1 | /** |
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2 | * Licensed to the Apache Software Foundation (ASF) under one |
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3 | * or more contributor license agreements. See the NOTICE file |
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4 | * distributed with this work for additional information |
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5 | * regarding copyright ownership. The ASF licenses this file |
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6 | * to you under the Apache License, Version 2.0 (the |
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7 | * "License"); you may not use this file except in compliance |
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8 | * with the License. You may obtain a copy of the License at |
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9 | * |
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10 | * http://www.apache.org/licenses/LICENSE-2.0 |
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11 | * |
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12 | * Unless required by applicable law or agreed to in writing, software |
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13 | * distributed under the License is distributed on an "AS IS" BASIS, |
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14 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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15 | * See the License for the specific language governing permissions and |
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16 | * limitations under the License. |
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17 | */ |
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18 | |
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19 | package org.apache.hadoop.mapreduce; |
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20 | |
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21 | /** |
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22 | * Partitions the key space. |
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23 | * |
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24 | * <p><code>Partitioner</code> controls the partitioning of the keys of the |
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25 | * intermediate map-outputs. The key (or a subset of the key) is used to derive |
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26 | * the partition, typically by a hash function. The total number of partitions |
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27 | * is the same as the number of reduce tasks for the job. Hence this controls |
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28 | * which of the <code>m</code> reduce tasks the intermediate key (and hence the |
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29 | * record) is sent for reduction.</p> |
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30 | * |
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31 | * @see Reducer |
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32 | */ |
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33 | public abstract class Partitioner<KEY, VALUE> { |
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34 | |
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35 | /** |
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36 | * Get the partition number for a given key (hence record) given the total |
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37 | * number of partitions i.e. number of reduce-tasks for the job. |
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38 | * |
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39 | * <p>Typically a hash function on a all or a subset of the key.</p> |
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40 | * |
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41 | * @param key the key to be partioned. |
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42 | * @param value the entry value. |
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43 | * @param numPartitions the total number of partitions. |
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44 | * @return the partition number for the <code>key</code>. |
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45 | */ |
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46 | public abstract int getPartition(KEY key, VALUE value, int numPartitions); |
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47 | |
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48 | } |
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