|
| 2740 | </td> |
---|
| 2741 | |
---|
| 2742 | </tr> |
---|
| 2743 | |
---|
| 2744 | <tr> |
---|
| 2745 | |
---|
| 2746 | <td colspan="1" rowspan="1">2.</td> |
---|
| 2747 | <td colspan="1" rowspan="1"></td> |
---|
| 2748 | |
---|
| 2749 | </tr> |
---|
| 2750 | |
---|
| 2751 | <tr> |
---|
| 2752 | |
---|
| 2753 | <td colspan="1" rowspan="1">3.</td> |
---|
| 2754 | <td colspan="1" rowspan="1"> |
---|
| 2755 | <span class="codefrag">import java.io.*;</span> |
---|
| 2756 | </td> |
---|
| 2757 | |
---|
| 2758 | </tr> |
---|
| 2759 | |
---|
| 2760 | <tr> |
---|
| 2761 | |
---|
| 2762 | <td colspan="1" rowspan="1">4.</td> |
---|
| 2763 | <td colspan="1" rowspan="1"> |
---|
| 2764 | <span class="codefrag">import java.util.*;</span> |
---|
| 2765 | </td> |
---|
| 2766 | |
---|
| 2767 | </tr> |
---|
| 2768 | |
---|
| 2769 | <tr> |
---|
| 2770 | |
---|
| 2771 | <td colspan="1" rowspan="1">5.</td> |
---|
| 2772 | <td colspan="1" rowspan="1"></td> |
---|
| 2773 | |
---|
| 2774 | </tr> |
---|
| 2775 | |
---|
| 2776 | <tr> |
---|
| 2777 | |
---|
| 2778 | <td colspan="1" rowspan="1">6.</td> |
---|
| 2779 | <td colspan="1" rowspan="1"> |
---|
| 2780 | <span class="codefrag">import org.apache.hadoop.fs.Path;</span> |
---|
| 2781 | </td> |
---|
| 2782 | |
---|
| 2783 | </tr> |
---|
| 2784 | |
---|
| 2785 | <tr> |
---|
| 2786 | |
---|
| 2787 | <td colspan="1" rowspan="1">7.</td> |
---|
| 2788 | <td colspan="1" rowspan="1"> |
---|
| 2789 | <span class="codefrag">import org.apache.hadoop.filecache.DistributedCache;</span> |
---|
| 2790 | </td> |
---|
| 2791 | |
---|
| 2792 | </tr> |
---|
| 2793 | |
---|
| 2794 | <tr> |
---|
| 2795 | |
---|
| 2796 | <td colspan="1" rowspan="1">8.</td> |
---|
| 2797 | <td colspan="1" rowspan="1"> |
---|
| 2798 | <span class="codefrag">import org.apache.hadoop.conf.*;</span> |
---|
| 2799 | </td> |
---|
| 2800 | |
---|
| 2801 | </tr> |
---|
| 2802 | |
---|
| 2803 | <tr> |
---|
| 2804 | |
---|
| 2805 | <td colspan="1" rowspan="1">9.</td> |
---|
| 2806 | <td colspan="1" rowspan="1"> |
---|
| 2807 | <span class="codefrag">import org.apache.hadoop.io.*;</span> |
---|
| 2808 | </td> |
---|
| 2809 | |
---|
| 2810 | </tr> |
---|
| 2811 | |
---|
| 2812 | <tr> |
---|
| 2813 | |
---|
| 2814 | <td colspan="1" rowspan="1">10.</td> |
---|
| 2815 | <td colspan="1" rowspan="1"> |
---|
| 2816 | <span class="codefrag">import org.apache.hadoop.mapred.*;</span> |
---|
| 2817 | </td> |
---|
| 2818 | |
---|
| 2819 | </tr> |
---|
| 2820 | |
---|
| 2821 | <tr> |
---|
| 2822 | |
---|
| 2823 | <td colspan="1" rowspan="1">11.</td> |
---|
| 2824 | <td colspan="1" rowspan="1"> |
---|
| 2825 | <span class="codefrag">import org.apache.hadoop.util.*;</span> |
---|
| 2826 | </td> |
---|
| 2827 | |
---|
| 2828 | </tr> |
---|
| 2829 | |
---|
| 2830 | <tr> |
---|
| 2831 | |
---|
| 2832 | <td colspan="1" rowspan="1">12.</td> |
---|
| 2833 | <td colspan="1" rowspan="1"></td> |
---|
| 2834 | |
---|
| 2835 | </tr> |
---|
| 2836 | |
---|
| 2837 | <tr> |
---|
| 2838 | |
---|
| 2839 | <td colspan="1" rowspan="1">13.</td> |
---|
| 2840 | <td colspan="1" rowspan="1"> |
---|
| 2841 | <span class="codefrag">public class WordCount extends Configured implements Tool {</span> |
---|
| 2842 | </td> |
---|
| 2843 | |
---|
| 2844 | </tr> |
---|
| 2845 | |
---|
| 2846 | <tr> |
---|
| 2847 | |
---|
| 2848 | <td colspan="1" rowspan="1">14.</td> |
---|
| 2849 | <td colspan="1" rowspan="1"></td> |
---|
| 2850 | |
---|
| 2851 | </tr> |
---|
| 2852 | |
---|
| 2853 | <tr> |
---|
| 2854 | |
---|
| 2855 | <td colspan="1" rowspan="1">15.</td> |
---|
| 2856 | <td colspan="1" rowspan="1"> |
---|
| 2857 | |
---|
| 2858 | <span class="codefrag"> |
---|
| 2859 | public static class Map extends MapReduceBase |
---|
| 2860 | implements Mapper<LongWritable, Text, Text, IntWritable> { |
---|
| 2861 | </span> |
---|
| 2862 | </td> |
---|
| 2863 | |
---|
| 2864 | </tr> |
---|
| 2865 | |
---|
| 2866 | <tr> |
---|
| 2867 | |
---|
| 2868 | <td colspan="1" rowspan="1">16.</td> |
---|
| 2869 | <td colspan="1" rowspan="1"></td> |
---|
| 2870 | |
---|
| 2871 | </tr> |
---|
| 2872 | |
---|
| 2873 | <tr> |
---|
| 2874 | |
---|
| 2875 | <td colspan="1" rowspan="1">17.</td> |
---|
| 2876 | <td colspan="1" rowspan="1"> |
---|
| 2877 | |
---|
| 2878 | <span class="codefrag"> |
---|
| 2879 | static enum Counters { INPUT_WORDS } |
---|
| 2880 | </span> |
---|
| 2881 | </td> |
---|
| 2882 | |
---|
| 2883 | </tr> |
---|
| 2884 | |
---|
| 2885 | <tr> |
---|
| 2886 | |
---|
| 2887 | <td colspan="1" rowspan="1">18.</td> |
---|
| 2888 | <td colspan="1" rowspan="1"></td> |
---|
| 2889 | |
---|
| 2890 | </tr> |
---|
| 2891 | |
---|
| 2892 | <tr> |
---|
| 2893 | |
---|
| 2894 | <td colspan="1" rowspan="1">19.</td> |
---|
| 2895 | <td colspan="1" rowspan="1"> |
---|
| 2896 | |
---|
| 2897 | <span class="codefrag"> |
---|
| 2898 | private final static IntWritable one = new IntWritable(1); |
---|
| 2899 | </span> |
---|
| 2900 | </td> |
---|
| 2901 | |
---|
| 2902 | </tr> |
---|
| 2903 | |
---|
| 2904 | <tr> |
---|
| 2905 | |
---|
| 2906 | <td colspan="1" rowspan="1">20.</td> |
---|
| 2907 | <td colspan="1" rowspan="1"> |
---|
| 2908 | |
---|
| 2909 | <span class="codefrag">private Text word = new Text();</span> |
---|
| 2910 | </td> |
---|
| 2911 | |
---|
| 2912 | </tr> |
---|
| 2913 | |
---|
| 2914 | <tr> |
---|
| 2915 | |
---|
| 2916 | <td colspan="1" rowspan="1">21.</td> |
---|
| 2917 | <td colspan="1" rowspan="1"></td> |
---|
| 2918 | |
---|
| 2919 | </tr> |
---|
| 2920 | |
---|
| 2921 | <tr> |
---|
| 2922 | |
---|
| 2923 | <td colspan="1" rowspan="1">22.</td> |
