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JIRA:  Unable to render Jira issues macro, execution error.

Discussion: here

Motivation

Currently Streams interactive queries (IQ) fail during the time period where there is a rebalance in progress. 

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.

Public Interfaces

  • StreamsMetadata

Proposed Changes

In the current code, t0 and t1 serve queries from Active(Running) partition. For case t2, we are planning to return List<StreamsMetadata> such that it returns <StreamsMetadata(A), StreamsMetadata(B)> so that if IQ fails on A, the replica on B can serve the data by enabling serving from replicas. This still does not solve case t3 and t4 since B has been promoted to active but it is in Restoring state to catchup till A’s last committed position as we don’t serve from Restoring state in active and new replica on R is building itself from scratch. Both these cases can be solved if we start serving from Restoring state of active as well, since it is almost equivalent to previous active.

There could be a case where all replicas of a partition become unavailable and active and all replicas of that partition are building themselves from scratch, in this case state in active is far behind even though it is in Restoring state. To cater to such cases that we don’t serve from this state we can either add another state before Restoring or check difference between last committed offset and current position. Only if it is within a permissible  range(say 10000) we will serve from Restoring state of active.

  • AssignmentInfo changes to include Map<HostInfo, Set<TopicPartition>> replicaPartitionsByHost;  so that each machine knows which machine holds which replica partitions along with the active partitions(which they currently do).

  • Changing singnature of setPartitionsByHostState(partitionsByHostState, replicaPartitionsByHost) and to onChange() and further rebuildMetadata() to add Set<TopicPartition> replicaTopicPartitions in StreamsMetadata. This will add replica partitions in the metadata.

  • To serve queries from this we will add a flag in *StreamsMetadataState::allMetadataForKey(boolean enableReplicaServing, String storeName, K key, Serializer<K> keySerializer) to get from users if they are okay to serve from replicas, default would be false. This function returns a list of StreamsMetadata which contains Active first and then all the replicas in the system for the partition. In allMetadataForKey we would have access to allMetadata containing activePartitions as well as ReplicaPartitions.

Open Questions

  • What if num.standby.replicas=0 or >1

Compatibility, Deprecation, and Migration Plan

This KIP affects StreamsMetadata and AssignmentInfo. Currently, StreamsMetadata is not following any versioning so we might have to upgrade the version for it. Also, since this would include AssignmentInfo changes to add replicaPartitionsByHost, we would need to upgrade AssignmentInfo as well.

Rejected Alternatives

  • None so far.


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