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MessageCombiner xref
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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10   *     http://www.apache.org/licenses/LICENSE-2.0
11   *
12   * Unless required by applicable law or agreed to in writing, software
13   * distributed under the License is distributed on an "AS IS" BASIS,
14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15   * See the License for the specific language governing permissions and
16   * limitations under the License.
17   */
18  
19  package org.apache.giraph.combiner;
20  
21  import org.apache.hadoop.io.Writable;
22  import org.apache.hadoop.io.WritableComparable;
23  
24  /**
25   * Abstract class to extend for combining messages sent to the same vertex.
26   * MessageCombiner for applications where each two messages for one
27   * vertex can be combined into one.
28   *
29   * @param <I> Vertex id
30   * @param <M> Message data
31   */
32  public interface MessageCombiner<I extends WritableComparable,
33      M extends Writable> {
34    /**
35     * Combine messageToCombine with originalMessage, by modifying
36     * originalMessage.  Note that the messageToCombine object
37     * may be reused by the infrastructure, therefore, you cannot directly
38     * use it or any objects from it in original message
39     *
40     * @param vertexIndex Index of the vertex getting these messages
41     * @param originalMessage The first message which we want to combine;
42     *                        put the result of combining in this message
43     * @param messageToCombine The second message which we want to combine
44     *                         (object may be reused - do not reference it or its
45     *                         member objects)
46     */
47    void combine(I vertexIndex, M originalMessage,
48        M messageToCombine);
49  
50    /**
51     * Get the initial message. When combined with any other message M,
52     * the result should be M.
53     *
54     * @return Initial message
55     */
56    M createInitialMessage();
57  }