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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 Tue, 02 Jul 2013 08:24:20 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

bq. How do we differentiate now between "it didn't work, because the row doesn't exist and
you specified non-empty columns" and "it did work?"

Right, that doesn't work. Well, the only solution I see is to create a special:
{noformat}
struct CASResult {
    1: required bool success,
    2: optional list<Column> current_values,
}
{noformat}
and to return that from the CAS in thrift. I updated the patch to do that (and updated the
test_cas accordingly).

bq. -0 on allowing both null and empty in parameters

Fair enough. But I'm -1 on allowing null but not empty because that would be extra weird API
wise, so changed the patch to only allow empty. I guess this is consistent with the rest of
the Thrift API were we don't allow null anywhere.
bq. How do we differentiate now between "it didn't work, because the row doesn't exist and
you specified non-empty columns" and "it did work?"

Right, that doesn't work. Well, the only solution I see is to create a special:
{noformat}
struct CASResult {
    1: required bool success,
    2: optional list<Column> current_values,
}
{noformat}
and to return that from the CAS in thrift. I updated the patch to do that (and updated the
test_cas accordingly).

bq. -0 on allowing both null and empty in parameters

Fair enough. But I'm -1 on allowing null but not empty because that would be extra weird API
wise, so changed the patch to only allow empty. I guess this is consistent with the rest of
the Thrift API were we don't allow null anywhere.
                
> 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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