Lee Y S created FLINK-29048:
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             Summary: WatermarksWithIdleness does not work with FLIP-27 sources
                 Key: FLINK-29048
                 URL: https://issues.apache.org/jira/browse/FLINK-29048
             Project: Flink
          Issue Type: Bug
          Components: API / DataStream
    Affects Versions: 1.15.1
            Reporter: Lee Y S


In org.apache.flink.streaming.api.operators.SourceOperator, there are separate 
instances of WatermarksWithIdleness created for each split output and the main 
output. There is multiplexing of watermarks between split outputs but no 
multiplexing between split output and main output in 
org.apache.flink.streaming.api.operators.source.ProgressiveTimestampsAndWatermarks.
 
For a source such as org.apache.flink.connector.kafka.source.KafkaSource, 
{color:#353833}there is only output from splits and no output from main. Hence 
the main output will (after an initial timeout) be marked as idle.{color}
{color:#353833} {color}
{color:#353833}The implementation of {color}WatermarksWithIdleness is such that 
once an output is idle, it will periodically re-mark the output as idle. Since 
there is no multiplexing between split outputs and main output, the idle marks 
coming from main output will repeatedly set the output to idle even though 
there are events from the splits. Result is that the entire source is 
repeatedly marked as idle.
 
One solution i can think of is to multiplex split and main output in 
org.apache.flink.streaming.api.operators.source.ProgressiveTimestampsAndWatermarks
 but I am not sure if there are other considerations.



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