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From "Alexandre Dutra (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-10786) Include hash of result set metadata in prepared statement id
Date Thu, 19 May 2016 12:24:13 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-10786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15291010#comment-15291010
] 

Alexandre Dutra commented on CASSANDRA-10786:
---------------------------------------------

It seems a bit artificial to draw a relationship between the algorithm for computing statement
hashes and the protocol version.

Granted, it would prevent _existing_ drivers from deadlocking. But because the protocol specs
are not affected directly, I fear that some driver implementors out there could add support
for protocol v5 and actually "forget" that with this version the algorithm changes, and all
of a sudden, all clients using such new drivers would deadlock just the same.

That's why the "opt-in" approach sounded better to me. It could be implemented, as [~ifesdjeen]
stated, as a config option; or alternatively, this could be negotiated between client and
server via the {{OPTIONS}}/{{SUPPORTED}} messages. [~ifesdjeen] correct me if I'm wrong but
as I understand your implementation, it is planned to keep both hashes (with and without metadata)
in memory, so this approach might be doable.

> 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, protocolv5
>             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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