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From "Eron Wright (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLINK-8533) Support MasterTriggerRestoreHook state reinitialization
Date Wed, 31 Jan 2018 08:02:00 GMT
Eron Wright  created FLINK-8533:
-----------------------------------

             Summary: Support MasterTriggerRestoreHook state reinitialization
                 Key: FLINK-8533
                 URL: https://issues.apache.org/jira/browse/FLINK-8533
             Project: Flink
          Issue Type: Bug
          Components: State Backends, Checkpointing
    Affects Versions: 1.3.0
            Reporter: Eron Wright 
            Assignee: Eron Wright 


{{MasterTriggerRestoreHook}} enables coordination with an external system for taking or restoring
checkpoints. When execution is restarted from a checkpoint, {{restoreCheckpoint}} is called
to restore or reinitialize the external system state. There's an edge case where the external
state is not adequately reinitialized, that is when execution fails _before the first checkpoint_.
In that case, the hook is not invoked and has no opportunity to restore the external state
to initial conditions.

The impact is a loss of exactly-once semantics in this case. For example, in the Pravega source
function, the reader group state (e.g. stream position data) is stored externally. In the
normal restore case, the reader group state is forcibly rewound to the checkpointed position.
In the edge case where no checkpoint has yet been successful, the reader group state is not
rewound and consequently some amount of stream data is not reprocessed.

A possible fix would be to introduce an {{initializeState}} method on the hook interface.
Similar to {{CheckpointedFunction::initializeState}}, this method would be invoked unconditionally
upon hook initialization. The Pravega hook would, for example, initialize or forcibly reinitialize
the reader group state.    



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