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From "Pavel Yaskevich (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-1391) Allow Concurrent Schema Migrations
Date Mon, 14 Nov 2011 15:38:52 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-1391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13149691#comment-13149691
] 

Pavel Yaskevich commented on CASSANDRA-1391:
--------------------------------------------

It seems like what we really want from migrations is "schema state" before any given migration
and actual modifications migration makes like "add keyspace <ks> with attributes = ...",
"update <cf> with attributes = ...." 

As all of the migrations are user initiated we can easily calculate what modifications migration
makes and propagate only them keeping TimeUUID as ID of the migration to identify "apply"
order. As it's okay for us to require full cluster update before accepting schema modifications
it makes merge a trivial task where modifications should be applied one-by-one on some "initial
state" of the schema (that also allows as to remove Avro overhead from migrations). Abandoning
Avro would make things less fragile because there would be no need to modify CFMetaData or
any other classes to support new (or deleted) attributes.
                
> Allow Concurrent Schema Migrations
> ----------------------------------
>
>                 Key: CASSANDRA-1391
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1391
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 0.7.0
>            Reporter: Stu Hood
>            Assignee: Pavel Yaskevich
>             Fix For: 1.1
>
>         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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