Matthias J. Sax created KAFKA-6730:
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Summary: Simplify state store recovery
Key: KAFKA-6730
URL: https://issues.apache.org/jira/browse/KAFKA-6730
Project: Kafka
Issue Type: Improvement
Components: streams
Reporter: Matthias J. Sax
Fix For: 1.2.0
In the current code base, we restore state stores in the main thread (in
contrast to older code that did restore state stored in the rebalance call
back). This has multiple advantages and allows us the further simplify restore
code.
In the original code base, during a long restore phase, it was possible that a
instance misses a rebalance and drops out of the consumer group. To detect this
case, we apply a check during the restore phase, that the end-offset of the
changelog topic does not change. A changed offset implies a missed rebalance as
another thread started to write into the changelog topic (ie, the current
thread does not own the task/store/changelog-topic anymore).
With the new code, that restores in the main-loop, it's ensured that poll() is
called regularly and thus, a rebalance will be detected automatically. This
make the check about an changing changleog-end-offset unnecessary.
We can simplify the restore logic, to just consuming until `pol()` does not
return any data. For this case, we fetch the end-offset to see if we did fully
restore. If yes, we resume processing, if not, we continue the restore.
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