/*

  * 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.aggregators;

 import java.io.IOException;

 import java.util.Map.Entry;

 import org.apache.giraph.conf.ImmutableClassesGiraphConfigurable;

 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(Iterable, long)} as the

  *  superstep value to signal the end of computation.

  */

 public interface AggregatorWriter extends ImmutableClassesGiraphConfigurable {

   /** Signal for last superstep */

   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 aggregatorMap Map from aggregator name to aggregator value

    * @param superstep Current superstep

    * @throws IOException

    */

   void writeAggregator(

       Iterable<Entry<String, 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(Iterable, long)}.

    *

    * @throws IOException

    */

   void close() throws IOException;

 }