Kamal Chandraprakash commented on KAFKA-6106:

[~guozhang] I was busy in other tasks. I try to complete it by this weekend.

> Postpone normal processing of tasks within a thread until restoration of all 
> tasks have completed
> -------------------------------------------------------------------------------------------------
>                 Key: KAFKA-6106
>                 URL: https://issues.apache.org/jira/browse/KAFKA-6106
>             Project: Kafka
>          Issue Type: Improvement
>          Components: streams
>    Affects Versions:, 1.0.0
>            Reporter: Guozhang Wang
>            Assignee: Kamal Chandraprakash
>            Priority: Major
>              Labels: newbie++
> Let's say a stream thread hosts multiple tasks, A and B. At the very 
> beginning when A and B are assigned to the thread, the thread state is 
> {{TASKS_ASSIGNED}}, and the thread start restoring these two tasks during 
> this state using the restore consumer while using normal consumer for 
> heartbeating.
> If task A's restoration has completed earlier than task B, then the thread 
> will start processing A immediately even when it is still in the 
> {{TASKS_ASSIGNED}} phase. But processing task A will slow down restoration of 
> task B since it is single-thread. So the thread's transition to {{RUNNING}} 
> when all of its assigned tasks have completed restoring and now can be 
> processed will be delayed.
> Note that the streams instance's state will only transit to {{RUNNING}} when 
> all of its threads have transit to {{RUNNING}}, so the instance's transition 
> will also be delayed by this scenario.
> We'd better to not start processing ready tasks immediately, but instead 
> focus on restoration during the {{TASKS_ASSIGNED}} state to shorten the 
> overall time of the instance's state transition.

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