...

Map<HostInfo, List<StoreLagInfo>> globalLagInformation = // collected by gossiping amongst the Streams App instances, using the local lag APIs in each. 
K key = // key which needs to be routed.
Collection<StreamsMetadata> hostMetadata = // active + standby replicas hosting the key , obtained by calling metadataForKey() and standbyMetadataForKey()

// filter out all the hosts with more than acceptable lag
Collection<StreamsMetdata>List<StreamsMetdata> inSyncHostMetadata = filter(hostMetadata, globalLagInforation)hostMetadata.stream().filter(metadata -> {
      List<StoreLagInfo> lagsForHost = globalLagInforation.get(metadata.hostInfo())
      ..
}).collect(Collectors.toList()); 

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 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.

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