src/main/java/org/apache/giraph/graph/AggregatorWriter.java

/*
 * 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.giraph.graph;

import java.io.IOException;
import java.util.Map;

import org.apache.hadoop.io.Writable;
import org.apache.hadoop.mapreduce.Mapper.Context;

/**
 *  An AggregatorWriter is used to export Aggregators during or at the end of 
 *  each computation. It runs on the master and it's called at the end of each
 *  superstep. The special signal {@link AggregatorWriter#LAST_SUPERSTEP} is 
 *  passed to {@link AggregatorWriter#writeAggregator(Map, long)} as the 
 *  superstep value to signal the end of computation.
 */
public interface AggregatorWriter {
    /** Signal for last superstep */
    public static final int LAST_SUPERSTEP = -1;

    /**
     * The method is called at the initialization of the AggregatorWriter.
     * More precisely, the aggregatorWriter is initialized each time a new
     * master is elected.
     * 
     * @param context Mapper Context where the master is running on
     * @param applicationAttempt ID of the applicationAttempt, used to
     *        disambiguate aggregator writes for different attempts
     * @throws IOException
     */
    @SuppressWarnings("rawtypes")
    void initialize(Context context, long applicationAttempt) throws IOException;

    /**
     * The method is called at the end of each superstep. The user might decide
     * whether to write the aggregators values for the current superstep. For 
     * the last superstep, {@link AggregatorWriter#LAST_SUPERSTEP} is passed.
     * 
     * @param map Map of aggregators to write
     * @param superstep Current superstep
     * @throws IOException
     */
    void writeAggregator(
            Map<String, Aggregator<Writable>> aggregatorMap, 
            long superstep) throws IOException;

    /**
     * The method is called at the end of a successful computation. The method
     * is not called when the job fails and a new master is elected. For this
     * reason it's advised to flush data at the end of 
     * {@link AggregatorWriter#writeAggregator(Map, long)}.
     * 
     * @throws IOException
     */
    void close() throws IOException;
}