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https://issues.apache.org/jira/browse/CASSANDRA-13992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16241847#comment-16241847
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Kurt Greaves commented on CASSANDRA-13992:
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bq. The next EXECUTE is sent with SKIP_METADATA = true, but the server appears
to ignore that
I believe this is because METADATA_CHANGED will take precedence. If C* thinks
the metadata changed it will set the METADATA_CHANGED flag and the driver
should need to update it's metadata. TBH this isn't super clear from the spec
but appears to be what the code achieves
[here|https://github.com/apache/cassandra/blob/922dbdb658b1693973926026b213153d05b4077c/src/java/org/apache/cassandra/transport/messages/ExecuteMessage.java#L174].
I may have no idea what I'm talking about but I think the simplest solution to
bq. never send a new_metadata_id in for a conditional update.
would be to simply always use the same digest for any LWT.
I think the following patch achieves this without breaking anything but I
haven't confirmed if it actually fixes the driver issue yet. If someone with
more understanding of the protocol and what not could have a glance and let me
know if this makes sense or point me in the right direction.
[trunk|https://github.com/apache/cassandra/compare/trunk...kgreav:13992-trunk]
> Don't send new_metadata_id for conditional updates
> --------------------------------------------------
>
> Key: CASSANDRA-13992
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13992
> Project: Cassandra
> Issue Type: Bug
> Reporter: Olivier Michallat
> Priority: Minor
>
> This is a follow-up to CASSANDRA-10786.
> Given the table
> {code}
> CREATE TABLE foo (k int PRIMARY KEY)
> {code}
> And the prepared statement
> {code}
> INSERT INTO foo (k) VALUES (?) IF NOT EXISTS
> {code}
> The result set metadata changes depending on the outcome of the update:
> * if the row didn't exist, there is only a single column \[applied] = true
> * if it did, the result contains \[applied] = false, plus the current value
> of column k.
> The way this was handled so far is that the PREPARED response contains no
> result set metadata, and therefore all EXECUTE messages have SKIP_METADATA =
> false, and the responses always include the full (and correct) metadata.
> CASSANDRA-10786 still sends the PREPARED response with no metadata, *but the
> response to EXECUTE now contains a {{new_metadata_id}}*. The driver thinks it
> is because of a schema change, and updates its local copy of the prepared
> statement's result metadata.
> The next EXECUTE is sent with SKIP_METADATA = true, but the server appears to
> ignore that, and still sends the metadata in the response. So each response
> includes the correct metadata, the driver uses it, and there is no visible
> issue for client code.
> The only drawback is that the driver updates its local copy of the metadata
> unnecessarily, every time. We can work around that by only updating if we had
> metadata before, at the cost of an extra volatile read. But I think the best
> thing to do would be to never send a {{new_metadata_id}} in for a conditional
> update.
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