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From "Stefan Podkowinski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13569) Schedule schema pulls just once per endpoint
Date Mon, 19 Jun 2017 08:48:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13569?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16053634#comment-16053634
] 

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

At this point it's probably more about personal coding preferences. What's bugging me a bit
about your version is that you're making the assumption that once scheduled, the runnable
will always be executed and thus will cleanup itself. But as tasks may get canceled, this
doesn't always have to be the case. Looking at ScheduledExecuter, this shouldn't be a problem,
but I'd rather have this implicitly  handled in the code to make it obvious and less brittle
in case of potential changes in the executor behavior. But again, it's probably more a matter
of personal coding style and both versions should work in practice. 



> 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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