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https://issues.apache.org/jira/browse/FLINK-5017?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15650400#comment-15650400
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Aljoscha Krettek commented on FLINK-5017:
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Possibly {{StreamStatus}}?
> Introduce WatermarkStatus stream element to allow for temporarily idle
> streaming sources
> ----------------------------------------------------------------------------------------
>
> Key: FLINK-5017
> URL: https://issues.apache.org/jira/browse/FLINK-5017
> Project: Flink
> Issue Type: New Feature
> Components: Streaming
> Reporter: Tzu-Li (Gordon) Tai
> Assignee: Tzu-Li (Gordon) Tai
> Priority: Blocker
> Fix For: 1.2.0
>
> Attachments: operator_chain_with_multiple_network_outputs.png
>
>
> A {{WatermarkStatus}} element informs receiving operators whether or not they
> should continue to expect watermarks from the sending operator. There are 2
> kinds of status, namely {{IDLE}} and {{ACTIVE}}. Watermark status elements
> are generated at the sources, and may be propagated through the operators of
> the topology using {{Output#emitWatermarkStatus(WatermarkStatus)}}.
> Sources and downstream operators should emit either of the status elements
> once it changes between "watermark-idle" and "watermark-active" states.
> A source is considered "watermark-idle" if it will not emit records for an
> indefinite amount of time. This is the case, for example, for Flink's Kafka
> Consumer, where sources might initially have no assigned partitions to read
> from, or no records can be read from the assigned partitions. Once the source
> detects that it will resume emitting data, it is considered
> "watermark-active".
> Downstream operators with multiple inputs (ex. head operators of a
> {{OneInputStreamTask}} or {{TwoInputStreamTask}}) should not wait for
> watermarks from an upstream operator that is "watermark-idle" when deciding
> whether or not to advance the operator's current watermark. When a downstream
> operator determines that all upstream operators are "watermark-idle" (i.e.
> when all input channels have received the watermark idle status element),
> then the operator is considered to also be "watermark-idle", as it will
> temporarily be unable to advance its own watermark. This is always the case
> for operators that only read from a single upstream operator. Once an
> operator is considered "watermark-idle", it should itself forward its idle
> status to inform downstream operators. The operator is considered to be back
> to "watermark-active" as soon as at least one of its upstream operators
> resume to be "watermark-active" (i.e. when at least one input channel
> receives the watermark active status element), and should also forward its
> active status to inform downstream operators.
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