/*
* 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.flink.api.common.accumulators;
import org.apache.flink.annotation.Public;
import java.io.Serializable;
/**
* Accumulators collect distributed statistics or aggregates in a from user functions
* and operators. Each parallel instance creates and updates its own accumulator object,
* and the different parallel instances of the accumulator are later merged.
* merged by the system at the end of the job. The result can be obtained from the
* result of a job execution, or from the web runtime monitor.
*
* The accumulators are inspired by the Hadoop/MapReduce counters.
*
* 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
public interface Accumulator<V, R extends Serializable> extends Serializable, Cloneable {
/**
* @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);
/**
* Duplicates the accumulator. All subclasses need to properly implement
* cloning and cannot throw a {@link java.lang.CloneNotSupportedException}
*
* @return The duplicated accumulator.
*/
Accumulator<V, R> clone();
}