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https://issues.apache.org/jira/browse/FLINK-5017?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15644949#comment-15644949
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Tzu-Li (Gordon) Tai commented on FLINK-5017:
--------------------------------------------
Hi [~aljoscha],
To ease discussion a bit, I've just pushed some of my current changes regarding
the IDLE watermark forwarding logic to a local branch:
https://github.com/tzulitai/flink/tree/FLINK-5017. It's WIP code, so might be a
bit sloppy / won't build yet as the output / operator interface implementations
aren't all updated yet.
{quote}
The only place where it is relevant is in the watermark advancement code in the
input processors. If all inputs are IDLE, then the input processor needs to
bypass the chain and directly output to all real outgoing connections, which in
itself is quite tricky.
{quote}
Concerning this: the approach I'm taking right now, is that operator's that are
IDLE are not supposed to receive any records or watermarks at all, before they
are triggered back to ACTIVE (using WatermarkStatus elements). It's basically
the sources' and intermediate timestamp extractors' responsibility to ensure
this.
So, regarding "If all inputs are IDLE, then the input processor needs to bypass
the chain and directly output to all real outgoing connections", I'm not really
sure if this is necessary. Once the whole input processor is IDLE, a IDLE
status will be propagated down the chain, which eventually is forwarded to a
real outgoing connection (output of last operator in chain, correct?). No
records or watermarks will be propagated down the chain while the input
processor remains IDLE. Once the input processor is ACTIVE again, it propagates
an ACTIVE down the chain, and records and watermarks are allowed to flow again.
Do you think the above makes sense? The WIP code implements this approach.
> 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
>
>
> 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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