Hi everyone, me again :) I'm still trying to implement my "remoting" layer that allows my clients to see the partitioned Kafka Streams state regardless of which instance they hit. Roughly, my lookup is:
Message get(Key key) {
RemoteInstance instance = selectPartition(key);
return instance.get(key); // http remoting
}
RemoteInstance.get(Key key) { // http endpoint
return readOnlyKeyValueStore.get(key);
}
However, the mapping of partitions to instances may change.
If you call KeyValueStore.get(K) where K is on a partition you
don't own, it returns null. This is indistinguishable from a
successful get on a key that doesn't exist.
If one instance selects a sibling instance right as the partition is failing
off of that instance, it may get routed there and by the time it gets
the request no longer "owns" the partition -- returns a false 'null'.
You can try re-checking after you get a null value, but that's susceptible
to the same race -- it's unlikely but possible that the data migrates *back*
before you do this re-check.
Is there any way to correctly implement this without races? I'd imagine
you need a new primitive like KeyValueStore#get that atomically finds
the key or throws an exception if it is not in an owned partition
at the time of lookup so you know to recheck the partition and retry.
Thoughts?
Thanks again,
Steven
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