Inconsistent CACHE_MIDDLEWARE_SECONDS = 0 meaning across cache backends

[Simon Law <simon@…>]

According to ​http://www.djangoproject.com/documentation/cache/#the-per-site-cache, settings.CACHE_MIDDLEWARE_SECONDS is "The number of seconds each page should be cached."

However, memcached takes a 0 second timeout to mean "cache forever" instead of "do not cache".

This patch enforces the Django meaning in django.core.cache.backends.memcached

[Simon Law <simon@…>]

The second patch is far more correct.

[James Bennett]

I guess I don't understand why someone would call cache.add() or cache.set() with something that's not meant to be cached...

[Philippe Raoult]

It is very common for 0 to mean "infinity" for those kind of settings. If you want to submit a patch to django's documentation to make that explicit, feel free to do so. If you want to disable caching in the settings then change the engine to dummy.

[Simon Law <simon@…>]

cache.add() and cache.set() with a timeout of 0 is used when CacheMiddleware is given a 0 second timeout.

You would want it in this case:

from django.conf import settings
from django.views.decorators.cache import cache_page

settings.CACHE_MIDDLEWARE_SECONDS = 300  # Obviously, this is done in settings.py

@cache_page(0)
def never_cache_this(request):
    ...

This works with all CACHE_BACKENDs other than memcached. This means that the semantics are different if you choose locmem:// instead of memcached://.../ which is not something that should be solved by documentation.

If 0 seconds means "cache forever", then I would be happy to prepare a patch to cause the other cache backends to behave the same way. But I believe that a 0 second timeout is valuable, because of CacheMiddleware's interface.

[Simon Law <simon@…>]

memcached.py has the same semantics as other django.core.cache.backends

[Marc Fargas]

I understand that the issue is not that "0" means infinity, but that 0 means different things depending on the backend you use.
So either 0 means infinity to all, or means "do not cache".

[otherjacob]

Eligible to be closed due to recent changes/tests no longer requiring 0 as a insta-invaliation. May be worth noting in the documentation the difference, but a fair warning of YMMV should be enough.

[otherjacob]

And closing to reflect the above.