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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-1391) Allow Concurrent Schema Migrations
Date Tue, 06 Sep 2011 18:13:12 GMT


Jonathan Ellis commented on CASSANDRA-1391:

bq. if we detect that current migration is outdated and should be merged we do the following

Why do we need to do two code paths here?

My preference would be to model this on our Row conflict resolution: for rows, we have a single
code path where distinct columns are simply merged, and for conflicting columns we pick a
winner based on user-provided timestamp and, if necessary, value contents.  So the result
is guaranteed to be the same on all replicas no matter what order updates were received in.

Similarly, I'd like to see schemas merge field-at-a-time in KSMetadata/CFMetadata, with commutative
conflict resolution.  (I suggested byte ordering of the field contents; Gary suggested using
some clock value created by coordinator.)

Seems to me this would make the "isolation" complexity go away.

> Allow Concurrent Schema Migrations
> ----------------------------------
>                 Key: CASSANDRA-1391
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>            Assignee: Pavel Yaskevich
>             Fix For: 1.0
>         Attachments: CASSANDRA-1391.patch
> 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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