Zhipeng Zhang created FLINK-31373:
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             Summary: PerRoundWrapperOperator should carry epoch information in 
watermark
                 Key: FLINK-31373
                 URL: https://issues.apache.org/jira/browse/FLINK-31373
             Project: Flink
          Issue Type: Bug
          Components: Library / Machine Learning
    Affects Versions: ml-2.2.0
            Reporter: Zhipeng Zhang


Currently we use `PerRoundWrapperOperator` to wrap the normal flink operators 
such that they can be used in iterations.


We already contained the epoch information in each record so that we know which 
iteration each record belongs to.

However, there is no epoch information when the stream element is a watermark. 
This works in most cases, but fail to address the following issue:

- In DataStreamUtils#withBroadcast, we will cache the elements (including 
watermarks) from non-broadcast inputs until the broadcast variables are ready. 
When the broadcast variables are ready, once we receive a stream element we 
will process the cached elements first. If the received element is a watermark, 
the current implementation of iteration module fails (`ProxyOutput#collect` 
throws NPE) since there is no epoch  information.



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