Re: CAS and Django cache

On Wed, Oct 26, 2011 at 11:29 PM, dmitry b <dmitry.maven@gmail.com> wrote:
> Can I do check-and-set operations using Django's cache api?
>
>
> Thanks
> D.
>

Memcached reuse existing terminology and then complain that this
causes confusion. Their CAS stands for 'compare and set', rather than
the 'compare and swap' that you would assume.

Django's cache API is designed to work with a number of backends, not
all of which offer an atomic compare and set method, so there is no
Django API for accessing compare and set on the memcached backend.

You can access the memcached client via django though:

>>> from django.core import cache
>>> c=cache.get_cache('default')
>>> help(c._client.cas)
Help on method cas in module memcache:

cas(self, key, val, time=0, min_compress_len=0) method of
memcache.Client instance
Sets a key to a given value in the memcache if it hasn't been
altered since last fetched. (See L{gets}).
... (do this yourself to see the whole docs)

Malcolm: compare and set is not equivalent to add. Compare and set is
used to change a cached value atomically, as long as it has not
already changed from the value you think it has. Add only sets a value
if it has not already been set.

Cheers

Tom

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