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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9131) Defining correct behavior during leap second insertion
Date Fri, 17 Apr 2015 15:00:59 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9131?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14499963#comment-14499963
] 

Benedict commented on CASSANDRA-9131:
-------------------------------------

I can't recall entirely, but I don't think that was quite the way the conversation resolved.
It's not exactly "expected" behaviour on either side, but obviously there's nothing we can
do about clients that are subjected to this bug, and we intend to deprecate server-side timestamps
(if perhaps never eliminate them entirely). So the question is

# if we should fix server side timestamps by making the clock universally monotonically increasing
(as opposed to only per-client connection) which would at least somewhat mitigate this problem
(but not eliminate it entirely, for any sequence of inserts hitting multiple servers)
# if we should update the client protocol spec/docs to make clear that this problem exists,
and that clients are expected to work around it
## if we should offer some sample code, and work with the Java Driver team to ensure this
problem doesn't affect it

> Defining correct behavior during leap second insertion
> ------------------------------------------------------
>
>                 Key: CASSANDRA-9131
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9131
>             Project: Cassandra
>          Issue Type: Bug
>         Environment: Linux ip-172-31-0-5 3.2.0-57-virtual #87-Ubuntu SMP Tue Nov 12 21:53:49
UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
>            Reporter: Jim Witschey
>            Assignee: Jim Witschey
>
> On Linux platforms, the insertion of a leap second breaks the monotonicity of timestamps.
This can make values appear to have been inserted into Cassandra in a different order than
they were. I want to know what behavior is expected and desirable for inserts over this discontinuity.
> From a timestamp perspective, an inserted leap second looks like a repeat of the previous
second:
> {code}
> $ while true ; do echo "`date +%s%N` `date -u`" ; sleep .5 ; done
> 1435708798171327029 Tue Jun 30 23:59:58 UTC 2015
> 1435708798679392477 Tue Jun 30 23:59:58 UTC 2015
> 1435708799187550335 Tue Jun 30 23:59:59 UTC 2015
> 1435708799695670453 Tue Jun 30 23:59:59 UTC 2015
> 1435708799203902068 Tue Jun 30 23:59:59 UTC 2015
> 1435708799712168566 Tue Jun 30 23:59:59 UTC 2015
> 1435708800220473932 Wed Jul 1 00:00:00 UTC 2015
> 1435708800728908190 Wed Jul 1 00:00:00 UTC 2015
> 1435708801237611983 Wed Jul 1 00:00:01 UTC 2015
> 1435708801746251996 Wed Jul 1 00:00:01 UTC 2015
> {code}
> Note that 23:59:59 repeats itself, and that the timestamps increase during the first
time through, then step back down to the beginning of the second and increase again.
> As a result, the timestamps on values inserted during these seconds will be out of order.
I set up a 4-node cluster running under Ubuntu 12.04.3 and synced them to shortly before the
leap second would be inserted. During the insertion of the leap second, I ran a test with
logic something like:
> {code}
> simple_insert = session.prepare(
>     'INSERT INTO test (foo, bar) VALUES (?, ?);')
> for i in itertools.count():
>     # stop after midnight
>     now = datetime.utcnow()
>     last_midnight = now.replace(hour=0, minute=0,
>                                 second=0, microsecond=0)
>     seconds_since_midnight = (now - last_midnight).total_seconds()
>     if 5 <= seconds_since_midnight <= 15:
>         break
>     session.execute(simple_insert, [i, i])
> result = session.execute("SELECT bar, WRITETIME(bar) FROM test;")
> {code}
> EDIT: This behavior occurs with server-generated timestamps; in this particular test,
I set {{use_client_timestamp}} to {{False}}.
> Under normal circumstances, the values and writetimes would increase together, but when
inserted over the leap second, they don't. These {{value, writetime}} pairs are sorted by
writetime:
> {code}
> (582, 1435708799285000)
> (579, 1435708799339000)
> (583, 1435708799593000)
> (580, 1435708799643000)
> (584, 1435708799897000)
> (581, 1435708799958000)
> {code}
> The values were inserted in increasing order, but their writetimes are in a different
order because of the repeated second. During the first instance of 23:59:59, the values 579,
580, and 581 were inserted at the beginning, middle, and end of the second. During the leap
second, which is also 23:59:59, 582, 583, and 584 were inserted, also at the beginning, middle,
and end of the second. However, since the two seconds are the same second, they appear interleaved
with respect to timestamps, as shown above.
> So, should I consider this behavior correct? If not, how should Cassandra correctly handle
the discontinuity introduced by the insertion of a leap second?



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