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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-5619) CAS UPDATE for a lost race: save round trip by returning column values
Date Mon, 01 Jul 2013 08:00:23 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-5619?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Sylvain Lebresne updated CASSANDRA-5619:
----------------------------------------

    Attachment: 5619_thrift_fixup.txt

Damn you thrift. I guess returning an empty list works as well so attaching a simple patch
to do that.

I changed it only on the thrift side, and not in the StorageProxy call cause it felt cleaner
internally to keep null, and I didn't saw the point of allocating an empty CF object in the
CQL3 case.

The patch also make is so that expected.isEmpty() tests existence (like expected == null).
It's what makes sense and I figured that maybe Thrift can't pass null as parameter either
so ...

                
> CAS UPDATE for a lost race: save round trip by returning column values
> ----------------------------------------------------------------------
>
>                 Key: CASSANDRA-5619
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5619
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 2.0 beta 1
>            Reporter: Blair Zajac
>            Assignee: Sylvain Lebresne
>             Fix For: 2.0 beta 1
>
>         Attachments: 5619_thrift_fixup.txt, 5619.txt
>
>
> Looking at the new CAS CQL3 support examples [1], if one lost a race for an UPDATE, to
save a round trip to get the current values to decide if you need to perform your work, could
the columns that were used in the IF clause also be returned to the caller?  Maybe the columns
values as part of the SET part could also be returned.
> I don't know if this is generally useful though.
> In the case of creating a new user account with a given username which is the partition
key, if one lost the race to another person creating an account with the same username, it
doesn't matter to the loser what the column values are, just that they lost.
> I'm new to Cassandra, so maybe there's other use cases, such as doing incremental amount
of work on a row.  In pure Java projects I've done while loops around AtomicReference.html#compareAndSet()
until the work was done on the referenced object to handle multiple threads each making forward
progress in updating the references object.
> [1] https://github.com/riptano/cassandra-dtest/blob/master/cql_tests.py#L3044

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