Django Sessions via Memcache: Cannot find session key manually

I recently migrated from database backed sessions to sessions stored via memcached using pylibmc.

Here is my CACHES, SESSION_CACHE_ALIAS & SESSION_ENGINE in my settings.py

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.PyLibMCCache',
        'LOCATION': ['127.0.0.1:11211'],
    }
}


SESSION_CACHE_ALIAS = 'default'


SESSION_ENGINE = "django.contrib.sessions.backends.cache"

Everything is working fine behind the scenes and I can see that it is using the new caching system. Running the get_stats() method from pylibmc shows me the number of current items in the cache and I can see that it has gone up by 1.

The issue is I'm unable to grab the session manually using pylibmc.

Upon inspecting the request session data in views.py:

def my_view(request):
    if request.user.is_authenticated():

        print request.session.session_key
        # the above prints something like this: "1ay2kcv7axb3nu5fwnwoyf85wkwsttz9"

        print request.session.cache_key
        # the above prints something like this: "django.contrib.sessions.cache1ay2kcv7axb3nu5fwnwoyf85wkwsttz9"

        return HttpResponse(status=200)
    else:
        return HttpResponse(status=401)

I noticed that when printing cache_key, it prints with the default KEY_PREFIX whereas for session_key it didn't. Take a look at the comments in the code to see what I mean.

So I figured, "Ok great, one of these key names should work. Let me try grabbing the session data manually just for educational purposes":

import pylibmc

mc = pylibmc.Client(['127.0.0.1:11211'])

# Let's try key "1ay2kcv7axb3nu5fwnwoyf85wkwsttz9"

mc.get("1ay2kcv7axb3nu5fwnwoyf85wkwsttz9")

Hmm nothing happens, no key exists by that name. Ok no worries, let's try the cache_key then, that should definitely work right?

mc.get("django.contrib.sessions.cache1ay2kcv7axb3nu5fwnwoyf85wkwsttz9")

What? How am I still getting nothing back? As I test I decide to set and get a random key value to see if it works and it does. I run get_stats() again just to make sure that the key does exist. I also test the web app to see if indeed my session is working and it does. So this leads me to conclude that there is a different naming scheme that I'm unaware of.

If so, what is the correct naming scheme?

Yes, the cache key used internally by Django is, in general, different to the key sent to the cache backend (in this case pylibmc / memcached). Let us call these two keys the django cache key and the final cache key respectively.

The django cache key given by request.session.cache_key is for use with Django's low-level cache API, e.g.:

>>> from django.core.cache import cache
>>> cache.get(request.session.cache_key)
{'_auth_user_hash': '1ay2kcv7axb3nu5fwnwoyf85wkwsttz9', '_auth_user_id': u'1', '_auth_user_backend': u'django.contrib.auth.backends.ModelBackend'}

The final cache key on the other hand, is a composition of the key prefix, the django cache key, and the cache version number. The make_key function (from Django docs) below demonstrates how these three values are composed to generate this key:

def make_key(key, key_prefix, version):
    return ':'.join([key_prefix, str(version), key])

By default, key_prefix is the empty string and version is 1.

Finally, by inspecting make_key we find that the correct final cache key to pass to mc.get is

:1:django.contrib.sessions.cache1ay2kcv7axb3nu5fwnwoyf85wkwsttz9

which has the form <KEY_PREFIX>:<VERSION>:<KEY>.

Note: the final cache key can be changed by defining KEY_FUNCTION in the cache settings.