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https://issues.apache.org/jira/browse/FLINK-1699?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14363049#comment-14363049
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Gábor Hermann commented on FLINK-1699:
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This was probably caused by using static non-thread-safe collections for
collecting the result which were accessed by multiple sinks at the same time.
> Co-task sometimes returns null
> ------------------------------
>
> Key: FLINK-1699
> URL: https://issues.apache.org/jira/browse/FLINK-1699
> Project: Flink
> Issue Type: Bug
> Components: Streaming
> Reporter: Gábor Hermann
> Assignee: Gábor Hermann
> Priority: Critical
>
> Connecting two grouped data stream coMapping them and sinking it in a custom
> sink sometimes results in null records.
> More specifically on in [this
> test|https://github.com/mbalassi/flink/blob/b6a2a8236404affb808267e47656791eceb57fe4/flink-staging/flink-streaming/flink-streaming-core/src/test/java/org/apache/flink/streaming/api/StreamCoTest.java]
> the result might be an array with null, thus getting
> java.lang.AssertionError:
> Expected :[1, 3, 5, 1, 3, 5]
> Actual :[null, 1, 5, 3, 1, 5]
> or not getting all the expected messages:
> java.lang.AssertionError:
> Expected :[1, 1, 3, 3, 5, 5]
> Actual :[1, 3, 5, 5]
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