Navinder Brar created KAFKA-9445:
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Summary: Allow fetching a key from a single partition rather than
iterating over all the stores on an instance
Key: KAFKA-9445
URL: https://issues.apache.org/jira/browse/KAFKA-9445
Project: Kafka
Issue Type: New Feature
Components: streams
Reporter: Navinder Brar
Assignee: Navinder Brar
Currently when expanding the KS cluster, the new node's partitions will be
unavailable during the rebalance, which for large states can take a very long
time, or for small state stores even more than a few ms can be a deal-breaker
for micro service use cases.
One workaround is to allow stale data to be read from the state stores when use
case allows. Adding the use case from KAFKA-8994 as it is more descriptive.
"Consider the following scenario in a three node Streams cluster with node A,
node S and node R, executing a stateful sub-topology/topic group with 1
partition and `_num.standby.replicas=1_`
* *t0*: A is the active instance owning the partition, B is the standby that
keeps replicating the A's state into its local disk, R just routes streams IQs
to active instance using StreamsMetadata
* *t1*: IQs pick node R as router, R forwards query to A, A responds back to R
which reverse forwards back the results.
* *t2:* Active A instance is killed and rebalance begins. IQs start failing to
A
* *t3*: Rebalance assignment happens and standby B is now promoted as active
instance. IQs continue to fail
* *t4*: B fully catches up to changelog tail and rewinds offsets to A's last
commit position, IQs continue to fail
* *t5*: IQs to R, get routed to B, which is now ready to serve results. IQs
start succeeding again
Depending on Kafka consumer group session/heartbeat timeouts, step t2,t3 can
take few seconds (~10 seconds based on defaults values). Depending on how laggy
the standby B was prior to A being killed, t4 can take few seconds-minutes.
While this behavior favors consistency over availability at all times, the long
unavailability window might be undesirable for certain classes of applications
(e.g simple caches or dashboards).
This issue aims to also expose information about standby B to R, during each
rebalance such that the queries can be routed by an application to a standby to
serve stale reads, choosing availability over consistency."
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