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From "Fabian Hueske (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLINK-4640) Serialization of the initialValue of a Fold on WindowedStream fails
Date Tue, 20 Sep 2016 09:16:20 GMT
Fabian Hueske created FLINK-4640:

             Summary: Serialization of the initialValue of a Fold on WindowedStream fails
                 Key: FLINK-4640
                 URL: https://issues.apache.org/jira/browse/FLINK-4640
             Project: Flink
          Issue Type: Bug
          Components: Streaming
    Affects Versions: 1.1.2, 1.2.0
            Reporter: Fabian Hueske
            Priority: Critical
             Fix For: 1.2.0, 1.1.3

The following program

DataStream<Tuple2<String, Long>> src = env.fromElements(new Tuple2<String,
Long>("a", 1L));

  .fold(TreeMultimap.<Long, String>create(), new FoldFunction<Tuple2<String, Long>,
TreeMultimap<Long, String>>() {
    public TreeMultimap<Long, String> fold(
        TreeMultimap<Long, String> topKSoFar, 
        Tuple2<String, Long> itemCount) throws Exception 
      String item = itemCount.f0;
      Long count = itemCount.f1;
      topKSoFar.put(count, item);
      if (topKSoFar.keySet().size() > 10) {
      return topKSoFar;

throws this exception

Caused by: java.lang.RuntimeException: Could not add value to folding state.
	at org.apache.flink.runtime.state.memory.MemFoldingState.add(MemFoldingState.java:91)
	at org.apache.flink.streaming.runtime.operators.windowing.WindowOperator.processElement(WindowOperator.java:382)
	at org.apache.flink.streaming.runtime.io.StreamInputProcessor.processInput(StreamInputProcessor.java:176)
	at org.apache.flink.streaming.runtime.tasks.OneInputStreamTask.run(OneInputStreamTask.java:66)
	at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:266)
	at org.apache.flink.runtime.taskmanager.Task.run(Task.java:584)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
	at com.google.common.collect.AbstractMapBasedMultimap.put(AbstractMapBasedMultimap.java:192)
	at com.google.common.collect.AbstractSetMultimap.put(AbstractSetMultimap.java:121)
	at com.google.common.collect.TreeMultimap.put(TreeMultimap.java:78)
	at org.apache.flink.streaming.examples.wordcount.WordCount$1.fold(WordCount.java:115)
	at org.apache.flink.streaming.examples.wordcount.WordCount$1.fold(WordCount.java:109)
	at org.apache.flink.runtime.state.memory.MemFoldingState.add(MemFoldingState.java:85)
	... 6 more

Using the same {{FoldFunction}} on a {{KeyedStream}} (without a window) works fine.

I tracked the problem down to the serialization of the {{StateDescriptor}}, i.e., the {{writeObject()}}
and {{readObject()}} methods. The methods use Flink's TypeSerializers to serialize the default
value. In case of the {{TreeMultiMap}} this is the {{KryoSerializer}} which fails to read
the serialized data for some reason.

A quick workaround to solve this issue would be to check if the default value implements {{Serializable}}
and use Java Serialization in this case. However, it would be good to track the root cause
of this problem.

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