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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 the standby 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 all standby 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  permissible range (say 10000) we will serve from Restoring state of active.

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

• Changing singnature of setPartitionsByHostState(partitionsByHostState, replicaPartitionsByHoststandbyPartitionsByHost) and to onChange() and further rebuildMetadata() to add Set<TopicPartition> replicaTopicPartitionsstandbyTopicPartitions in StreamsMetadata. This will add replica add standby 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 replicasstandbys, default would be false. This function returns a list of StreamsMetadata which contains Active first and then all the replicas the standbys in the system for the partition. In allMetadataForKey we would have access to allMetadata containing activePartitions as well as ReplicaPartitions.

Open Questions

We need to distinguish between genuine state Restoration (in case a replica becomes active) and Buildup of state from Scratch since we can serve data from the prior but not from the latter.  Let's discuss this case by case basis.

What if

• num.standby.replicas=0

or >1

• : The system should be unavailable for reads even if there is 1 failure. Since now this partition has to recreate the whole state from scratch.

• 
  

   num.standby.replicas=1 : In case of 1 failure, we can serve from the available replica (if it doesn't lag too much) till the available replica is promoted to active and from this point we can start serving from this new active(which will undergo state changes from restoring to running).

• num.standby.replicas>1 : If the user decides to shutdown all 3 instances, then only outcome should be unavailability until all of them come back or state is rebuilt on other nodes in the cluster. In normal operations, f <= 2 and when a failure does happen we can provide a list of all available in-sync standbys which don't lag too much and user can decide to query from any of those.

Future Enhancements

Once KIP-441 is completed, the lag on each of the standby would be available in the assignment itself. This would enable us to add only those standby's in the metadata which have lag less than 10000 from the active as well send a sorted list back as a result which contains active node first and then the standbys sorted by freshness of data they have.

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 farAdding a StreamsConfig to have a universal enableReplicaServing flag for the application. This would restrict the functionality as there could be multiple stores in an application and we need to have the flexibility to extend different consistency guarantees in such cases, which would be restricted by having a StreamsConfig.