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Alexandre Dutra commented on CASSANDRA-10786:
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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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