CAS in cache framework

[Alexey Moskvin]

Hi all,
there is a usefull (for me now :)) CAS operation in memcache: http://neopythonic.blogspot.ru/2011/08/compare-and-set-in-memcache.html, but I can't find it in django cache api.
Are there any reasons for it not to be implemented there?
Thank you!

[Russell Keith-Magee]

I can't see any reason that we *shouldn't* support it - it seems like a reasonable primitive to support. 

The only real issue is ensuring it either is, or can be, implemented on all backends and supported memcache libraries. If you can produce a patch that does this, I'd say it has a good chance of being merged into trunk.

Yours,

Russ Magee %-)

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[Curtis Maloney]

Is it CAS specifically that you want?  Or a Locking primitive?  Or some other operation?

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Curtis

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[ipcam...@gmail.com]

Currently `PylibMC` includes support for CAS: https://sendapatch.se/projects/pylibmc/reference.html#pylibmc.Client.cas

Would a PR be welcome that would implement the `cas` and `gets` methods on the `PylibMCCache` cache backend? This would also require setting the `cas` behavior to `True` in the `CACHES` setting; I could include documentation explaining this in the `PylibMCCache` docs as well.

Thanks so much for maintaining Django. I'd love to help contribute my first PR to Django if this would be welcomed!

[Adam Johnson]

pymemcache also supports a cas() method, so that's two backends that could have the other method.

I think the main problem is as Russell highlighted - supporting other backends. It would be nice to see research into emulating cas(), at least on LocMemCache which is often used as a fake during testing. Have you got any input there?

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[ipcam...@gmail.com]

Thanks Adam! That makes sense to me - I believe Redis transactions are atomic by default so the `cas` method wouldn't apply there. I took a look at the LocMemCache code, and I think that the need for cas in this instance also doesn't exist - because the code uses a threading Lock() to prevent race conditions that cas addresses, see https://github.com/django/django/blob/main/django/core/cache/backends/locmem.py#L38. Perhaps for testing we could simply implement a `cas` method for `LocMemCache` that simply calls `LocMemCache.get`?

I'd be happy to implement the cas() method for both Memcached backends and the dummy method for LocMemCache if that makes sense!