---|
| 2924 | <td colspan="1" rowspan="1"> |
---|
| 2925 | |
---|
| 2926 | <span class="codefrag">private boolean caseSensitive = true;</span> |
---|
| 2927 | </td> |
---|
| 2928 | |
---|
| 2929 | </tr> |
---|
| 2930 | |
---|
| 2931 | <tr> |
---|
| 2932 | |
---|
| 2933 | <td colspan="1" rowspan="1">23.</td> |
---|
| 2934 | <td colspan="1" rowspan="1"> |
---|
| 2935 | |
---|
| 2936 | <span class="codefrag">private Set<String> patternsToSkip = new HashSet<String>();</span> |
---|
| 2937 | </td> |
---|
| 2938 | |
---|
| 2939 | </tr> |
---|
| 2940 | |
---|
| 2941 | <tr> |
---|
| 2942 | |
---|
| 2943 | <td colspan="1" rowspan="1">24.</td> |
---|
| 2944 | <td colspan="1" rowspan="1"></td> |
---|
| 2945 | |
---|
| 2946 | </tr> |
---|
| 2947 | |
---|
| 2948 | <tr> |
---|
| 2949 | |
---|
| 2950 | <td colspan="1" rowspan="1">25.</td> |
---|
| 2951 | <td colspan="1" rowspan="1"> |
---|
| 2952 | |
---|
| 2953 | <span class="codefrag">private long numRecords = 0;</span> |
---|
| 2954 | </td> |
---|
| 2955 | |
---|
| 2956 | </tr> |
---|
| 2957 | |
---|
| 2958 | <tr> |
---|
| 2959 | |
---|
| 2960 | <td colspan="1" rowspan="1">26.</td> |
---|
| 2961 | <td colspan="1" rowspan="1"> |
---|
| 2962 | |
---|
| 2963 | <span class="codefrag">private String inputFile;</span> |
---|
| 2964 | </td> |
---|
| 2965 | |
---|
| 2966 | </tr> |
---|
| 2967 | |
---|
| 2968 | <tr> |
---|
| 2969 | |
---|
| 2970 | <td colspan="1" rowspan="1">27.</td> |
---|
| 2971 | <td colspan="1" rowspan="1"></td> |
---|
| 2972 | |
---|
| 2973 | </tr> |
---|
| 2974 | |
---|
| 2975 | <tr> |
---|
| 2976 | |
---|
| 2977 | <td colspan="1" rowspan="1">28.</td> |
---|
| 2978 | <td colspan="1" rowspan="1"> |
---|
| 2979 | |
---|
| 2980 | <span class="codefrag">public void configure(JobConf job) {</span> |
---|
| 2981 | </td> |
---|
| 2982 | |
---|
| 2983 | </tr> |
---|
| 2984 | |
---|
| 2985 | <tr> |
---|
| 2986 | |
---|
| 2987 | <td colspan="1" rowspan="1">29.</td> |
---|
| 2988 | <td colspan="1" rowspan="1"> |
---|
| 2989 | |
---|
| 2990 | <span class="codefrag"> |
---|
| 2991 | caseSensitive = job.getBoolean("wordcount.case.sensitive", true); |
---|
| 2992 | </span> |
---|
| 2993 | </td> |
---|
| 2994 | |
---|
| 2995 | </tr> |
---|
| 2996 | |
---|
| 2997 | <tr> |
---|
| 2998 | |
---|
| 2999 | <td colspan="1" rowspan="1">30.</td> |
---|
| 3000 | <td colspan="1" rowspan="1"> |
---|
| 3001 | |
---|
| 3002 | <span class="codefrag">inputFile = job.get("map.input.file");</span> |
---|
| 3003 | </td> |
---|
| 3004 | |
---|
| 3005 | </tr> |
---|
| 3006 | |
---|
| 3007 | <tr> |
---|
| 3008 | |
---|
| 3009 | <td colspan="1" rowspan="1">31.</td> |
---|
| 3010 | <td colspan="1" rowspan="1"></td> |
---|
| 3011 | |
---|
| 3012 | </tr> |
---|
| 3013 | |
---|
| 3014 | <tr> |
---|
| 3015 | |
---|
| 3016 | <td colspan="1" rowspan="1">32.</td> |
---|
| 3017 | <td colspan="1" rowspan="1"> |
---|
| 3018 | |
---|
| 3019 | <span class="codefrag">if (job.getBoolean("wordcount.skip.patterns", false)) {</span> |
---|
| 3020 | </td> |
---|
| 3021 | |
---|
| 3022 | </tr> |
---|
| 3023 | |
---|
| 3024 | <tr> |
---|
| 3025 | |
---|
| 3026 | <td colspan="1" rowspan="1">33.</td> |
---|
| 3027 | <td colspan="1" rowspan="1"> |
---|
| 3028 | |
---|
| 3029 | <span class="codefrag">Path[] patternsFiles = new Path[0];</span> |
---|
| 3030 | </td> |
---|
| 3031 | |
---|
| 3032 | </tr> |
---|
| 3033 | |
---|
| 3034 | <tr> |
---|
| 3035 | |
---|
| 3036 | <td colspan="1" rowspan="1">34.</td> |
---|
| 3037 | <td colspan="1" rowspan="1"> |
---|
| 3038 | |
---|
| 3039 | <span class="codefrag">try {</span> |
---|
| 3040 | </td> |
---|
| 3041 | |
---|
| 3042 | </tr> |
---|
| 3043 | |
---|
| 3044 | <tr> |
---|
| 3045 | |
---|
| 3046 | <td colspan="1" rowspan="1">35.</td> |
---|
| 3047 | <td colspan="1" rowspan="1"> |
---|
| 3048 | |
---|
| 3049 | <span class="codefrag"> |
---|
| 3050 | patternsFiles = DistributedCache.getLocalCacheFiles(job); |
---|
| 3051 | </span> |
---|
| 3052 | </td> |
---|
| 3053 | |
---|
| 3054 | </tr> |
---|
| 3055 | |
---|
| 3056 | <tr> |
---|
| 3057 | |
---|
| 3058 | <td colspan="1" rowspan="1">36.</td> |
---|
| 3059 | <td colspan="1" rowspan="1"> |
---|
| 3060 | |
---|
| 3061 | <span class="codefrag">} catch (IOException ioe) {</span> |
---|
| 3062 | </td> |
---|
| 3063 | |
---|
| 3064 | </tr> |
---|
| 3065 | |
---|
| 3066 | <tr> |
---|
| 3067 | |
---|
| 3068 | <td colspan="1" rowspan="1">37.</td> |
---|
| 3069 | <td colspan="1" rowspan="1"> |
---|
| 3070 | |
---|
| 3071 | <span class="codefrag"> |
---|
| 3072 | System.err.println("Caught exception while getting cached files: " |
---|
| 3073 | + StringUtils.stringifyException(ioe)); |
---|
| 3074 | </span> |
---|
| 3075 | </td> |
---|
| 3076 | |
---|
| 3077 | </tr> |
---|
| 3078 | |
---|
| 3079 | <tr> |
---|
| 3080 | |
---|
| 3081 | <td colspan="1" rowspan="1">38.</td> |
---|
| 3082 | <td colspan="1" rowspan="1"> |
---|
| 3083 | |
---|
| 3084 | <span class="codefrag">}</span> |
---|
| 3085 | </td> |
---|
| 3086 | |
---|
| 3087 | </tr> |
---|
| 3088 | |
---|
| 3089 | <tr> |
---|
| 3090 | |
---|
| 3091 | <td colspan="1" rowspan="1">39.</td> |
---|
| 3092 | <td colspan="1" rowspan="1"> |
---|
| 3093 | |
---|
| 3094 | <span class="codefrag">for (Path patternsFile : patternsFiles) {</span> |
---|
| 3095 | </td> |
---|
| 3096 | |
---|
| 3097 | </tr> |
---|
| 3098 | |
---|
| 3099 | <tr> |
---|
| 3100 | |
---|
| 3101 | <td colspan="1" rowspan="1">40.</td> |
---|
| 3102 | <td colspan="1" rowspan="1"> |
---|
| 3103 | |
---|
| 3104 | <span class="codefrag">parseSkipFile(patternsFile);</span> |
