[jira] [Created] (FLINK-14109) Improve javadocs and tests for high-availability backend

["Brendan MacDonell (Jira)" <ji...@apache.org>]

Brendan MacDonell created FLINK-14109:
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             Summary: Improve javadocs and tests for high-availability backend
                 Key: FLINK-14109
                 URL: https://issues.apache.org/jira/browse/FLINK-14109
             Project: Flink
          Issue Type: Improvement
          Components: Runtime / Coordination
            Reporter: Brendan MacDonell


My team at Sight Machine just finished building a custom HA backend for our infrastructure. The process was mostly painless, but we ran into a few pain points on the way:
 * {{CompletedCheckpointStore#getLatestCheckpoint}} is not marked as {{@Nullable, }}so there was some confusion about whether the Javadocs or (lack of) annotation is correct. The interface would be clearer if the annotation was present.
 * The javadocs for {{CompletedCheckpointStore#recover}} disagree with the documentation for {{ZooKeeperCompletedCheckpointStore#recover}}. It's not immediately clear to someone working on the code that the ZK javadoc is outdated and the interface documentation is correct.
 * The base {{CompletedCheckpointStore}} tests only work with high-availability backends that keep a list of checkpoints in memory. If the backend persists and retrieves data from another source the tests will fail. It's fairly simple to change the tests to lift this requirement though. See [this gist|https://gist.github.com/bremac/1b3365bc0257dfbd33bcd0b7a7627c00] for an example.

We've got patches for the points above that we'd be happy to contribute. :)



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