Performance comparison of using django signed cookie session over django db + cache based session?

Django 1.4 offers several ways to maintain django sessions :

My viewpoint on using :

i) cache only : Not preferable, users sessions may get purged out of memcache.

ii) Db + cache (cached_db): Preferable , simple and secure solution.

iii) Signed cookie sessions : Preferable, no database hit .

Assuming that the session data is very small i.e. no problem in storing it in cookie. Is there any performance benefit of using signed cookie over a database + memcache driven session engine ? As for N concurrent new users, there would be order of N database hits saved. This will reduce total queries that the database server has to handle per unit time.

UPDATE : We found redis or aerospike as really robust and high throughput session backends for django. We are using redis as sessions backend

Solution

You must be able to handle N concurrent queries (N - concurrent new users).

You see the difference at a very large scale only. You must be pessimist (server is down, bad buckup, amazon bancrupcy ) and your database must be able to handle all users in rush hours.

10 000 000 users with cookie session will cost you 0$ extra, problems with iframes and mobiles
10 000 000 users with cache session will cost you hundreds or less than hundred of dollars daily, lost sessions in case of restart
10 000 000 users with cache_db session will cost you thousands of dollars daily (any persistent storage able to handle 1 000 000 hits in one time, 10% of users)

I use redis backend (periodic save to disk) with the possibility of switching to a cluster of memcached services.

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