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From "Kelvin Kakugawa (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-580) vector clock support
Date Tue, 01 Dec 2009 22:51:20 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-580?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12784493#action_12784493
] 

Kelvin Kakugawa commented on CASSANDRA-580:
-------------------------------------------

Ah, I see what you were going after.  That's a more concise interface change.

My understanding about including a timestamp along w/ the logical clock is to limit the potential
growth of the vector.  Basically, whenever a new node updates a value, the vector size grows
by one.  However, the problem is that if many different nodes happen to update a given value
(for various reasons--failure scenarios, etc.), the potential size of a vector could grow
to an unmanageable length and it would keep that length forever.  So, the Dynamo authors chose
to tag each update with a timestamp, so they could truncate the vector to only the last 10
nodes to update the value.  There is a possibility that an inconsistency could arise, because
of the truncation.  However, the paper said in practice, it was a non-issue.

In summary, the timestamp is not there to help resolve consistency problems, it's there to
make the vector more manageable.

> vector clock support
> --------------------
>
>                 Key: CASSANDRA-580
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-580
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core
>         Environment: N/A
>            Reporter: Kelvin Kakugawa
>            Assignee: Kelvin Kakugawa
>         Attachments: 580-interface-1-add-vector-clock.diff, 580-interface-2-add-vector-clock.diff,
580-thrift-v3.patch
>
>   Original Estimate: 672h
>  Remaining Estimate: 672h
>
> Allow a ColumnFamily to be versioned via vector clocks, instead of long timestamps. 
Purpose: enable incr/decr; flexible conflict resolution.

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