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Sylvain Lebresne commented on CASSANDRA-5619:
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I suppose we could change cas() to return a ColumnFamily instead of a boolean.
We would then return null to mean that the CAS applied (in that case all
conditions were met so there is not need to return anything), and if the result
is not null then it would contain the current value for the condition (and thus
would mean the CAS didn't applied).
We would carry that on to thrift and CQL pretty much directly. For CQL3 in
particular, it would replace the current boolean. Instead checking if the CAS
is successful would just be checking if the resultSet is empty or not.
Sounds reasonable?
> 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
> Reporter: Blair Zajac
>
> 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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