| /** |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you under the Apache License, Version 2.0 (the |
| * "License"); you may not use this file except in compliance |
| * with the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| package org.apache.hadoop.mapreduce; |
| |
| /** |
| * Partitions the key space. |
| * |
| * <p><code>Partitioner</code> controls the partitioning of the keys of the |
| * intermediate map-outputs. The key (or a subset of the key) is used to derive |
| * the partition, typically by a hash function. The total number of partitions |
| * is the same as the number of reduce tasks for the job. Hence this controls |
| * which of the <code>m</code> reduce tasks the intermediate key (and hence the |
| * record) is sent for reduction.</p> |
| * |
| * @see Reducer |
| */ |
| public abstract class Partitioner<KEY, VALUE> { |
| |
| /** |
| * Get the partition number for a given key (hence record) given the total |
| * number of partitions i.e. number of reduce-tasks for the job. |
| * |
| * <p>Typically a hash function on a all or a subset of the key.</p> |
| * |
| * @param key the key to be partioned. |
| * @param value the entry value. |
| * @param numPartitions the total number of partitions. |
| * @return the partition number for the <code>key</code>. |
| */ |
| public abstract int getPartition(KEY key, VALUE value, int numPartitions); |
| |
| } |