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From Sriram Srinivasan <sri...@malhar.net>
Subject Re: Atomic Compare and Swap
Date Mon, 21 Jun 2010 04:47:37 GMT

I too am interested in a CAS facility.

I like Rishi's proposal. One could simply use a version number as the  
logical timestamp. If we promote CAS to a consistency level, it would  
rate higher than a quorum. One pays the price for a more complex write  
path to obtain the requisite guarantee.


On Jun 21, 2010, at 4:03 AM, Rishi Bhardwaj wrote:

>
> Heres another thought I had, if say the user always wrote with  
> quorum (or to all) nodes then can't we implement CAS (compare and  
> swap) assuming that user employs logical timestamp and Cassandra  
> doesn't allow writes to a column with same or older timestamp.  
> Here's the scenario I am thinking about:
> Say we use logical timestamp for a column value and lets assume the  
> current timestamp is t. Now say two clients read this column and  
> generate concurrent CAS (compare and swap) operations on timestamp t  
> and for both the writes the resulting new timestamp would become (t 
> +1). Now if we don't allow writes to a column with same timestamp  
> then only one of these writes would succeed. Of course another  
> assumption is that if a third CAS write with compare on logical  
> timestamp (t - 1) came in, that would be denied as I believe  
> Cassandra doesn't allow "older" writes to win over "newer" writes.  
> Do you think such a thing can be accomplished?


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