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From "Stefan Podkowinski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13569) Schedule schema pulls just once per endpoint
Date Fri, 16 Jun 2017 08:25:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13569?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16051582#comment-16051582
] 

Stefan Podkowinski commented on CASSANDRA-13569:
------------------------------------------------

The main point of the patch was to prevent runnables from pilling up in {{maybeScheduleSchemaPull}}
as scheduled by:
{{ScheduledExecutors.nonPeriodicTasks.schedule(runnable, MIGRATION_DELAY_IN_MS, TimeUnit.MILLISECONDS);}}

Your patch is undoing this change and is instead now preventing concurrent execution per endpoint
of MigrationTask. But keep in mind here that MigrationTask itself is short lived and non-blocking.
Using your suggested approach would not be very effective to throttle migration requests and
would probably not help a lot for preventing the problem stated in the first paragraph.


> Schedule schema pulls just once per endpoint
> --------------------------------------------
>
>                 Key: CASSANDRA-13569
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13569
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Distributed Metadata
>            Reporter: Stefan Podkowinski
>            Assignee: Stefan Podkowinski
>             Fix For: 3.0.x, 3.11.x, 4.x
>
>
> Schema mismatches detected through gossip will get resolved by calling {{MigrationManager.maybeScheduleSchemaPull}}.
This method may decide to schedule execution of {{MigrationTask}}, but only after using a
{{MIGRATION_DELAY_IN_MS = 60000}} delay (for reasons unclear to me). Meanwhile, as long as
the migration task hasn't been executed, we'll continue to have schema mismatches reported
by gossip and will have corresponding {{maybeScheduleSchemaPull}} calls, which will schedule
further tasks with the mentioned delay. Some local testing shows that dozens of tasks for
the same endpoint will eventually be executed and causing the same, stormy behavior for this
very endpoints.
> My proposal would be to simply not schedule new tasks for the same endpoint, in case
we still have pending tasks waiting for execution after {{MIGRATION_DELAY_IN_MS}}.



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