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From "Damian Guy (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-6555) Making state store queryable during restoration
Date Wed, 28 Feb 2018 13:21:00 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-6555?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16380293#comment-16380293

Damian Guy commented on KAFKA-6555:

I think if we are going to allow stale reads from restoring tasks then it probably should
be configurable as this may not always be desirable. Imagine a situation where there is a
lot of data to restore, you could be reading different incorrect values for quite a while.
Sure there may be situations where you don't care, but I'm sure there are situations when
you do care.

> 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
>            Assignee: 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 resulting in the partition
downtime. 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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