---|
| 3105 | </td> |
---|
| 3106 | |
---|
| 3107 | </tr> |
---|
| 3108 | |
---|
| 3109 | <tr> |
---|
| 3110 | |
---|
| 3111 | <td colspan="1" rowspan="1">41.</td> |
---|
| 3112 | <td colspan="1" rowspan="1"> |
---|
| 3113 | |
---|
| 3114 | <span class="codefrag">}</span> |
---|
| 3115 | </td> |
---|
| 3116 | |
---|
| 3117 | </tr> |
---|
| 3118 | |
---|
| 3119 | <tr> |
---|
| 3120 | |
---|
| 3121 | <td colspan="1" rowspan="1">42.</td> |
---|
| 3122 | <td colspan="1" rowspan="1"> |
---|
| 3123 | |
---|
| 3124 | <span class="codefrag">}</span> |
---|
| 3125 | </td> |
---|
| 3126 | |
---|
| 3127 | </tr> |
---|
| 3128 | |
---|
| 3129 | <tr> |
---|
| 3130 | |
---|
| 3131 | <td colspan="1" rowspan="1">43.</td> |
---|
| 3132 | <td colspan="1" rowspan="1"> |
---|
| 3133 | |
---|
| 3134 | <span class="codefrag">}</span> |
---|
| 3135 | </td> |
---|
| 3136 | |
---|
| 3137 | </tr> |
---|
| 3138 | |
---|
| 3139 | <tr> |
---|
| 3140 | |
---|
| 3141 | <td colspan="1" rowspan="1">44.</td> |
---|
| 3142 | <td colspan="1" rowspan="1"></td> |
---|
| 3143 | |
---|
| 3144 | </tr> |
---|
| 3145 | |
---|
| 3146 | <tr> |
---|
| 3147 | |
---|
| 3148 | <td colspan="1" rowspan="1">45.</td> |
---|
| 3149 | <td colspan="1" rowspan="1"> |
---|
| 3150 | |
---|
| 3151 | <span class="codefrag">private void parseSkipFile(Path patternsFile) {</span> |
---|
| 3152 | </td> |
---|
| 3153 | |
---|
| 3154 | </tr> |
---|
| 3155 | |
---|
| 3156 | <tr> |
---|
| 3157 | |
---|
| 3158 | <td colspan="1" rowspan="1">46.</td> |
---|
| 3159 | <td colspan="1" rowspan="1"> |
---|
| 3160 | |
---|
| 3161 | <span class="codefrag">try {</span> |
---|
| 3162 | </td> |
---|
| 3163 | |
---|
| 3164 | </tr> |
---|
| 3165 | |
---|
| 3166 | <tr> |
---|
| 3167 | |
---|
| 3168 | <td colspan="1" rowspan="1">47.</td> |
---|
| 3169 | <td colspan="1" rowspan="1"> |
---|
| 3170 | |
---|
| 3171 | <span class="codefrag"> |
---|
| 3172 | BufferedReader fis = |
---|
| 3173 | new BufferedReader(new FileReader(patternsFile.toString())); |
---|
| 3174 | </span> |
---|
| 3175 | </td> |
---|
| 3176 | |
---|
| 3177 | </tr> |
---|
| 3178 | |
---|
| 3179 | <tr> |
---|
| 3180 | |
---|
| 3181 | <td colspan="1" rowspan="1">48.</td> |
---|
| 3182 | <td colspan="1" rowspan="1"> |
---|
| 3183 | |
---|
| 3184 | <span class="codefrag">String pattern = null;</span> |
---|
| 3185 | </td> |
---|
| 3186 | |
---|
| 3187 | </tr> |
---|
| 3188 | |
---|
| 3189 | <tr> |
---|
| 3190 | |
---|
| 3191 | <td colspan="1" rowspan="1">49.</td> |
---|
| 3192 | <td colspan="1" rowspan="1"> |
---|
| 3193 | |
---|
| 3194 | <span class="codefrag">while ((pattern = fis.readLine()) != null) {</span> |
---|
| 3195 | </td> |
---|
| 3196 | |
---|
| 3197 | </tr> |
---|
| 3198 | |
---|
| 3199 | <tr> |
---|
| 3200 | |
---|
| 3201 | <td colspan="1" rowspan="1">50.</td> |
---|
| 3202 | <td colspan="1" rowspan="1"> |
---|
| 3203 | |
---|
| 3204 | <span class="codefrag">patternsToSkip.add(pattern);</span> |
---|
| 3205 | </td> |
---|
| 3206 | |
---|
| 3207 | </tr> |
---|
| 3208 | |
---|
| 3209 | <tr> |
---|
| 3210 | |
---|
| 3211 | <td colspan="1" rowspan="1">51.</td> |
---|
| 3212 | <td colspan="1" rowspan="1"> |
---|
| 3213 | |
---|
| 3214 | <span class="codefrag">}</span> |
---|
| 3215 | </td> |
---|
| 3216 | |
---|
| 3217 | </tr> |
---|
| 3218 | |
---|
| 3219 | <tr> |
---|
| 3220 | |
---|
| 3221 | <td colspan="1" rowspan="1">52.</td> |
---|
| 3222 | <td colspan="1" rowspan="1"> |
---|
| 3223 | |
---|
| 3224 | <span class="codefrag">} catch (IOException ioe) {</span> |
---|
| 3225 | </td> |
---|
| 3226 | |
---|
| 3227 | </tr> |
---|
| 3228 | |
---|
| 3229 | <tr> |
---|
| 3230 | |
---|
| 3231 | <td colspan="1" rowspan="1">53.</td> |
---|
| 3232 | <td colspan="1" rowspan="1"> |
---|
| 3233 | |
---|
| 3234 | <span class="codefrag"> |
---|
| 3235 | System.err.println("Caught exception while parsing the cached file '" + |
---|
| 3236 | patternsFile + "' : " + |
---|
| 3237 | StringUtils.stringifyException(ioe)); |
---|
| 3238 | |
---|
| 3239 | </span> |
---|
| 3240 | </td> |
---|
| 3241 | |
---|
| 3242 | </tr> |
---|
| 3243 | |
---|
| 3244 | <tr> |
---|
| 3245 | |
---|
| 3246 | <td colspan="1" rowspan="1">54.</td> |
---|
| 3247 | <td colspan="1" rowspan="1"> |
---|
| 3248 | |
---|
| 3249 | <span class="codefrag">}</span> |
---|
| 3250 | </td> |
---|
| 3251 | |
---|
| 3252 | </tr> |
---|
| 3253 | |
---|
| 3254 | <tr> |
---|
| 3255 | |
---|
| 3256 | <td colspan="1" rowspan="1">55.</td> |
---|
| 3257 | <td colspan="1" rowspan="1"> |
---|
| 3258 | |
---|
| 3259 | <span class="codefrag">}</span> |
---|
| 3260 | </td> |
---|
| 3261 | |
---|
| 3262 | </tr> |
---|
| 3263 | |
---|
| 3264 | <tr> |
---|
| 3265 | |
---|
| 3266 | <td colspan="1" rowspan="1">56.</td> |
---|
| 3267 | <td colspan="1" rowspan="1"></td> |
---|
| 3268 | |
---|
| 3269 | </tr> |
---|
| 3270 | |
---|
| 3271 | <tr> |
---|
| 3272 | |
---|
| 3273 | <td colspan="1" rowspan="1">57.</td> |
---|
| 3274 | <td colspan="1" rowspan="1"> |
---|
| 3275 | |
---|
| 3276 | <span class="codefrag"> |
---|
| 3277 | public void map(LongWritable key, Text value, |
---|
| 3278 | OutputCollector<Text, IntWritable> output, |
---|
| 3279 | Reporter reporter) throws IOException { |
---|
| 3280 | </span> |
---|
| 3281 | </td> |
---|
| 3282 | |
---|
| 3283 | </tr> |
---|
| 3284 | |
---|
| 3285 | <tr> |
---|
| 3286 | |
---|
| 3287 | <td colspan="1" rowspan="1">58.</td> |
---|
| 3288 | <td colspan="1" rowspan="1"> |
---|
| 3289 | |
---|
| 3290 | <span class="codefrag"> |
---|
| 3291 | String line = |
---|
| 3292 | (caseSensitive) ? value.toString() : |
