Ashish Surana commented on KAFKA-6555:

Hi Matthias,

  Thanks for pointing to KAFKA-6144. As I go through it, it looks very similar 
ticket but with minor difference:
 * I am suggesting to allow stale reads only from PARTITION_ASSIGNED (not from 
PARTITION_REVOKED) primarily as it's going to be the one in RUNNING state, and 
this is the minimum we need to do keep serving request for this partition. We 
still have one instance doing write or read and still want to have pure standby 
replicas. This approach is good if we continue with current design of one 
active write/read instance.

I have made the changes, and can share it in few days.

> Making state store queryable during restoration
> -----------------------------------------------
>                 Key: KAFKA-6555
>                 URL: https://issues.apache.org/jira/browse/KAFKA-6555
>             Project: Kafka
>          Issue Type: Improvement
>          Components: streams
>            Reporter: Ashish Surana
>            Priority: Major
> State store in Kafka streams are currently only queryable when StreamTask is 
> in RUNNING state. The idea is to make it queryable even in the RESTORATION 
> (PARTITION_ASSIGNED) state as the time spend on restoration can be huge and 
> making the data inaccessible during this time could be downtime not suitable 
> for many applications.
> When the active partition goes down then one of the following occurs:
>  # One of the standby replica partition gets promoted to active: Replica task 
> has to restore the remaining state from the changelog topic before it can 
> become RUNNING. The time taken for this depends on how much the replica is 
> lagging behind. During this restoration time the state store for that 
> partition is currently not queryable giving making the partition down. We can 
> make the state store partition queryable for the data already present in the 
> state store.
>  # When there is no replica or standby task, then active task will be started 
> in one of the existing node. That node has to build the entire state from the 
> changelog topic which can take lot of time depending on how big is the 
> changelog topic, and keeping state store not queryable during this time is 
> the downtime for the parition.
> It's very important improvement as it could simply improve the availability 
> of microservices developed using kafka streams.
> I am working on a patch for this change. Any feedback or comments are welcome.

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