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From Ville Lautanala <lau...@gmail.com>
Subject Re: create only - no update
Date Wed, 16 Dec 2009 20:53:38 GMT

On Dec 16, 2009, at 22:31, gabriele renzi wrote:

> On Wed, Dec 16, 2009 at 12:34 AM, Brandon Williams <driftx@gmail.com> wrote:
>> On Tue, Dec 15, 2009 at 5:19 PM, Brian Burruss <bburruss@real.com> wrote:
>>> 
>>> can the cassandra client (java specifically) specify that a particular
>>> "put" should be "create only, do not update"?  If the value already exists
>>> in the database, i want the put to fail.  for instance, two users want the
>>> exact same username, so they both do a "get" to determine if the username
>>> already exists, it doesn't, so they create.  the last one to create wins,
>>> correct?
>> 
>> Correct.  You would need to implement a locking mechanism such as Zookeeper
>>  in your application to get around this.
> 
> sorry for my usual stupid question, but I'm still trying to wrap my
> head around cassandra...
> is there a fundamental reason for not being able to provide a
> putIfAbsent-like abstraction in cassandra?
> 
> If I understand correctly this can't be done in the case of network
> partitioning  but it could work while the network is not split with an
> operation requiring ConsistencyLevel.ALL. But this seems like it would
> be still the case with any other system.
> So, for unique login ids  in the case of partition there would  be a
> temporary impossibility to register but in the (normal?) non
> partitioned case Brian would still be able to let users register via
> cassandra.

Using decrementing timestamps you could get "first insert wins" behavior. Then you'd have
to check after insert if it really wrote any data using ConsistencyLevel.ALL.

This is a really bad use case for Cassandra and you really should use some external locking
mechanism.

-Ville


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