---|
| 3293 | value.toString().toLowerCase(); |
---|
| 3294 | </span> |
---|
| 3295 | </td> |
---|
| 3296 | |
---|
| 3297 | </tr> |
---|
| 3298 | |
---|
| 3299 | <tr> |
---|
| 3300 | |
---|
| 3301 | <td colspan="1" rowspan="1">59.</td> |
---|
| 3302 | <td colspan="1" rowspan="1"></td> |
---|
| 3303 | |
---|
| 3304 | </tr> |
---|
| 3305 | |
---|
| 3306 | <tr> |
---|
| 3307 | |
---|
| 3308 | <td colspan="1" rowspan="1">60.</td> |
---|
| 3309 | <td colspan="1" rowspan="1"> |
---|
| 3310 | |
---|
| 3311 | <span class="codefrag">for (String pattern : patternsToSkip) {</span> |
---|
| 3312 | </td> |
---|
| 3313 | |
---|
| 3314 | </tr> |
---|
| 3315 | |
---|
| 3316 | <tr> |
---|
| 3317 | |
---|
| 3318 | <td colspan="1" rowspan="1">61.</td> |
---|
| 3319 | <td colspan="1" rowspan="1"> |
---|
| 3320 | |
---|
| 3321 | <span class="codefrag">line = line.replaceAll(pattern, "");</span> |
---|
| 3322 | </td> |
---|
| 3323 | |
---|
| 3324 | </tr> |
---|
| 3325 | |
---|
| 3326 | <tr> |
---|
| 3327 | |
---|
| 3328 | <td colspan="1" rowspan="1">62.</td> |
---|
| 3329 | <td colspan="1" rowspan="1"> |
---|
| 3330 | |
---|
| 3331 | <span class="codefrag">}</span> |
---|
| 3332 | </td> |
---|
| 3333 | |
---|
| 3334 | </tr> |
---|
| 3335 | |
---|
| 3336 | <tr> |
---|
| 3337 | |
---|
| 3338 | <td colspan="1" rowspan="1">63.</td> |
---|
| 3339 | <td colspan="1" rowspan="1"></td> |
---|
| 3340 | |
---|
| 3341 | </tr> |
---|
| 3342 | |
---|
| 3343 | <tr> |
---|
| 3344 | |
---|
| 3345 | <td colspan="1" rowspan="1">64.</td> |
---|
| 3346 | <td colspan="1" rowspan="1"> |
---|
| 3347 | |
---|
| 3348 | <span class="codefrag">StringTokenizer tokenizer = new StringTokenizer(line);</span> |
---|
| 3349 | </td> |
---|
| 3350 | |
---|
| 3351 | </tr> |
---|
| 3352 | |
---|
| 3353 | <tr> |
---|
| 3354 | |
---|
| 3355 | <td colspan="1" rowspan="1">65.</td> |
---|
| 3356 | <td colspan="1" rowspan="1"> |
---|
| 3357 | |
---|
| 3358 | <span class="codefrag">while (tokenizer.hasMoreTokens()) {</span> |
---|
| 3359 | </td> |
---|
| 3360 | |
---|
| 3361 | </tr> |
---|
| 3362 | |
---|
| 3363 | <tr> |
---|
| 3364 | |
---|
| 3365 | <td colspan="1" rowspan="1">66.</td> |
---|
| 3366 | <td colspan="1" rowspan="1"> |
---|
| 3367 | |
---|
| 3368 | <span class="codefrag">word.set(tokenizer.nextToken());</span> |
---|
| 3369 | </td> |
---|
| 3370 | |
---|
| 3371 | </tr> |
---|
| 3372 | |
---|
| 3373 | <tr> |
---|
| 3374 | |
---|
| 3375 | <td colspan="1" rowspan="1">67.</td> |
---|
| 3376 | <td colspan="1" rowspan="1"> |
---|
| 3377 | |
---|
| 3378 | <span class="codefrag">output.collect(word, one);</span> |
---|
| 3379 | </td> |
---|
| 3380 | |
---|
| 3381 | </tr> |
---|
| 3382 | |
---|
| 3383 | <tr> |
---|
| 3384 | |
---|
| 3385 | <td colspan="1" rowspan="1">68.</td> |
---|
| 3386 | <td colspan="1" rowspan="1"> |
---|
| 3387 | |
---|
| 3388 | <span class="codefrag">reporter.incrCounter(Counters.INPUT_WORDS, 1);</span> |
---|
| 3389 | </td> |
---|
| 3390 | |
---|
| 3391 | </tr> |
---|
| 3392 | |
---|
| 3393 | <tr> |
---|
| 3394 | |
---|
| 3395 | <td colspan="1" rowspan="1">69.</td> |
---|
| 3396 | <td colspan="1" rowspan="1"> |
---|
| 3397 | |
---|
| 3398 | <span class="codefrag">}</span> |
---|
| 3399 | </td> |
---|
| 3400 | |
---|
| 3401 | </tr> |
---|
| 3402 | |
---|
| 3403 | <tr> |
---|
| 3404 | |
---|
| 3405 | <td colspan="1" rowspan="1">70.</td> |
---|
| 3406 | <td colspan="1" rowspan="1"></td> |
---|
| 3407 | |
---|
| 3408 | </tr> |
---|
| 3409 | |
---|
| 3410 | <tr> |
---|
| 3411 | |
---|
| 3412 | <td colspan="1" rowspan="1">71.</td> |
---|
| 3413 | <td colspan="1" rowspan="1"> |
---|
| 3414 | |
---|
| 3415 | <span class="codefrag">if ((++numRecords % 100) == 0) {</span> |
---|
| 3416 | </td> |
---|
| 3417 | |
---|
| 3418 | </tr> |
---|
| 3419 | |
---|
| 3420 | <tr> |
---|
| 3421 | |
---|
| 3422 | <td colspan="1" rowspan="1">72.</td> |
---|
| 3423 | <td colspan="1" rowspan="1"> |
---|
| 3424 | |
---|
| 3425 | <span class="codefrag"> |
---|
| 3426 | reporter.setStatus("Finished processing " + numRecords + |
---|
| 3427 | " records " + "from the input file: " + |
---|
| 3428 | inputFile); |
---|
| 3429 | </span> |
---|
| 3430 | </td> |
---|
| 3431 | |
---|
| 3432 | </tr> |
---|
| 3433 | |
---|
| 3434 | <tr> |
---|
| 3435 | |
---|
| 3436 | <td colspan="1" rowspan="1">73.</td> |
---|
| 3437 | <td colspan="1" rowspan="1"> |
---|
| 3438 | |
---|
| 3439 | <span class="codefrag">}</span> |
---|
| 3440 | </td> |
---|
| 3441 | |
---|
| 3442 | </tr> |
---|
| 3443 | |
---|
| 3444 | <tr> |
---|
| 3445 | |
---|
| 3446 | <td colspan="1" rowspan="1">74.</td> |
---|
| 3447 | <td colspan="1" rowspan="1"> |
---|
| 3448 | |
---|
| 3449 | <span class="codefrag">}</span> |
---|
| 3450 | </td> |
---|
| 3451 | |
---|
| 3452 | </tr> |
---|
| 3453 | |
---|
| 3454 | <tr> |
---|
| 3455 | |
---|
| 3456 | <td colspan="1" rowspan="1">75.</td> |
---|
| 3457 | <td colspan="1" rowspan="1"> |
---|
| 3458 | |
---|
| 3459 | <span class="codefrag">}</span> |
---|
| 3460 | </td> |
---|
| 3461 | |
---|
| 3462 | </tr> |
---|
| 3463 | |
---|
| 3464 | <tr> |
---|
| 3465 | |
---|
| 3466 | <td colspan="1" rowspan="1">76.</td> |
---|
| 3467 | <td colspan="1" rowspan="1"></td> |
---|
| 3468 | |
---|
| 3469 | </tr> |
---|
| 3470 | |
---|
| 3471 | <tr> |
---|
| 3472 | |
---|
| 3473 | <td colspan="1" rowspan="1">77.</td> |
---|
| 3474 | <td colspan="1" rowspan="1"> |
---|
| 3475 | |
---|
| 3476 | <span class="codefrag"> |
---|
| 3477 | public static class Reduce extends MapReduceBase implements |
---|
| 3478 | Reducer<Text, IntWritable, Text, IntWritable> { |
