CAS operation

Dustin Sallings dustin at
Sat Sep 8 07:27:33 UTC 2007

	I just prototyped a pair of operations for implementing CAS in  
memcached.  This provides atomic updates of arbitrary objects without  
having to rely on locks.

	The idea is that every object will have some unique 64-bit value  
that can be retrieved with a new command (I called it ``gets'' for  
some reason).  This value needs to be unique and consistent for a  
given value within memcached.  It can be a counter or memory address  
or whatever.

	After doing a ``gets'' for a key and getting a response, the client  
can manipulate the data structure as fit, and use a ``cas'' operation  
to put it back iff it has the same value as retrieved (based on the  
identifier).  It returns a ``NOT FOUND'' error when there's no value  
for the key (and then you can try it with an ``add''), or ``EXISTS''  
when the identifier does not match (and then you can try again from  
the ``gets'').


Dustin Sallings

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