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From "Jason Brown (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-11547) Add background thread to check for clock drift
Date Fri, 22 Apr 2016 14:48:12 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-11547?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15254043#comment-15254043
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Jason Brown edited comment on CASSANDRA-11547 at 4/22/16 2:47 PM:
------------------------------------------------------------------

Hmm, starting to think I should have titled this ticket referencing "clock skew" rather than
"clock drift". "drift" seems to indicate capture the minuscule differences that arise over
time, rather than the more generic "skew", which would cover all clock differences, regardless
of size. It is the larger differences, measured in minutes, for which this ticket was indicated
to catch.


was (Author: jasobrown):
Hmm, starting to think I should have titled this ticket referencing "clock skew" rather than
"clock drift". "drift" seems to indicate capture the minuscule differences that arise over
time, rather than the more generic "skew", which would cover all clock differences, regardless
of size. (It is the larger differences, measured in minutes, for which this ticket was indicated
to catch.)

> Add background thread to check for clock drift
> ----------------------------------------------
>
>                 Key: CASSANDRA-11547
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11547
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Jason Brown
>            Assignee: Jason Brown
>            Priority: Minor
>              Labels: clocks, time
>
> The system clock has the potential to drift while a system is running. As a simple way
to check if this occurs, we can run a background thread that wakes up every n seconds, reads
the system clock, and checks to see if, indeed, n seconds have passed. 
> * If the clock's current time is less than the last recorded time (captured n seconds
in the past), we know the clock has jumped backward.
> * If n seconds have not elapsed, we know the system clock is running slow or has moved
backward (by a value less than n)
> * If (n + a small offset) seconds have elapsed, we can assume we are within an acceptable
window of clock movement. Reasons for including an offset are the clock checking thread might
not have been scheduled on time, or garbage collection, and so on.
> * If the clock is greater than (n + a small offset) seconds, we can assume the clock
jumped forward.
> In the unhappy cases, we can write a message to the log and increment some metric that
the user's monitoring systems can trigger/alert on.



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