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From "Sylvain Lebresne (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-5619) CAS UPDATE for a lost race: save round trip by returning column values
Date Sat, 06 Jul 2013 12:43:48 GMT


Sylvain Lebresne commented on CASSANDRA-5619:

bq. The problem with adding a column to the resultset is that it can conflict with a user-defined
column of the same name

I agree. Though for the record I'll note that we already have that problem in another context.
Namely, when you prepare a timestamp or a ttl, in the metadata returned, we use [timestamp]
and [ttl] for the name of the prepared variables returned (since CQL does return the column
names for the prepared variables in that case), which can conflict in theory with existing
column names. So anyway, not saying this is something we should do too often, but just wanted
to mention it.

bq. an exception for failure at the protocol level

I can live with that, I'll rework the patch that way. Do you want to change it on the thrift
side (I'd rather not bother personally since as you said earlier, nobody uses raw thrift anyway,
but I'm fine changing it if that's you preference).
> CAS UPDATE for a lost race: save round trip by returning column values
> ----------------------------------------------------------------------
>                 Key: CASSANDRA-5619
>                 URL:
>             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: 0001-Add-back-boolean-result-column.txt, 5619_thrift_fixup.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]

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