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Date: Wed, 15 Nov 2017 10:11:00 +0000 (UTC)
From: "Kurt Greaves (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (CASSANDRA-13992) Don't send new_metadata_id for
 conditional updates
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archived-at: Wed, 15 Nov 2017 10:11:08 -0000


    [ https://issues.apache.org/jira/browse/CASSANDRA-13992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16253220#comment-16253220 ] 

Kurt Greaves commented on CASSANDRA-13992:
------------------------------------------

> then I have to change the last test to if (METADATA_CHANGED || new_metadata_id == special_value)
In Alex's last version, yes, but if the metadata ID from the prepare and the exec are the same (because they are generated with the same value) this is not the case. You wouldn't have to worry about METADATA_CHANGED being set because a LWT will always have the same ID as the initial preparation. <bikeshedding>

> 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
>            Assignee: Kurt Greaves
>            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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