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| 80 | |
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| 82 | <H2> |
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| 83 | Package org.apache.hadoop.metrics |
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| 84 | </H2> |
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| 85 | This package defines an API for reporting performance metric information. |
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| 86 | <P> |
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| 87 | <B>See:</B> |
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| 88 | <BR> |
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| 89 | <A HREF="#package_description"><B>Description</B></A> |
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| 90 | <P> |
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| 91 | |
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| 93 | <TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"> |
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| 94 | <TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2"> |
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| 95 | <B>Interface Summary</B></FONT></TH> |
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| 96 | </TR> |
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| 97 | <TR BGCOLOR="white" CLASS="TableRowColor"> |
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| 98 | <TD WIDTH="15%"><B><A HREF="../../../../org/apache/hadoop/metrics/MetricsContext.html" title="interface in org.apache.hadoop.metrics">MetricsContext</A></B></TD> |
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| 99 | <TD>The main interface to the metrics package.</TD> |
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| 100 | </TR> |
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| 101 | <TR BGCOLOR="white" CLASS="TableRowColor"> |
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| 102 | <TD WIDTH="15%"><B><A HREF="../../../../org/apache/hadoop/metrics/MetricsRecord.html" title="interface in org.apache.hadoop.metrics">MetricsRecord</A></B></TD> |
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| 103 | <TD>A named and optionally tagged set of records to be sent to the metrics |
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| 104 | system.</TD> |
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| 105 | </TR> |
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| 106 | <TR BGCOLOR="white" CLASS="TableRowColor"> |
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| 107 | <TD WIDTH="15%"><B><A HREF="../../../../org/apache/hadoop/metrics/Updater.html" title="interface in org.apache.hadoop.metrics">Updater</A></B></TD> |
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| 108 | <TD>Call-back interface.</TD> |
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| 109 | </TR> |
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| 110 | </TABLE> |
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| 111 | |
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| 112 | |
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| 113 | <P> |
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| 119 | </TR> |
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| 120 | <TR BGCOLOR="white" CLASS="TableRowColor"> |
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| 121 | <TD WIDTH="15%"><B><A HREF="../../../../org/apache/hadoop/metrics/ContextFactory.html" title="class in org.apache.hadoop.metrics">ContextFactory</A></B></TD> |
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| 122 | <TD>Factory class for creating MetricsContext objects.</TD> |
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| 123 | </TR> |
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| 124 | <TR BGCOLOR="white" CLASS="TableRowColor"> |
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| 125 | <TD WIDTH="15%"><B><A HREF="../../../../org/apache/hadoop/metrics/MetricsUtil.html" title="class in org.apache.hadoop.metrics">MetricsUtil</A></B></TD> |
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| 126 | <TD>Utility class to simplify creation and reporting of hadoop metrics.</TD> |
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| 127 | </TR> |
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| 129 | |
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| 130 | |
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| 136 | <B>Exception Summary</B></FONT></TH> |
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| 137 | </TR> |
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| 138 | <TR BGCOLOR="white" CLASS="TableRowColor"> |
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| 139 | <TD WIDTH="15%"><B><A HREF="../../../../org/apache/hadoop/metrics/MetricsException.html" title="class in org.apache.hadoop.metrics">MetricsException</A></B></TD> |
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| 140 | <TD>General-purpose, unchecked metrics exception.</TD> |
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| 141 | </TR> |
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| 142 | </TABLE> |
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| 143 | |
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| 144 | |
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| 145 | <P> |
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| 146 | <A NAME="package_description"><!-- --></A><H2> |
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| 147 | Package org.apache.hadoop.metrics Description |
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| 148 | </H2> |
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| 149 | |
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| 150 | <P> |
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| 151 | This package defines an API for reporting performance metric information. |
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| 152 | <p/> |
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| 153 | The API is abstract so that it can be implemented on top of |
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| 154 | a variety of metrics client libraries. The choice of |
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| 155 | client library is a configuration option, and different |
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| 156 | modules within the same application can use |
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| 157 | different metrics implementation libraries. |
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| 158 | <p/> |
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| 159 | Sub-packages: |
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| 160 | <dl> |
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| 161 | <dt><code>org.apache.hadoop.metrics.spi</code></dt> |
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| 162 | <dd>The abstract Server Provider Interface package. Those wishing to |
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| 163 | integrate the metrics API with a particular metrics client library should |
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| 164 | extend this package.</dd> |
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| 165 | |
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| 166 | <dt><code>org.apache.hadoop.metrics.file</code></dt> |
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| 167 | <dd>An implementation package which writes the metric data to |
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| 168 | a file, or sends it to the standard output stream.</dd> |
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| 169 | |
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| 170 | <dt> <code>org.apache.hadoop.metrics.ganglia</code></dt> |
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| 171 | <dd>An implementation package which sends metric data to |
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| 172 | <a href="http://ganglia.sourceforge.net/">Ganglia</a>.</dd> |
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| 173 | </dl> |
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| 174 | |
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| 175 | <h3>Introduction to the Metrics API</h3> |
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| 176 | |
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| 177 | Here is a simple example of how to use this package to report a single |
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| 178 | metric value: |
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| 179 | <pre> |
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| 180 | private ContextFactory contextFactory = ContextFactory.getFactory(); |
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| 181 | |
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| 182 | void reportMyMetric(float myMetric) { |
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| 183 | MetricsContext myContext = contextFactory.getContext("myContext"); |
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| 184 | MetricsRecord myRecord = myContext.getRecord("myRecord"); |
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| 185 | myRecord.setMetric("myMetric", myMetric); |
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| 186 | myRecord.update(); |
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| 187 | } |
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| 188 | </pre> |
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| 189 | |
