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From "Aljoscha Krettek (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-5017) Introduce WatermarkStatus stream element to allow for temporarily idle streaming sources
Date Wed, 09 Nov 2016 09:07:58 GMT

    [ https://issues.apache.org/jira/browse/FLINK-5017?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15650400#comment-15650400

Aljoscha Krettek commented on FLINK-5017:

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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