[jira] [Created] (FLINK-3761) Introduce key group state backend

[Shang Yuanchun (Jira)]

Till Rohrmann created FLINK-3761:
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             Summary: Introduce key group state backend
                 Key: FLINK-3761
                 URL: https://issues.apache.org/jira/browse/FLINK-3761
             Project: Flink
          Issue Type: Sub-task
          Components: state backends
            Reporter: Till Rohrmann
            Assignee: Till Rohrmann


After an off-line discussion with [~aljoscha], we came to the conclusion that it would be beneficial to reflect the differences between a keyed and a non-keyed stream also in the state backends. A state backend which is used for a keyed stream offers a value, list, folding and value state and has to group its keys into key groups.

A state backend for non-keyed streams can only offer a union state to make it work with dynamic scaling. A union state is a state which is broadcasted to all tasks in case of a recovery. The state backends can then select what information they need to recover from the whole state (formerly distributed).



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