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| 190 | In this example there are three names: |
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| 191 | <dl> |
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| 192 | <dt><i>myContext</i></dt> |
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| 193 | <dd>The context name will typically identify either the application, or else a |
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| 194 | module within an application or library.</dd> |
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| 195 | |
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| 196 | <dt><i>myRecord</i></dt> |
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| 197 | <dd>The record name generally identifies some entity for which a set of |
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| 198 | metrics are to be reported. For example, you could have a record named |
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| 199 | "cacheStats" for reporting a number of statistics relating to the usage of |
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| 200 | some cache in your application.</dd> |
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| 201 | |
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| 202 | <dt><i>myMetric</i></dt> |
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| 203 | <dd>This identifies a particular metric. For example, you might have metrics |
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| 204 | named "cache_hits" and "cache_misses". |
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| 205 | </dd> |
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| 206 | </dl> |
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| 207 | |
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| 208 | <h3>Tags</h3> |
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| 209 | |
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| 210 | In some cases it is useful to have multiple records with the same name. For |
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| 211 | example, suppose that you want to report statistics about each disk on a computer. |
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| 212 | In this case, the record name would be something like "diskStats", but you also |
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| 213 | need to identify the disk which is done by adding a <i>tag</i> to the record. |
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| 214 | The code could look something like this: |
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| 215 | <pre> |
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| 216 | private MetricsRecord diskStats = |
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| 217 | contextFactory.getContext("myContext").getRecord("diskStats"); |
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| 218 | |
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| 219 | void reportDiskMetrics(String diskName, float diskBusy, float diskUsed) { |
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| 220 | diskStats.setTag("diskName", diskName); |
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| 221 | diskStats.setMetric("diskBusy", diskBusy); |
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| 222 | diskStats.setMetric("diskUsed", diskUsed); |
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| 223 | diskStats.update(); |
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| 224 | } |
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| 225 | </pre> |
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| 226 | |
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| 227 | <h3>Buffering and Callbacks</h3> |
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| 228 | |
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| 229 | Data is not sent immediately to the metrics system when |
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| 230 | <code>MetricsRecord.update()</code> is called. Instead it is stored in an |
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| 231 | internal table, and the contents of the table are sent periodically. |
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| 232 | This can be important for two reasons: |
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| 233 | <ol> |
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| 234 | <li>It means that a programmer is free to put calls to this API in an |
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| 235 | inner loop, since updates can be very frequent without slowing down |
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| 236 | the application significantly.</li> |
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| 237 | <li>Some implementations can gain efficiency by combining many metrics |
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| 238 | into a single UDP message.</li> |
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| 239 | </ol> |
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| 240 | |
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| 241 | The API provides a timer-based callback via the |
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| 242 | <code>registerUpdater()</code> method. The benefit of this |
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| 243 | versus using <code>java.util.Timer</code> is that the callbacks will be done |
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| 244 | immediately before sending the data, making the data as current as possible. |
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| 245 | |
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| 246 | <h3>Configuration</h3> |
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| 247 | |
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| 248 | It is possible to programmatically examine and modify configuration data |
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| 249 | before creating a context, like this: |
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| 250 | <pre> |
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| 251 | ContextFactory factory = ContextFactory.getFactory(); |
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| 252 | ... examine and/or modify factory attributes ... |
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| 253 | MetricsContext context = factory.getContext("myContext"); |
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| 254 | </pre> |
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| 255 | The factory attributes can be examined and modified using the following |
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| 256 | <code>ContextFactory</code>methods: |
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| 257 | <ul> |
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| 258 | <li><code>Object getAttribute(String attributeName)</code></li> |
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| 259 | <li><code>String[] getAttributeNames()</code></li> |
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| 260 | <li><code>void setAttribute(String name, Object value)</code></li> |
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| 261 | <li><code>void removeAttribute(attributeName)</code></li> |
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| 262 | </ul> |
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| 263 | |
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| 264 | <p/> |
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| 265 | <code>ContextFactory.getFactory()</code> initializes the factory attributes by |
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| 266 | reading the properties file <code>hadoop-metrics.properties</code> if it exists |
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| 267 | on the class path. |
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| 268 | |
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| 269 | <p/> |
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| 270 | A factory attribute named: |
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| 271 | <pre> |
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| 272 | <i>contextName</i>.class |
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| 273 | </pre> |
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| 274 | should have as its value the fully qualified name of the class to be |
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| 275 | instantiated by a call of the <code>CodeFactory</code> method |
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| 276 | <code>getContext(<i>contextName</i>)</code>. If this factory attribute is not |
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| 277 | specified, the default is to instantiate |
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| 278 | <code>org.apache.hadoop.metrics.file.FileContext</code>. |
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| 279 | |
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| 280 | <p/> |
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| 281 | Other factory attributes are specific to a particular implementation of this |
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| 282 | API and are documented elsewhere. For example, configuration attributes for |
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| 283 | the file and Ganglia implementations can be found in the javadoc for |
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| 284 | their respective packages. |
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