---|
| 3479 | </span> |
---|
| 3480 | </td> |
---|
| 3481 | |
---|
| 3482 | </tr> |
---|
| 3483 | |
---|
| 3484 | <tr> |
---|
| 3485 | |
---|
| 3486 | <td colspan="1" rowspan="1">78.</td> |
---|
| 3487 | <td colspan="1" rowspan="1"> |
---|
| 3488 | |
---|
| 3489 | <span class="codefrag"> |
---|
| 3490 | public void reduce(Text key, Iterator<IntWritable> values, |
---|
| 3491 | OutputCollector<Text, IntWritable> output, |
---|
| 3492 | Reporter reporter) throws IOException { |
---|
| 3493 | </span> |
---|
| 3494 | </td> |
---|
| 3495 | |
---|
| 3496 | </tr> |
---|
| 3497 | |
---|
| 3498 | <tr> |
---|
| 3499 | |
---|
| 3500 | <td colspan="1" rowspan="1">79.</td> |
---|
| 3501 | <td colspan="1" rowspan="1"> |
---|
| 3502 | |
---|
| 3503 | <span class="codefrag">int sum = 0;</span> |
---|
| 3504 | </td> |
---|
| 3505 | |
---|
| 3506 | </tr> |
---|
| 3507 | |
---|
| 3508 | <tr> |
---|
| 3509 | |
---|
| 3510 | <td colspan="1" rowspan="1">80.</td> |
---|
| 3511 | <td colspan="1" rowspan="1"> |
---|
| 3512 | |
---|
| 3513 | <span class="codefrag">while (values.hasNext()) {</span> |
---|
| 3514 | </td> |
---|
| 3515 | |
---|
| 3516 | </tr> |
---|
| 3517 | |
---|
| 3518 | <tr> |
---|
| 3519 | |
---|
| 3520 | <td colspan="1" rowspan="1">81.</td> |
---|
| 3521 | <td colspan="1" rowspan="1"> |
---|
| 3522 | |
---|
| 3523 | <span class="codefrag">sum += values.next().get();</span> |
---|
| 3524 | </td> |
---|
| 3525 | |
---|
| 3526 | </tr> |
---|
| 3527 | |
---|
| 3528 | <tr> |
---|
| 3529 | |
---|
| 3530 | <td colspan="1" rowspan="1">82.</td> |
---|
| 3531 | <td colspan="1" rowspan="1"> |
---|
| 3532 | |
---|
| 3533 | <span class="codefrag">}</span> |
---|
| 3534 | </td> |
---|
| 3535 | |
---|
| 3536 | </tr> |
---|
| 3537 | |
---|
| 3538 | <tr> |
---|
| 3539 | |
---|
| 3540 | <td colspan="1" rowspan="1">83.</td> |
---|
| 3541 | <td colspan="1" rowspan="1"> |
---|
| 3542 | |
---|
| 3543 | <span class="codefrag">output.collect(key, new IntWritable(sum));</span> |
---|
| 3544 | </td> |
---|
| 3545 | |
---|
| 3546 | </tr> |
---|
| 3547 | |
---|
| 3548 | <tr> |
---|
| 3549 | |
---|
| 3550 | <td colspan="1" rowspan="1">84.</td> |
---|
| 3551 | <td colspan="1" rowspan="1"> |
---|
| 3552 | |
---|
| 3553 | <span class="codefrag">}</span> |
---|
| 3554 | </td> |
---|
| 3555 | |
---|
| 3556 | </tr> |
---|
| 3557 | |
---|
| 3558 | <tr> |
---|
| 3559 | |
---|
| 3560 | <td colspan="1" rowspan="1">85.</td> |
---|
| 3561 | <td colspan="1" rowspan="1"> |
---|
| 3562 | |
---|
| 3563 | <span class="codefrag">}</span> |
---|
| 3564 | </td> |
---|
| 3565 | |
---|
| 3566 | </tr> |
---|
| 3567 | |
---|
| 3568 | <tr> |
---|
| 3569 | |
---|
| 3570 | <td colspan="1" rowspan="1">86.</td> |
---|
| 3571 | <td colspan="1" rowspan="1"></td> |
---|
| 3572 | |
---|
| 3573 | </tr> |
---|
| 3574 | |
---|
| 3575 | <tr> |
---|
| 3576 | |
---|
| 3577 | <td colspan="1" rowspan="1">87.</td> |
---|
| 3578 | <td colspan="1" rowspan="1"> |
---|
| 3579 | |
---|
| 3580 | <span class="codefrag">public int run(String[] args) throws Exception {</span> |
---|
| 3581 | </td> |
---|
| 3582 | |
---|
| 3583 | </tr> |
---|
| 3584 | |
---|
| 3585 | <tr> |
---|
| 3586 | |
---|
| 3587 | <td colspan="1" rowspan="1">88.</td> |
---|
| 3588 | <td colspan="1" rowspan="1"> |
---|
| 3589 | |
---|
| 3590 | <span class="codefrag"> |
---|
| 3591 | JobConf conf = new JobConf(getConf(), WordCount.class); |
---|
| 3592 | </span> |
---|
| 3593 | </td> |
---|
| 3594 | |
---|
| 3595 | </tr> |
---|
| 3596 | |
---|
| 3597 | <tr> |
---|
| 3598 | |
---|
| 3599 | <td colspan="1" rowspan="1">89.</td> |
---|
| 3600 | <td colspan="1" rowspan="1"> |
---|
| 3601 | |
---|
| 3602 | <span class="codefrag">conf.setJobName("wordcount");</span> |
---|
| 3603 | </td> |
---|
| 3604 | |
---|
| 3605 | </tr> |
---|
| 3606 | |
---|
| 3607 | <tr> |
---|
| 3608 | |
---|
| 3609 | <td colspan="1" rowspan="1">90.</td> |
---|
| 3610 | <td colspan="1" rowspan="1"></td> |
---|
| 3611 | |
---|
| 3612 | </tr> |
---|
| 3613 | |
---|
| 3614 | <tr> |
---|
| 3615 | |
---|
| 3616 | <td colspan="1" rowspan="1">91.</td> |
---|
| 3617 | <td colspan="1" rowspan="1"> |
---|
| 3618 | |
---|
| 3619 | <span class="codefrag">conf.setOutputKeyClass(Text.class);</span> |
---|
| 3620 | </td> |
---|
| 3621 | |
---|
| 3622 | </tr> |
---|
| 3623 | |
---|
| 3624 | <tr> |
---|
| 3625 | |
---|
| 3626 | <td colspan="1" rowspan="1">92.</td> |
---|
| 3627 | <td colspan="1" rowspan="1"> |
---|
| 3628 | |
---|
| 3629 | <span class="codefrag">conf.setOutputValueClass(IntWritable.class);</span> |
---|
| 3630 | </td> |
---|
| 3631 | |
---|
| 3632 | </tr> |
---|
| 3633 | |
---|
| 3634 | <tr> |
---|
| 3635 | |
---|
| 3636 | <td colspan="1" rowspan="1">93.</td> |
---|
| 3637 | <td colspan="1" rowspan="1"></td> |
---|
| 3638 | |
---|
| 3639 | </tr> |
---|
| 3640 | |
---|
| 3641 | <tr> |
---|
| 3642 | |
---|
| 3643 | <td colspan="1" rowspan="1">94.</td> |
---|
| 3644 | <td colspan="1" rowspan="1"> |
---|
| 3645 | |
---|
| 3646 | <span class="codefrag">conf.setMapperClass(Map.class);</span> |
---|
| 3647 | </td> |
---|
| 3648 | |
---|
| 3649 | </tr> |
---|
| 3650 | |
---|
| 3651 | <tr> |
---|
| 3652 | |
---|
| 3653 | <td colspan="1" rowspan="1">95.</td> |
---|
| 3654 | <td colspan="1" rowspan="1"> |
---|
| 3655 | |
---|
| 3656 | <span class="codefrag">conf.setCombinerClass(Reduce.class);</span> |
---|
| 3657 | </td> |
---|
| 3658 | |
---|
| 3659 | </tr> |
---|
| 3660 | |
---|
| 3661 | <tr> |
---|
| 3662 | |
---|
| 3663 | <td colspan="1" rowspan="1">96.</td> |
---|
| 3664 | <td colspan="1" rowspan="1"> |
