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From "Andy Tolbert (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-10786) Include hash of result set metadata in prepared statement id
Date Wed, 18 May 2016 16:18:12 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-10786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15289228#comment-15289228
] 

Andy Tolbert commented on CASSANDRA-10786:
------------------------------------------

With the issue that [~ifesdjeen] described on the driver end, there are some things to worry
about:

# this will be an issue in the case where a user has an older client install with a newer
C* install and they have this case where altering a table causes the prepared statement hash
to change, causing the repeating prepare as described.
# this could also be a problem for cases of a cluster with mixed versions.  If you have some
nodes at a newer version using this calculation, and others using the previous calculation
you will get different prepared ids depending on which node you hit.   Depending on the driver
implementation this could be an issue.

What do you think about the possibility of tying prepared statement id calculation to protocol
versions?  This way older clients that don't know to handle such a case where the prepared
id can change will continue to work, and newer clients can be updated to support these scenarios
as part of also supporting a new protocol version (as [~ifesdjeen] is doing for the java driver).
  This also would solve #2.    The downside of tying this change to a protocol version bump
is that I imagine it defers this change quite a bit, and this fix would be really nice to
have as it will fix a problem a lot of users have run into.

> Include hash of result set metadata in prepared statement id
> ------------------------------------------------------------
>
>                 Key: CASSANDRA-10786
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10786
>             Project: Cassandra
>          Issue Type: Bug
>          Components: CQL
>            Reporter: Olivier Michallat
>            Assignee: Alex Petrov
>            Priority: Minor
>              Labels: client-impacting
>             Fix For: 3.x
>
>
> This is a follow-up to CASSANDRA-7910, which was about invalidating a prepared statement
when the table is altered, to force clients to update their local copy of the metadata.
> There's still an issue if multiple clients are connected to the same host. The first
client to execute the query after the cache was invalidated will receive an UNPREPARED response,
re-prepare, and update its local metadata. But other clients might miss it entirely (the MD5
hasn't changed), and they will keep using their old metadata. For example:
> # {{SELECT * ...}} statement is prepared in Cassandra with md5 abc123, clientA and clientB
both have a cache of the metadata (columns b and c) locally
> # column a gets added to the table, C* invalidates its cache entry
> # clientA sends an EXECUTE request for md5 abc123, gets UNPREPARED response, re-prepares
on the fly and updates its local metadata to (a, b, c)
> # prepared statement is now in C*’s cache again, with the same md5 abc123
> # clientB sends an EXECUTE request for id abc123. Because the cache has been populated
again, the query succeeds. But clientB still has not updated its metadata, it’s still (b,c)
> One solution that was suggested is to include a hash of the result set metadata in the
md5. This way the md5 would change at step 3, and any client using the old md5 would get an
UNPREPARED, regardless of whether another client already reprepared.



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