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https://issues.apache.org/jira/browse/CASSANDRA-10786?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Alex Petrov updated CASSANDRA-10786:
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Reviewer: (was: Tyler Hobbs)
> 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: Sub-task
> Components: CQL
> Reporter: Olivier Michallat
> Assignee: Alex Petrov
> Priority: Minor
> Labels: client-impacting, doc-impacting, protocolv5
> Fix For: 4.x
>
>
> *_Initial description:_*
> 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.
> -----
> *_Resolution (2017/02/13):_*
> The following changes were made to native protocol v5:
> - the PREPARED response includes {{result_metadata_id}}, a hash of the result
> set metadata.
> - every EXECUTE message must provide {{result_metadata_id}} in addition to
> the prepared statement id. If it doesn't match the current one on the server,
> it means the client is operating on a stale schema.
> - to notify the client, the server returns a ROWS response with a new
> {{Metadata_changed}} flag, the new {{result_metadata_id}} and the updated
> result metadata (this overrides the {{No_metadata}} flag, even if the client
> had requested it)
> - the client updates its copy of the result metadata before it decodes the
> results.
> So the scenario above would now look like:
> # {{SELECT * ...}} statement is prepared in Cassandra with md5 abc123, and
> result set (b, c) that hashes to cde456
> # column a gets added to the table, C* does not invalidate its cache entry,
> but only updates the result set to (a, b, c) which hashes to fff789
> # client sends an EXECUTE request for (statementId=abc123, resultId=cde456)
> and skip_metadata flag
> # cde456!=fff789, so C* responds with ROWS(..., no_metadata=false,
> metadata_changed=true, new_metadata_id=fff789,col specs for (a,b,c))
> # client updates its column specifications, and will send the next execute
> queries with (statementId=abc123, resultId=fff789)
> This works the same with multiple clients.
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