---|
| 3665 | |
---|
| 3666 | <span class="codefrag">conf.setReducerClass(Reduce.class);</span> |
---|
| 3667 | </td> |
---|
| 3668 | |
---|
| 3669 | </tr> |
---|
| 3670 | |
---|
| 3671 | <tr> |
---|
| 3672 | |
---|
| 3673 | <td colspan="1" rowspan="1">97.</td> |
---|
| 3674 | <td colspan="1" rowspan="1"></td> |
---|
| 3675 | |
---|
| 3676 | </tr> |
---|
| 3677 | |
---|
| 3678 | <tr> |
---|
| 3679 | |
---|
| 3680 | <td colspan="1" rowspan="1">98.</td> |
---|
| 3681 | <td colspan="1" rowspan="1"> |
---|
| 3682 | |
---|
| 3683 | <span class="codefrag">conf.setInputFormat(TextInputFormat.class);</span> |
---|
| 3684 | </td> |
---|
| 3685 | |
---|
| 3686 | </tr> |
---|
| 3687 | |
---|
| 3688 | <tr> |
---|
| 3689 | |
---|
| 3690 | <td colspan="1" rowspan="1">99.</td> |
---|
| 3691 | <td colspan="1" rowspan="1"> |
---|
| 3692 | |
---|
| 3693 | <span class="codefrag">conf.setOutputFormat(TextOutputFormat.class);</span> |
---|
| 3694 | </td> |
---|
| 3695 | |
---|
| 3696 | </tr> |
---|
| 3697 | |
---|
| 3698 | <tr> |
---|
| 3699 | |
---|
| 3700 | <td colspan="1" rowspan="1">100.</td> |
---|
| 3701 | <td colspan="1" rowspan="1"></td> |
---|
| 3702 | |
---|
| 3703 | </tr> |
---|
| 3704 | |
---|
| 3705 | <tr> |
---|
| 3706 | |
---|
| 3707 | <td colspan="1" rowspan="1">101.</td> |
---|
| 3708 | <td colspan="1" rowspan="1"> |
---|
| 3709 | |
---|
| 3710 | <span class="codefrag"> |
---|
| 3711 | List<String> other_args = new ArrayList<String>(); |
---|
| 3712 | </span> |
---|
| 3713 | </td> |
---|
| 3714 | |
---|
| 3715 | </tr> |
---|
| 3716 | |
---|
| 3717 | <tr> |
---|
| 3718 | |
---|
| 3719 | <td colspan="1" rowspan="1">102.</td> |
---|
| 3720 | <td colspan="1" rowspan="1"> |
---|
| 3721 | |
---|
| 3722 | <span class="codefrag">for (int i=0; i < args.length; ++i) {</span> |
---|
| 3723 | </td> |
---|
| 3724 | |
---|
| 3725 | </tr> |
---|
| 3726 | |
---|
| 3727 | <tr> |
---|
| 3728 | |
---|
| 3729 | <td colspan="1" rowspan="1">103.</td> |
---|
| 3730 | <td colspan="1" rowspan="1"> |
---|
| 3731 | |
---|
| 3732 | <span class="codefrag">if ("-skip".equals(args[i])) {</span> |
---|
| 3733 | </td> |
---|
| 3734 | |
---|
| 3735 | </tr> |
---|
| 3736 | |
---|
| 3737 | <tr> |
---|
| 3738 | |
---|
| 3739 | <td colspan="1" rowspan="1">104.</td> |
---|
| 3740 | <td colspan="1" rowspan="1"> |
---|
| 3741 | |
---|
| 3742 | <span class="codefrag"> |
---|
| 3743 | DistributedCache.addCacheFile(new Path(args[++i]).toUri(), conf); |
---|
| 3744 | </span> |
---|
| 3745 | </td> |
---|
| 3746 | |
---|
| 3747 | </tr> |
---|
| 3748 | |
---|
| 3749 | <tr> |
---|
| 3750 | |
---|
| 3751 | <td colspan="1" rowspan="1">105.</td> |
---|
| 3752 | <td colspan="1" rowspan="1"> |
---|
| 3753 | |
---|
| 3754 | <span class="codefrag"> |
---|
| 3755 | conf.setBoolean("wordcount.skip.patterns", true); |
---|
| 3756 | </span> |
---|
| 3757 | </td> |
---|
| 3758 | |
---|
| 3759 | </tr> |
---|
| 3760 | |
---|
| 3761 | <tr> |
---|
| 3762 | |
---|
| 3763 | <td colspan="1" rowspan="1">106.</td> |
---|
| 3764 | <td colspan="1" rowspan="1"> |
---|
| 3765 | |
---|
| 3766 | <span class="codefrag">} else {</span> |
---|
| 3767 | </td> |
---|
| 3768 | |
---|
| 3769 | </tr> |
---|
| 3770 | |
---|
| 3771 | <tr> |
---|
| 3772 | |
---|
| 3773 | <td colspan="1" rowspan="1">107.</td> |
---|
| 3774 | <td colspan="1" rowspan="1"> |
---|
| 3775 | |
---|
| 3776 | <span class="codefrag">other_args.add(args[i]);</span> |
---|
| 3777 | </td> |
---|
| 3778 | |
---|
| 3779 | </tr> |
---|
| 3780 | |
---|
| 3781 | <tr> |
---|
| 3782 | |
---|
| 3783 | <td colspan="1" rowspan="1">108.</td> |
---|
| 3784 | <td colspan="1" rowspan="1"> |
---|
| 3785 | |
---|
| 3786 | <span class="codefrag">}</span> |
---|
| 3787 | </td> |
---|
| 3788 | |
---|
| 3789 | </tr> |
---|
| 3790 | |
---|
| 3791 | <tr> |
---|
| 3792 | |
---|
| 3793 | <td colspan="1" rowspan="1">109.</td> |
---|
| 3794 | <td colspan="1" rowspan="1"> |
---|
| 3795 | |
---|
| 3796 | <span class="codefrag">}</span> |
---|
| 3797 | </td> |
---|
| 3798 | |
---|
| 3799 | </tr> |
---|
| 3800 | |
---|
| 3801 | <tr> |
---|
| 3802 | |
---|
| 3803 | <td colspan="1" rowspan="1">110.</td> |
---|
| 3804 | <td colspan="1" rowspan="1"></td> |
---|
| 3805 | |
---|
| 3806 | </tr> |
---|
| 3807 | |
---|
| 3808 | <tr> |
---|
| 3809 | |
---|
| 3810 | <td colspan="1" rowspan="1">111.</td> |
---|
| 3811 | <td colspan="1" rowspan="1"> |
---|
| 3812 | |
---|
| 3813 | <span class="codefrag">FileInputFormat.setInputPaths(conf, new Path(other_args.get(0)));</span> |
---|
| 3814 | </td> |
---|
| 3815 | |
---|
| 3816 | </tr> |
---|
| 3817 | |
---|
| 3818 | <tr> |
---|
| 3819 | |
---|
| 3820 | <td colspan="1" rowspan="1">112.</td> |
---|
| 3821 | <td colspan="1" rowspan="1"> |
---|
| 3822 | |
---|
| 3823 | <span class="codefrag">FileOutputFormat.setOutputPath(conf, new Path(other_args.get(1)));</span> |
---|
| 3824 | </td> |
---|
| 3825 | |
---|
| 3826 | </tr> |
---|
| 3827 | |
---|
| 3828 | <tr> |
---|
| 3829 | |
---|
| 3830 | <td colspan="1" rowspan="1">113.</td> |
---|
| 3831 | <td colspan="1" rowspan="1"></td> |
---|
| 3832 | |
---|
| 3833 | </tr> |
---|
| 3834 | |
---|
| 3835 | <tr> |
---|
| 3836 | |
---|
| 3837 | <td colspan="1" rowspan="1">114.</td> |
---|
| 3838 | <td colspan="1" rowspan="1"> |
---|
| 3839 | |
---|
| 3840 | <span class="codefrag">JobClient.runJob(conf);</span> |
---|
| 3841 | </td> |
---|
| 3842 | |
---|
| 3843 | </tr> |
---|
| 3844 | |
---|
| 3845 | <tr> |
---|
| 3846 | |
---|
| 3847 | <td colspan="1" rowspan="1">115.</td> |
---|
| 3848 | <td colspan="1" rowspan="1"> |
---|
| 3849 | |
