[jira] [Created] (FLINK-12121) Use composition instead of inheritance for the InternalKeyContext logic in backend

[Shang Yuanchun (Jira)]

Yu Li created FLINK-12121:
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             Summary: Use composition instead of inheritance for the InternalKeyContext logic in backend
                 Key: FLINK-12121
                 URL: https://issues.apache.org/jira/browse/FLINK-12121
             Project: Flink
          Issue Type: Improvement
          Components: Runtime / State Backends
            Reporter: Yu Li
            Assignee: Yu Li


Commonly it's [recommended|https://stackoverflow.com/questions/2399544/difference-between-inheritance-and-composition] to favor composition over inheritance in java design, but currently in keyed backend we're using inheritance for the {{InternalKeyContext}} logic, and here we propose to change to the composition way.

Another advantage of changing to the composition way is that we could remove the requirement of a heap backend instance when constructing {{HeapRestoreOperation}}, and further making sure all fields are final when constructing the {{HeapKeyedStateBackend}}



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