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From "Guozhang Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-6555) Making state store queryable during restoration
Date Thu, 01 Mar 2018 00:07:00 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-6555?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16381278#comment-16381278
] 

Guozhang Wang commented on KAFKA-6555:
--------------------------------------

First of all, as I mentioned before this change itself deserves a KIP discussion given its
impact and scope so that we can bring this to a more broad audience's attention for feedbacks.

And here is my two cents: although this ticket may be orthogonal to 6144 and 6145, they are
trying to tackle the same general issue, which is during a rebalance, the stores may be unavailable
for queries during restoration, and how long that unavailability would be depends on whether
or not you have standby replicas, and if the rebalance is triggered by fail-over or scaling
out, etc. But ultimately we'd like to reduce that unavailability window as much as possible
by trading some data consistency off.

My take then is that, if eventually we are going to support KAFKA-6145 (which I think we should),
then the case of scaling out should be well covered, and hence I'd prefer to just read from
the restoring active task for simplicity, and additionally make it configurable as Damian
suggested. So the situation becomes:

1. For scaling out scenario, KAFKA-6145 will make sure we have a close-to-latest replica when
the actual rebalance happens, so this scenario is effectively reduced to a "controlled fail
over" scenario.
2. For fail-over scenario, we can assume that the restoring state should be the closest to
latest, and hence have a knob to allow users to read stale data from it during restoring to
reduce unavailability gap.

I.e. we will drop KAFKA-6144, and only do this JIRA and KAFKA-6145 by the end of the day.

> 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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