[ 
https://issues.apache.org/jira/browse/FLINK-4391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15574716#comment-15574716
 ] 

ASF GitHub Bot commented on FLINK-4391:
---------------------------------------

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

    https://github.com/apache/flink/pull/2629#discussion_r83380141
  
    --- Diff: 
flink-examples/flink-examples-streaming/src/main/java/org/apache/flink/streaming/examples/async/AsyncIOExample.java
 ---
    @@ -0,0 +1,186 @@
    +/*
    + * 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.streaming.examples.async;
    +
    +import org.apache.flink.api.common.functions.FlatMapFunction;
    +import org.apache.flink.api.java.tuple.Tuple2;
    +import org.apache.flink.runtime.state.filesystem.FsStateBackend;
    +import org.apache.flink.streaming.api.CheckpointingMode;
    +import org.apache.flink.streaming.api.TimeCharacteristic;
    +import org.apache.flink.streaming.api.checkpoint.Checkpointed;
    +import org.apache.flink.streaming.api.datastream.AsyncDataStream;
    +import org.apache.flink.streaming.api.datastream.DataStream;
    +import 
org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
    +import org.apache.flink.streaming.api.functions.async.AsyncFunction;
    +import org.apache.flink.streaming.api.functions.async.RichAsyncFunction;
    +import org.apache.flink.streaming.api.functions.source.SourceFunction;
    +import org.apache.flink.streaming.api.operators.async.AsyncCollector;
    +import org.apache.flink.util.Collector;
    +
    +import java.io.IOException;
    +import java.util.ArrayList;
    +import java.util.List;
    +import java.util.Random;
    +import java.util.concurrent.ExecutorService;
    +import java.util.concurrent.Executors;
    +
    +/**
    + * Example to illustrates how to use {@link 
org.apache.flink.streaming.api.functions.async.AsyncFunction}
    + */
    +public class AsyncIOExample {
    +
    +   /**
    +    * A checkpointed source.
    +    */
    +   private static class SimpleSource implements SourceFunction<Integer>, 
Checkpointed<Integer> {
    +           private static final long serialVersionUID = 1L;
    +
    +           private volatile boolean isRunning = true;
    +           private int counter = 0;
    +           private int start = 0;
    +
    +           @Override
    +           public void restoreState(Integer state) throws Exception {
    +                   this.start = state;
    +           }
    +
    +           @Override
    +           public Integer snapshotState(long checkpointId, long 
checkpointTimestamp) throws Exception {
    +                   return start;
    +           }
    +
    +           public SimpleSource(int maxNum) {
    +                   this.counter = maxNum;
    +           }
    +
    +           @Override
    +           public void run(SourceContext<Integer> ctx) throws Exception {
    +                   while (start < counter && isRunning) {
    +                           synchronized (ctx.getCheckpointLock()) {
    +                                   ctx.collect(start);
    +                                   ++start;
    +                                   Thread.sleep(10);
    --- End diff --
    
    Can you put the `Thread.sleep(...)` outside the lock? Otherwise, it will be 
hard for the checkpoint to ever acquire the lock.


> Provide support for asynchronous operations over streams
> --------------------------------------------------------
>
>                 Key: FLINK-4391
>                 URL: https://issues.apache.org/jira/browse/FLINK-4391
>             Project: Flink
>          Issue Type: New Feature
>          Components: DataStream API
>            Reporter: Jamie Grier
>            Assignee: david.wang
>
> Many Flink users need to do asynchronous processing driven by data from a 
> DataStream.  The classic example would be joining against an external 
> database in order to enrich a stream with extra information.
> It would be nice to add general support for this type of operation in the 
> Flink API.  Ideally this could simply take the form of a new operator that 
> manages async operations, keeps so many of them in flight, and then emits 
> results to downstream operators as the async operations complete.



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