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Date: Thu, 24 Apr 2014 14:11:17 +0000 (UTC)
From: "Benedict (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (CASSANDRA-6106) Provide timestamp with true
 microsecond resolution
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    [ https://issues.apache.org/jira/browse/CASSANDRA-6106?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13979758#comment-13979758 ] 

Benedict commented on CASSANDRA-6106:
-------------------------------------

Well, what I should have made clear is that I am willing to drop the monotonicity guarantees, however I am -1 on your extra thread.

But I still think the monotonicity guarantees are good, and not so difficult to prove, so if we can get somebody who doesn't have a newborn to contend with to take a look maybe that wouldn't be a bad thing :)

In case it helps, here's a quick proof we can never give a whack value:

{noformat}
1. -100000<= adjustMicros<=100000
2. expire-adjustFrom=1000000000
2a. expireMicros-adjustFromMicros=1000000
3. adjustFromMicros<=micros<=expireMicros
4. delta = (adjustMicros * (micros-adjustFromMicros)) / (expireMicros-adjustFromMicros)
5. 2a ^ 3 ^ 4 -> expireMicros-adjustFromMicros > micros-adjustFromMicros -> |delta| <= |adjustMicros|
{noformat}

i.e. the adjustment is definitely always less than adjustMicros, which is itself always less than 100ms per second (per 1 and 2). So we can never give a totally whack result. Can do more thorough proofs of other criteria, but I think this plus my other statement is enough to demonstrate its safety.

> Provide timestamp with true microsecond resolution
> --------------------------------------------------
>
>                 Key: CASSANDRA-6106
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6106
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>         Environment: DSE Cassandra 3.1, but also HEAD
>            Reporter: Christopher Smith
>            Assignee: Benedict
>            Priority: Minor
>              Labels: timestamps
>             Fix For: 2.1 beta2
>
>         Attachments: microtimstamp.patch, microtimstamp_random.patch, microtimstamp_random_rev2.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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