---|
| 3850 | <span class="codefrag">return 0;</span> |
---|
| 3851 | </td> |
---|
| 3852 | |
---|
| 3853 | </tr> |
---|
| 3854 | |
---|
| 3855 | <tr> |
---|
| 3856 | |
---|
| 3857 | <td colspan="1" rowspan="1">116.</td> |
---|
| 3858 | <td colspan="1" rowspan="1"> |
---|
| 3859 | |
---|
| 3860 | <span class="codefrag">}</span> |
---|
| 3861 | </td> |
---|
| 3862 | |
---|
| 3863 | </tr> |
---|
| 3864 | |
---|
| 3865 | <tr> |
---|
| 3866 | |
---|
| 3867 | <td colspan="1" rowspan="1">117.</td> |
---|
| 3868 | <td colspan="1" rowspan="1"></td> |
---|
| 3869 | |
---|
| 3870 | </tr> |
---|
| 3871 | |
---|
| 3872 | <tr> |
---|
| 3873 | |
---|
| 3874 | <td colspan="1" rowspan="1">118.</td> |
---|
| 3875 | <td colspan="1" rowspan="1"> |
---|
| 3876 | |
---|
| 3877 | <span class="codefrag"> |
---|
| 3878 | public static void main(String[] args) throws Exception { |
---|
| 3879 | </span> |
---|
| 3880 | </td> |
---|
| 3881 | |
---|
| 3882 | </tr> |
---|
| 3883 | |
---|
| 3884 | <tr> |
---|
| 3885 | |
---|
| 3886 | <td colspan="1" rowspan="1">119.</td> |
---|
| 3887 | <td colspan="1" rowspan="1"> |
---|
| 3888 | |
---|
| 3889 | <span class="codefrag"> |
---|
| 3890 | int res = ToolRunner.run(new Configuration(), new WordCount(), |
---|
| 3891 | args); |
---|
| 3892 | </span> |
---|
| 3893 | </td> |
---|
| 3894 | |
---|
| 3895 | </tr> |
---|
| 3896 | |
---|
| 3897 | <tr> |
---|
| 3898 | |
---|
| 3899 | <td colspan="1" rowspan="1">120.</td> |
---|
| 3900 | <td colspan="1" rowspan="1"> |
---|
| 3901 | |
---|
| 3902 | <span class="codefrag">System.exit(res);</span> |
---|
| 3903 | </td> |
---|
| 3904 | |
---|
| 3905 | </tr> |
---|
| 3906 | |
---|
| 3907 | <tr> |
---|
| 3908 | |
---|
| 3909 | <td colspan="1" rowspan="1">121.</td> |
---|
| 3910 | <td colspan="1" rowspan="1"> |
---|
| 3911 | |
---|
| 3912 | <span class="codefrag">}</span> |
---|
| 3913 | </td> |
---|
| 3914 | |
---|
| 3915 | </tr> |
---|
| 3916 | |
---|
| 3917 | <tr> |
---|
| 3918 | |
---|
| 3919 | <td colspan="1" rowspan="1">122.</td> |
---|
| 3920 | <td colspan="1" rowspan="1"> |
---|
| 3921 | <span class="codefrag">}</span> |
---|
| 3922 | </td> |
---|
| 3923 | |
---|
| 3924 | </tr> |
---|
| 3925 | |
---|
| 3926 | <tr> |
---|
| 3927 | |
---|
| 3928 | <td colspan="1" rowspan="1">123.</td> |
---|
| 3929 | <td colspan="1" rowspan="1"></td> |
---|
| 3930 | |
---|
| 3931 | </tr> |
---|
| 3932 | |
---|
| 3933 | </table> |
---|
| 3934 | <a name="N11732"></a><a name="Sample+Runs"></a> |
---|
| 3935 | <h3 class="h4">Sample Runs</h3> |
---|
| 3936 | <p>Sample text-files as input:</p> |
---|
| 3937 | <p> |
---|
| 3938 | |
---|
| 3939 | <span class="codefrag">$ bin/hadoop dfs -ls /usr/joe/wordcount/input/</span> |
---|
| 3940 | <br> |
---|
| 3941 | |
---|
| 3942 | <span class="codefrag">/usr/joe/wordcount/input/file01</span> |
---|
| 3943 | <br> |
---|
| 3944 | |
---|
| 3945 | <span class="codefrag">/usr/joe/wordcount/input/file02</span> |
---|
| 3946 | <br> |
---|
| 3947 | |
---|
| 3948 | <br> |
---|
| 3949 | |
---|
| 3950 | <span class="codefrag">$ bin/hadoop dfs -cat /usr/joe/wordcount/input/file01</span> |
---|
| 3951 | <br> |
---|
| 3952 | |
---|
| 3953 | <span class="codefrag">Hello World, Bye World!</span> |
---|
| 3954 | <br> |
---|
| 3955 | |
---|
| 3956 | <br> |
---|
| 3957 | |
---|
| 3958 | <span class="codefrag">$ bin/hadoop dfs -cat /usr/joe/wordcount/input/file02</span> |
---|
| 3959 | <br> |
---|
| 3960 | |
---|
| 3961 | <span class="codefrag">Hello Hadoop, Goodbye to hadoop.</span> |
---|
| 3962 | |
---|
| 3963 | </p> |
---|
| 3964 | <p>Run the application:</p> |
---|
| 3965 | <p> |
---|
| 3966 | |
---|
| 3967 | <span class="codefrag"> |
---|
| 3968 | $ bin/hadoop jar /usr/joe/wordcount.jar org.myorg.WordCount |
---|
| 3969 | /usr/joe/wordcount/input /usr/joe/wordcount/output |
---|
| 3970 | </span> |
---|
| 3971 | |
---|
| 3972 | </p> |
---|
| 3973 | <p>Output:</p> |
---|
| 3974 | <p> |
---|
| 3975 | |
---|
| 3976 | <span class="codefrag"> |
---|
| 3977 | $ bin/hadoop dfs -cat /usr/joe/wordcount/output/part-00000 |
---|
| 3978 | </span> |
---|
| 3979 | |
---|
| 3980 | <br> |
---|
| 3981 | |
---|
| 3982 | <span class="codefrag">Bye 1</span> |
---|
| 3983 | <br> |
---|
| 3984 | |
---|
| 3985 | <span class="codefrag">Goodbye 1</span> |
---|
| 3986 | <br> |
---|
| 3987 | |
---|
| 3988 | <span class="codefrag">Hadoop, 1</span> |
---|
| 3989 | <br> |
---|
| 3990 | |
---|
| 3991 | <span class="codefrag">Hello 2</span> |
---|
| 3992 | <br> |
---|
| 3993 | |
---|
| 3994 | <span class="codefrag">World! 1</span> |
---|
| 3995 | <br> |
---|
| 3996 | |
---|
| 3997 | <span class="codefrag">World, 1</span> |
---|
| 3998 | <br> |
---|
| 3999 | |
---|
| 4000 | <span class="codefrag">hadoop. 1</span> |
---|
| 4001 | <br> |
---|
| 4002 | |
---|
| 4003 | <span class="codefrag">to 1</span> |
---|
| 4004 | <br> |
---|
| 4005 | |
---|
| 4006 | </p> |
---|
| 4007 | <p>Notice that the inputs differ from the first version we looked at, |
---|
| 4008 | and how they affect the outputs.</p> |
---|
| 4009 | <p>Now, lets plug-in a pattern-file which lists the word-patterns to be |
---|
| 4010 | ignored, via the <span class="codefrag">DistributedCache</span>.</p> |
---|
| 4011 | <p> |
---|
| 4012 | |
---|
| 4013 | <span class="codefrag">$ hadoop dfs -cat /user/joe/wordcount/patterns.txt</span> |
---|
| 4014 | <br> |
---|
| 4015 | |
---|
| 4016 | <span class="codefrag">\.</span> |
---|
| 4017 | <br> |
---|
| 4018 | |
---|
| 4019 | <span class="codefrag">\,</span> |
---|
| 4020 | <br> |
---|
| 4021 | |
---|
| 4022 | <span class="codefrag">\!</span> |
