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From "Olivier Michallat (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-13992) Don't send new_metadata_id for conditional updates
Date Mon, 13 Nov 2017 17:19:03 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16249838#comment-16249838
] 

Olivier Michallat edited comment on CASSANDRA-13992 at 11/13/17 5:18 PM:
-------------------------------------------------------------------------

[~ifesdjeen] that would work, the driver can treat an empty {{new_metadata_id}} as "don't
update my local copy". Namely, changing [this line|https://github.com/datastax/java-driver/blob/6eeb8b2193ab5b50b73b0d9a533e775265f11007/driver-core/src/main/java/com/datastax/driver/core/ArrayBackedResultSet.java#L83]
to:
{code}
if (newMetadataId != null && newMetadataId.bytes.length > 0) {
{code}
However that feels kind of hacky. Consider how we would have to explain that in the protocol
spec:
{quote}
        - <new_metadata_id> is \[short bytes] representing the new, changed resultset
           metadata. The new metadata ID must also be used in subsequent executions of
           the corresponding prepared statement, if any, *except if it is empty*.
{quote}
It would make so much more sense to force {{METADATA_CHANGED}} to *false* for conditional
updates, isn't there any way we can do that?


was (Author: omichallat):
[~ifesdjeen] that would work, the driver can treat an empty {{new_metadata_id}} as "don't
update my local copy". Namely, changing [this line|https://github.com/datastax/java-driver/blob/6eeb8b2193ab5b50b73b0d9a533e775265f11007/driver-core/src/main/java/com/datastax/driver/core/ArrayBackedResultSet.java#L83]
to:
{code}
if (newMetadataId != null && newMetadataId.bytes.length > 0) {
{code}
However that feels kind of hacky. Consider how we would have to update the protocol spec to
explain this:
{quote}
        - <new_metadata_id> is \[short bytes] representing the new, changed resultset
           metadata. The new metadata ID must also be used in subsequent executions of
           the corresponding prepared statement, if any, *except if it is empty*.
{quote}
It would make so much more sense to force {{METADATA_CHANGED}} to *false* for conditional
statements, isn't there any way we can do that?

> 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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