/***********************************************************************************************************************
* Copyright (C) 2010-2013 by the Stratosphere project (http://stratosphere.eu)
*
* Licensed 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 eu.stratosphere.api.common.accumulators;
import java.io.Serializable;
import eu.stratosphere.core.io.IOReadableWritable;
/**
* Interface for custom accumulator objects. Data are written to in a UDF and
* merged by the system at the end of the job. The result can be read at the end
* of the job from the calling client. Inspired by Hadoop/MapReduce counters.<br>
* <br>
*
* The type added to the accumulator might differ from the type returned. This
* is the case e.g. for a set-accumulator: We add single objects, but the result
* is a set of objects.
*
* @param <V>
* Type of values that are added to the accumulator
* @param <R>
* Type of the accumulator result as it will be reported to the
* client
*/
public interface Accumulator<V, R> extends IOReadableWritable, Serializable {
/**
* @param value
* The value to add to the accumulator object
*/
void add(V value);
/**
* @return local The local value from the current UDF context
*/
R getLocalValue();
/**
* Reset the local value. This only affects the current UDF context.
*/
void resetLocal();
/**
* Used by system internally to merge the collected parts of an accumulator
* at the end of the job.
*
* @param other Reference to accumulator to merge in.
*/
void merge(Accumulator<V, R> other);
}