Github user vipul1409 commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2922#discussion_r90598293
  
    --- Diff: 
flink-contrib/flink-streaming-contrib/src/main/java/org/apache/flink/contrib/streaming/MultiThreadedFlatMapFunction.java
 ---
    @@ -0,0 +1,91 @@
    +package org.apache.flink.contrib.streaming;
    +
    +import org.apache.flink.api.common.functions.FlatMapFunction;
    +import org.apache.flink.api.common.functions.RichFlatMapFunction;
    +import org.apache.flink.api.common.functions.util.ListCollector;
    +import org.apache.flink.configuration.Configuration;
    +import org.apache.flink.runtime.util.NamedThreadFactory;
    +import org.apache.flink.util.Collector;
    +
    +import java.util.ArrayList;
    +import java.util.List;
    +import java.util.concurrent.*;
    +
    +/**
    + * Created by vipulmodi on 11/30/16.
    + */
    +
    +//To return Futures to calling fucntion we can do OUT extends Future.
    +public class MultiThreadedFlatMapFunction<IN, OUT> extends 
RichFlatMapFunction<IN, OUT> {
    +
    +    private static final long serialVersionUID;
    +    private ExecutorService flatMapExecutors;
    +    private Integer numberOfthreads;
    +    private List<Callable<ArrayList<OUT>>> callables;
    +    private List<Future<ArrayList<OUT>>> futures;
    +    static {
    +        serialVersionUID = 1L;
    +    }
    +
    +    public FlatMapFunction flatMapFunction;
    +
    +    public MultiThreadedFlatMapFunction(FlatMapFunction flatMapFunction, 
Integer numberOfthreads) {
    +        this.flatMapFunction = flatMapFunction;
    +        this.numberOfthreads = numberOfthreads;
    +        this.callables = new ArrayList<>();
    +        this.futures = new ArrayList<>();
    +    }
    +
    +    // Understand side effects of making out final.
    +    @Override
    +    public void flatMap(final IN value, final Collector<OUT> out) throws 
Exception {
    +        // Figure out a better way to call
    +        /*callables.add(new Callable<ArrayList<OUT>>() {
    +            @Override
    +            public ArrayList<OUT> call() throws Exception {
    +                ArrayList<OUT> result = new ArrayList<OUT>();
    +                Collector<OUT> collector = new ListCollector<OUT>(result);
    +                flatMapFunction.flatMap(value, collector);
    +                return result;
    +            }
    +        });*/
    +
    +        futures.add(flatMapExecutors.submit(new Callable<ArrayList<OUT>>() 
{
    +            @Override
    +            public ArrayList<OUT> call() throws Exception {
    +                ArrayList<OUT> result = new ArrayList<OUT>();
    --- End diff --
    
    As each call may take significant time we need to ensure that all the 
results objects of active threads can stay in memory.


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