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https://issues.apache.org/jira/browse/CASSANDRA-3884?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13214634#comment-13214634
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Pavel Yaskevich commented on CASSANDRA-3884:
--------------------------------------------

The answer to your main question is - compaction, everything works well when 
you add new or modify columns but when you e.g. delete cf columns from keyspace 
and compaction kicks in before you grabbed the whole schema that schema will be 
missing updates for that columns so they won't be pushed to the remote nodes 
leaving cf attributes in their schema_columnfamilies.

bq. In MigrationHelper if withSchemaRecord is false the mutations will be null, 
and most function will return a list containing null. We should return an empty 
list instead or null (but in that last case, Migration.apply() should deal with 
null). Also MigrationHelper.dropColumnFamily() directly return null, so we 
should make it match whatever we do for the other method

Sure, I will make it return Collections.singleton()

bq. It's slightly more efficient to use Collections.singleton() than 
Arrays.asList with one element.

Sure, will change it in updated v2.

bq. Why does the tests now need to start gossip?

I have experienced gossip related NPE exceptions (in isEnabled() method for 
example) in the MM.passiveAnnounce method when Gossiper wasn't started.
                
> Intermittent SchemaDisagreementException
> ----------------------------------------
>
>                 Key: CASSANDRA-3884
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3884
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 1.1.0
>         Environment: using ccm on ubuntu. 
>            Reporter: Tyler Patterson
>            Assignee: Pavel Yaskevich
>             Fix For: 1.1.0
>
>         Attachments: CASSANDRA-3884.patch
>
>
> Set up a cluster of two nodes (on cassandra-1.1), create some keyspaces and 
> column families, and then make several schema changes. Everything is being 
> done through only one of the nodes.  About once every 10 times (on my setup) 
> I get a SchemaDisagreementException when creating and dropping keyspaces. 
> There is a dtest for this: schema_changes_test.py. If your environment 
> behaves like mine, you might need to run it 10 times to get the error.

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