[jira] [Created] (FLINK-21935) Remove "state.backend.async" option.

[Shang Yuanchun (Jira)]

Stephan Ewen created FLINK-21935:
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             Summary: Remove "state.backend.async" option.
                 Key: FLINK-21935
                 URL: https://issues.apache.org/jira/browse/FLINK-21935
             Project: Flink
          Issue Type: Improvement
          Components: Runtime / State Backends
            Reporter: Stephan Ewen
             Fix For: 1.13.0


Checkpoints are always asynchronous, there is no case ever for a synchronous checkpoint.
The RocksDB state backend doesn't even support synchronous snapshots, and the HashMap Heap backend also has no good use case for synchronous snapshots (other than a very minor reduction in heap objects).

Most importantly, we should not expose this option in the constructors of the new state backend API classes, like {{HashMapStateBackend}}.

I marked this a blocker because it is part of the new user-facing State Backend API and I would like to avoid that this option enters this API and causes confusion when we eventually remove it.

/cc [~sjwiesman] and [~liyu]



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