pltbkd opened a new pull request, #28305:
URL: https://github.com/apache/flink/pull/28305
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## What is the purpose of the change
When AsyncWaitOperator processes input, if the async queue is full, it
yields and waits for a callback to complete to free up a slot. During this
time, unaligned checkpoints cannot be processed.
Soft backpressure is used to ensure UC can be processed in time. In the
non-async path, `processInput` produces data written to the ResultPartition. If
the RP is full, `processInput` can be blocked by soft backpressure so that
mails can be executed.
The same idea applies to AsyncWaitOperator: on the async path,
`processInput` produces requests written to the AsyncWaitOperator's queue —
when the queue is full, `processInput` should be blocked. Similarly, results
from AsyncWaitOperator produce data written to downstream (e.g., RP) — if
downstream is full, AsyncWaitOperator's `emitResult` should also be blocked.
## Brief change log
- Add `SupportsSoftBackpressure` interface extending AvailabilityProvider,
allowing operators to declare soft-backpressure capability and receive a
downstream AvailabilityProvider.
- OperatorChain checks `instanceof SupportsSoftBackpressure` and wires
downstream availability to the operator; StreamTask feeds it into the mailbox
soft-backpressure path.
- AsyncWaitOperator implements `SupportsSoftBackpressure`, checks
downstream availability before emitting and re-yields if not available.
## Verifying this change
- StreamOperatorChainingTest verifies chain availability provider wiring
for both head and chained SupportsSoftBackpressure operators.
- CompositeAvailabilityProviderTest covers the AND semantics.
- AsyncWaitOperatorTest covers soft-backpressure pause/resume under
simulated downstream unavailability.
- Ordered/UnorderedStreamElementQueueTest cover the emit-side availability
hook.
## 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): yes
- Anything that affects deployment or recovery: 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
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