---|
| 4023 | <br> |
---|
| 4024 | |
---|
| 4025 | <span class="codefrag">to</span> |
---|
| 4026 | <br> |
---|
| 4027 | |
---|
| 4028 | </p> |
---|
| 4029 | <p>Run it again, this time with more options:</p> |
---|
| 4030 | <p> |
---|
| 4031 | |
---|
| 4032 | <span class="codefrag"> |
---|
| 4033 | $ bin/hadoop jar /usr/joe/wordcount.jar org.myorg.WordCount |
---|
| 4034 | -Dwordcount.case.sensitive=true /usr/joe/wordcount/input |
---|
| 4035 | /usr/joe/wordcount/output -skip /user/joe/wordcount/patterns.txt |
---|
| 4036 | </span> |
---|
| 4037 | |
---|
| 4038 | </p> |
---|
| 4039 | <p>As expected, the output:</p> |
---|
| 4040 | <p> |
---|
| 4041 | |
---|
| 4042 | <span class="codefrag"> |
---|
| 4043 | $ bin/hadoop dfs -cat /usr/joe/wordcount/output/part-00000 |
---|
| 4044 | </span> |
---|
| 4045 | |
---|
| 4046 | <br> |
---|
| 4047 | |
---|
| 4048 | <span class="codefrag">Bye 1</span> |
---|
| 4049 | <br> |
---|
| 4050 | |
---|
| 4051 | <span class="codefrag">Goodbye 1</span> |
---|
| 4052 | <br> |
---|
| 4053 | |
---|
| 4054 | <span class="codefrag">Hadoop 1</span> |
---|
| 4055 | <br> |
---|
| 4056 | |
---|
| 4057 | <span class="codefrag">Hello 2</span> |
---|
| 4058 | <br> |
---|
| 4059 | |
---|
| 4060 | <span class="codefrag">World 2</span> |
---|
| 4061 | <br> |
---|
| 4062 | |
---|
| 4063 | <span class="codefrag">hadoop 1</span> |
---|
| 4064 | <br> |
---|
| 4065 | |
---|
| 4066 | </p> |
---|
| 4067 | <p>Run it once more, this time switch-off case-sensitivity:</p> |
---|
| 4068 | <p> |
---|
| 4069 | |
---|
| 4070 | <span class="codefrag"> |
---|
| 4071 | $ bin/hadoop jar /usr/joe/wordcount.jar org.myorg.WordCount |
---|
| 4072 | -Dwordcount.case.sensitive=false /usr/joe/wordcount/input |
---|
| 4073 | /usr/joe/wordcount/output -skip /user/joe/wordcount/patterns.txt |
---|
| 4074 | </span> |
---|
| 4075 | |
---|
| 4076 | </p> |
---|
| 4077 | <p>Sure enough, the output:</p> |
---|
| 4078 | <p> |
---|
| 4079 | |
---|
| 4080 | <span class="codefrag"> |
---|
| 4081 | $ bin/hadoop dfs -cat /usr/joe/wordcount/output/part-00000 |
---|
| 4082 | </span> |
---|
| 4083 | |
---|
| 4084 | <br> |
---|
| 4085 | |
---|
| 4086 | <span class="codefrag">bye 1</span> |
---|
| 4087 | <br> |
---|
| 4088 | |
---|
| 4089 | <span class="codefrag">goodbye 1</span> |
---|
| 4090 | <br> |
---|
| 4091 | |
---|
| 4092 | <span class="codefrag">hadoop 2</span> |
---|
| 4093 | <br> |
---|
| 4094 | |
---|
| 4095 | <span class="codefrag">hello 2</span> |
---|
| 4096 | <br> |
---|
| 4097 | |
---|
| 4098 | <span class="codefrag">world 2</span> |
---|
| 4099 | <br> |
---|
| 4100 | |
---|
| 4101 | </p> |
---|
| 4102 | <a name="N11806"></a><a name="Highlights"></a> |
---|
| 4103 | <h3 class="h4">Highlights</h3> |
---|
| 4104 | <p>The second version of <span class="codefrag">WordCount</span> improves upon the |
---|
| 4105 | previous one by using some features offered by the Map/Reduce framework: |
---|
| 4106 | </p> |
---|
| 4107 | <ul> |
---|
| 4108 | |
---|
| 4109 | <li> |
---|
| 4110 | Demonstrates how applications can access configuration parameters |
---|
| 4111 | in the <span class="codefrag">configure</span> method of the <span class="codefrag">Mapper</span> (and |
---|
| 4112 | <span class="codefrag">Reducer</span>) implementations (lines 28-43). |
---|
| 4113 | </li> |
---|
| 4114 | |
---|
| 4115 | <li> |
---|
| 4116 | Demonstrates how the <span class="codefrag">DistributedCache</span> can be used to |
---|
| 4117 | distribute read-only data needed by the jobs. Here it allows the user |
---|
| 4118 | to specify word-patterns to skip while counting (line 104). |
---|
| 4119 | </li> |
---|
| 4120 | |
---|
| 4121 | <li> |
---|
| 4122 | Demonstrates the utility of the <span class="codefrag">Tool</span> interface and the |
---|
| 4123 | <span class="codefrag">GenericOptionsParser</span> to handle generic Hadoop |
---|
| 4124 | command-line options (lines 87-116, 119). |
---|
| 4125 | </li> |
---|
| 4126 | |
---|
| 4127 | <li> |
---|
| 4128 | Demonstrates how applications can use <span class="codefrag">Counters</span> (line 68) |
---|
| 4129 | and how they can set application-specific status information via |
---|
| 4130 | the <span class="codefrag">Reporter</span> instance passed to the <span class="codefrag">map</span> (and |
---|
| 4131 | <span class="codefrag">reduce</span>) method (line 72). |
---|
| 4132 | </li> |
---|
| 4133 | |
---|
| 4134 | </ul> |
---|
| 4135 | </div> |
---|
| 4136 | |
---|
| 4137 | |
---|
| 4138 | <p> |
---|
| 4139 | |
---|
| 4140 | <em>Java and JNI are trademarks or registered trademarks of |
---|
| 4141 | Sun Microsystems, Inc. in the United States and other countries.</em> |
---|
| 4142 | |
---|
| 4143 | </p> |
---|
| 4144 | |
---|
| 4145 | |
---|
| 4146 | </div> |
---|
| 4147 | <!--+ |
---|
| 4148 | |end content |
---|
| 4149 | +--> |
---|
| 4150 | <div class="clearboth"> </div> |
---|
| 4151 | </div> |
---|
| 4152 | <div id="footer"> |
---|
| 4153 | <!--+ |
---|
| 4154 | |start bottomstrip |
---|
| 4155 | +--> |
---|
| 4156 | <div class="lastmodified"> |
---|
| 4157 | <script type="text/javascript"><!-- |
---|
| 4158 | document.write("Last Published: " + document.lastModified); |
---|
| 4159 | // --></script> |
---|
| 4160 | </div> |
---|
| 4161 | <div class="copyright"> |
---|
| 4162 | Copyright © |
---|
| 4163 | 2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a> |
---|
| 4164 | </div> |
---|
| 4165 | <!--+ |
---|
| 4166 | |end bottomstrip |
---|
| 4167 | +--> |
---|
| 4168 | </div> |
---|
| 4169 | </body> |
---|
| 4170 | </html> |
---|