[jira] [Issue Comment Edited] (CASSANDRA-1391) Allow Concurrent Schema Migrations

["Gary Dusbabek (JIRA)" <ji...@apache.org>]

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

Gary Dusbabek edited comment on CASSANDRA-1391 at 7/13/11 3:04 PM:
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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 not able to see because the merge-conflict problem makes it a non-starter for me.

      was (Author: gdusbabek):
    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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