GitHub user StephanEwen opened a pull request:
https://github.com/apache/flink/pull/6066
[FLINK-9428] [checkpointing] Allow operators to flush data on checkpoint
pre-barrier
## What is the purpose of the change
Some operators maintain some small transient state that may be inefficient
to checkpoint, especially when it would need to be checkpointed also in a
re-scalable way.
An example are opportunistic pre-aggregation operators, which have small
the pre-aggregation state that is frequently flushed downstream.
Rather that persisting that state in a checkpoint, it can make sense to
flush the data downstream upon a checkpoint, to let it be part of the
downstream operator's state.
This feature is sensitive, because flushing state has a clean implication
on the downstream operator's checkpoint alignment. However, used with care, and
with the new back-pressure-based checkpoint alignment, this feature can be very
useful.
Because it is sensitive, this PR makes this an internal feature (accessible
to operators) and does NOT expose it in the public API.
## Brief change log
- Adds the `prepareSnapshotPreBarrier(long checkpointId)` call to
`(Abstract)StreamOperator`, with an empty default implementation.
- Adds a call on `OperatorChain` to call this in front-to-back order on
the operators.
## Verifying this change
- This change does not yet alter any behavior, it adds only a plug point
for future stream operators.
- The `OperatorChainTest` Unit Test validates that the call happens, and
that operators are called in the right order.
## Does this pull request potentially affect one of the following parts:
- Dependencies (does it add or upgrade a dependency): **no**
- The public API, i.e., is any changed class annotated with
`@Public(Evolving)`: **no**
- The serializers: **no**
- The runtime per-record code paths (performance sensitive): **no**
- Anything that affects deployment or recovery: JobManager (and its
components), Checkpointing, Yarn/Mesos, ZooKeeper: **yes**
- The S3 file system connector: **no**
## Documentation
- Does this pull request introduce a new feature? **no**
- If yes, how is the feature documented? **not applicable**
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/StephanEwen/incubator-flink pre_barrier
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/flink/pull/6066.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #6066
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