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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7979) Acceptable time skew for C*
Date Fri, 19 Sep 2014 06:26:34 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14140078#comment-14140078
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Benedict commented on CASSANDRA-7979:
-------------------------------------

I think this is an excellent idea. We can ensure low cost by only taking the minimum delta
for all columns in a given update. Once we deliver improved latency metrics there will be
effectively no cost/competition for maintaining this, but in the meantime this will keep it
low enough.

> Acceptable time skew for C*
> ---------------------------
>
>                 Key: CASSANDRA-7979
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7979
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: sankalp kohli
>            Assignee: sankalp kohli
>            Priority: Minor
>
> It is very hard to know the bounds on clock skew required for C* to work properly. Since
the resolution is based on time and is at thrift column level, it depends on the application.
How fast is the application updating the same column. If you update a column say after 5 millisecond
and the clock skew is more than that, you might not see the updates in correct order. 
> In this JIRA, I am proposing a change which will answer this question: "How much clock
skew is acceptable for a given application". This will help answer the question whether the
system needs some alternate NTP algorithms to keep time in sync. 
> If we measure the time difference between two updates to the same column,  we will be
able to answer the question on clock skew. 
> We can implement this in memtable(AtomicSortedColumns.addColumn). If we find that a column
is updated within say 100 millisecond, add the diff to a histogram. Since this might have
performance issues, we might want to have some throttling like randomization or only enable
it for a small time via nodetool. 
> With this histogram, we will know what is an acceptable clock skew. 
> Also apart from column resolution, is there any other area which will be affected by
clock skew? 
> Note: For the sake of argument, I am not talking about back date deletes or application
modified timestamps. 



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