Hi all, I think the code example in following section has a thread safety issue: https://ci.apache.org/projects/flink/flink-docs-release-1.9/dev/stream/testing.html#junit-rule-miniclusterwithclientresource
The class CollectSink is not thread safe as only the write to the values collection are synchronized but not the read: CollectSink.values.containsAll(...). Maybe if you use a CopyOnWriteArrayList instead of an ArrayList and remove the synchronized key work it should be ok. Romain
