AHeise opened a new pull request #16817:
URL: https://github.com/apache/flink/pull/16817
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## What is the purpose of the change
StreamStatus is currently defined as follows:
> A Stream Status element informs stream tasks whether or not they should
continue to expect records and watermarks from the input stream that sent them.
There are 2 kinds of status, namely IDLE and ACTIVE. Stream Status elements are
generated at the sources, and may be propagated through the tasks of the
topology. They directly infer the current status of the emitting task; a
SourceStreamTask or StreamTask emits a IDLE if it will temporarily halt to emit
any records or watermarks (i.e. is idle), and emits a ACTIVE once it resumes to
do so (i.e. is active).
However that definition is quite problematic. Operators with delayed
responses, such as AsyncIO, windows, or time-based process functions, might
output elements when they are actually idle. So by that definition, they would
need to switch from idle to active, emit an element, and go back to idle. It's
not entirely clear when they would go back to idle (after each element or after
some time?). Furthermore, under heavy backpressure, these additional
StreamStatuses delay progress quite a bit.
At this point, it makes sense to revise the original intent of StreamStatus:
being able to advance the watermark without data. Indeed, there is no reason to
expand the definition towards records; we could simply limit the definition to
watermarks as this is the only use case for StreamStatus in the code base
currently. Moreover, that's exactly how it's implemented in Flink so far. So,
in this FLIP we propose to adjust the definition to reality and act on that.
This is the implementation PR for
[FLIP-180](https://cwiki.apache.org/confluence/display/FLINK/FLIP-180%3A+Adjust+StreamStatus+and+Idleness+definition).
## Brief change log
- Update definition of StreamStatus.
- Rename StreamStatus to WatermarkStatus.
- Add WatermarkOutput#markActive.
## Verifying this change
Mostly renaming that is covered by existing tests.
## Does this pull request potentially affect one of the following parts:
- Dependencies (does it add or upgrade a dependency): (yes / **no**)
- The public API, i.e., is any changed class annotated with
`@Public(Evolving)`: (**yes** / no)
- The serializers: (yes / **no** / don't know)
- The runtime per-record code paths (performance sensitive): (**yes** / no
/ don't know)
- Anything that affects deployment or recovery: JobManager (and its
components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / **no** / don't
know)
- The S3 file system connector: (yes / **no** / don't know)
## Documentation
- Does this pull request introduce a new feature? (yes / **no**)
- If yes, how is the feature documented? (not applicable / docs /
**JavaDocs** / not documented)
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