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Posted to issues@flink.apache.org by GitBox <gi...@apache.org> on 2021/08/13 12:35:09 UTC

[GitHub] [flink] AHeise opened a new pull request #16817: [FLINK-23767][streaming] FLIP-180: Adjust StreamStatus and Idleness definition.

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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