StephanEwen commented on pull request #12234:
URL: https://github.com/apache/flink/pull/12234#issuecomment-636172362
Updated the PR to address the issue raised by @becketqin
This required a change in the `OperatorCoordinator` interface. To explain
why, let's revisit the semantics we want to offer.
The exactly-once semantics for the `OperatorCoordinator` are defined as
follows:
- The point in time when the checkpoint future is completed is considered
the point in time when the coordinator's checkpoint takes place.
- The OperatorCoordinator implementation must have a way of strictly
ordering the sending of events and the completion of the checkpoint future (for
example the same thread does both actions, or both actions are guarded by a
mutex).
- Every event sent before the checkpoint future is completed is considered
before the checkpoint.
- Every event sent after the checkpoint future is completed is considered
to be after the checkpoint.
The previous interface did not allow us to observe this point accurately.
The future was created inside the application-specific OperatorCoordinator code
and returned from the methods. By the time that the scheduler/checkpointing
code could observe the future (attach handlers to it), some (small amount of)
time had inevitably passed in the meantime.
Within that time, the future could already be complete and some events could
have been sent, and in that case the scheduler/checkpointing code could not
determin which events were before the completion of the future, and which
events were after the completion of the future.
We hence need to change the checkpointing method from
```java
CompletableFuture checkpointCoordinator(long checkpointId) throws
Exception;
```
to
```java
void checkpointCoordinator(long checkpointId, CompletableFuture
result) throws Exception;
```
The changed interface passes the future from the scheduler/checkpointing
code into the coordinator. The future already has synchronous handlers attached
to it which exactly mark the point when the future was completed, allowing the
scheduler/checkpointing code to observe the correct order in which the
Checkpoint Coordinator implementation performed its actions (event sending,
future completion).
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