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From "Christopher Smith (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6106) QueryState.getTimestamp() & FBUtilities.timestampMicros() reads current timestamp with System.currentTimeMillis() * 1000 instead of System.nanoTime() / 1000
Date Fri, 27 Sep 2013 03:11:07 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6106?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13779600#comment-13779600
] 

Christopher Smith commented on CASSANDRA-6106:
----------------------------------------------

Jonathan:
If you look at my patch, it calibrates against currentTimeMillis() so that you do get about
as proper a "microseconds since the epoch" as is possible. The one thing you could do to improve
it would be to periodically recalibrate with currentTimeMillis, but I'd argue that is actually
a *bad* thing, as that would introduce the possibility of timestamps that go back in time.

What this does is dramatically reduce the probability that two concurrent writes sent to two
different nodes will collide (and therefore Cassandra violates its atomicity "guarantee").
                
> QueryState.getTimestamp() & FBUtilities.timestampMicros() reads current timestamp
with System.currentTimeMillis() * 1000 instead of System.nanoTime() / 1000
> ------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-6106
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6106
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: DSE Cassandra 3.1, but also HEAD
>            Reporter: Christopher Smith
>            Priority: Minor
>              Labels: collision, conflict, timestamp
>         Attachments: microtimstamp.patch
>
>
> I noticed this blog post: http://aphyr.com/posts/294-call-me-maybe-cassandra mentioned
issues with millisecond rounding in timestamps and was able to reproduce the issue. If I specify
a timestamp in a mutating query, I get microsecond precision, but if I don't, I get timestamps
rounded to the nearest millisecond, at least for my first query on a given connection, which
substantially increases the possibilities of collision.
> I believe I found the offending code, though I am by no means sure this is comprehensive.
I think we probably need a fairly comprehensive replacement of all uses of System.currentTimeMillis()
with System.nanoTime().

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