ifndef-SleePy opened a new pull request #10332: [FLINK-13905][checkpointing] 
Separate checkpoint triggering into several asynchronous stages
URL: https://github.com/apache/flink/pull/10332
 
 
   ## What is the purpose of the change
   
   * This PR aims to separating checkpoint triggering into several asynchronous 
stages.
   * This is the heart part of threading model refactoring.
   
   ## Brief change log
   
   * Unify all error handling of checkpoint failure.
   * Change the semantic from queued to suspended if there are too many 
in-flight checkpoints.
   * Separate the triggering from synchronous to asynchronous.
     * The last commit is a bit large although I have done some preparations in 
the prior and this PR :(
     * Introduces an `isTriggering` variable to avoid competition between 
in-flight checkpoint trigger and the later one. Because we have to keep the 
order of triggering and avoid concurrent competition.
     * Introduces a `triggerRequestQueue` which is used to queue the trigger 
request which can't be triggered at that moment.
     * There are only two exits of triggering, `onTriggerSuccess` and 
`OnTriggerFailure`. Either of them must be called at the end of triggering 
lifecycle. In there, we maintain the status of `isTriggering`.
     * Do not invoke 
`CheckpointFailureManager#handleJobLevelCheckpointException` when trigger 
failure. It's useless for now. And it's also a blocking operation to get the 
current checkpoint id.
     * There are lots of test cases need to change because `triggerCheckpoint` 
becomes an asynchronous operation. We need to manually trigger the asynchronous 
trigger stages.
     * `triggerCheckpoint` will not throw exception directly even it fails at 
the very beginning. All error handling outside checkpoint coordinator should be 
through return value of `triggerCheckpoint`. In this way, we don't need "try 
catch" everywhere.
   
   ## Verifying this change
   
   * There are a lot of existing test cases and this change also added tests
   
   ## 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: it should be no
     - 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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