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From "Gary Dusbabek (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-1391) Allow Concurrent Schema Migrations
Date Wed, 13 Jul 2011 14:21:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-1391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13064581#comment-13064581
] 

Gary Dusbabek commented on CASSANDRA-1391:
------------------------------------------

Bottom line: we need to solve the problem of how to handle conflicts.  I view this as a similar
problem of handling merge conflicts in change sets: a good subset of the conflicts can be
merged automatically because they are independent of each other.  But every once a while there
is a conflict that needs a manual edit-the solution is not computable because it isn't deterministic.

This is currently addressed right now by strictly enforcing the relationship of a migration
with its predecessor to ensure that all migrations are applied serially.

There is probably a pragmatic approach that I'm able to see because the merge-conflict problem
makes it a non-starter for me.

> Allow Concurrent Schema Migrations
> ----------------------------------
>
>                 Key: CASSANDRA-1391
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1391
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>            Assignee: Gary Dusbabek
>
> CASSANDRA-1292 fixed multiple migrations started from the same node to properly queue
themselves, but it is still possible for migrations initiated on different nodes to conflict
and leave the cluster in a bad state. Since the system_add/drop/rename methods are accessible
directly from the client API, they should be completely safe for concurrent use.
> It should be possible to allow for most types of concurrent migrations by converting
the UUID schema ID into a VersionVectorClock (as provided by CASSANDRA-580).

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