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From "Olivier Michallat (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-13992) Don't send new_metadata_id for conditional updates
Date Fri, 03 Nov 2017 21:36:00 GMT
Olivier Michallat created CASSANDRA-13992:
---------------------------------------------

             Summary: 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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