Hi Steven,
Do you know beforehand if a key exists? If you know that and are getting null()
the code will have to retry by refreshing the metadata and going to the new
instance. If you don’t know beforehand if a key exists or not you might have to
check all instances of a store to make sure.
Eno
> On Jun 5, 2017, at 10:12 PM, Steven Schlansker <sschlans...@opentable.com>
> wrote:
>
> 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
>
