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From: shaoxuan-wang <git@git.apache.org>
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References: <git-pr-3186-flink@git.apache.org>
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Subject: [GitHub] flink pull request #3186: [FLINK-5582] [streaming] dd a general distributive...
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Message-Id: <20170122162505.CA71CDFCBC@git1-us-west.apache.org>
Date: Sun, 22 Jan 2017 16:25:05 +0000 (UTC)
archived-at: Sun, 22 Jan 2017 16:25:11 -0000

Github user shaoxuan-wang commented on a diff in the pull request:

    https://github.com/apache/flink/pull/3186#discussion_r97226958
  
    --- Diff: flink-core/src/main/java/org/apache/flink/api/common/functions/AggregateFunction.java ---
    @@ -0,0 +1,94 @@
    +/*
    + * 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.functions;
    +
    +import java.io.Serializable;
    +
    +/**
    + * 
    + * <p>Aggregation functions must be {@link Serializable} because they are sent around
    + * between distributed processes during distributed execution.
    + * 
    + * <p>An example how to use this interface is below:
    + * 
    + * <pre>{@code
    + * // the accumulator, which holds the state of the in-flight aggregate
    + * public class AverageAccumulator {
    + *     long count;
    + *     long sum;
    + * }
    + * 
    + * // implementation of an aggregation function for an 'average'
    + * public class Average implements AggregateFunction<Integer, AverageAccumulator, Double> {
    + * 
    + *     public AverageAccumulator createAccumulator() {
    + *         return new AverageAccumulator();
    + *     }
    + * 
    + *     public AverageAccumulator merge(AverageAccumulator a, AverageAccumulator b) {
    + *         a.count += b.count;
    + *         a.sum += b.sum;
    + *         return a;
    + *     }
    + * 
    + *     public void add(Integer value, AverageAccumulator acc) {
    + *         acc.sum += value;
    + *         acc.count++;
    + *     }
    + * 
    + *     public Double getResult(AverageAccumulator acc) {
    + *         return acc.sum / (double) acc.count;
    + *     }
    + * }
    + * 
    + * // implementation of a weighted average
    + * // this reuses the same accumulator type as the aggregate function for 'average'
    + * public class WeightedAverage implements AggregateFunction<Datum, AverageAccumulator, Double> {
    + *
    + *     public AverageAccumulator createAccumulator() {
    + *         return new AverageAccumulator();
    + *     }
    + *
    + *     public AverageAccumulator merge(AverageAccumulator a, AverageAccumulator b) {
    + *         a.count += b.count;
    + *         a.sum += b.sum;
    + *         return a;
    + *     }
    + *
    + *     public void add(Datum value, AverageAccumulator acc) {
    + *         acc.count += value.getWeight();
    + *         acc.sum += value.getValue();
    + *     }
    + *
    + *     public Double getResult(AverageAccumulator acc) {
    + *         return acc.sum / (double) acc.count;
    + *     }
    + * }
    + * }</pre>
    + */
    +public interface AggregateFunction<IN, ACC, OUT> extends Function, Serializable {
    +
    +	ACC createAccumulator();
    +
    +	void add(IN value, ACC accumulator);
    +
    +	OUT getResult(ACC accumulator);
    +
    +	ACC merge(ACC a, ACC b);
    --- End diff --
    
    Do you think it is useful to extend merge function to accept a list of ACC:  ACC merge(List<ACC> a). There are cases where the group merging a list of instances is much more efficient than just merge only two instances. 


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