Question

I have a social network type app I have been working on for at least 2 years now, I need it to scale well so I have put a lot of effort into perfecting the code of this app. I use sessions very often to cache database results for a user when a user logs into the site I cache there userID number, username, urer status/role, photo URL, online status, last activity time, and several other things into session variables/array. Now I currently have 2 seperate servers to handle this site, 1 server for apache webserver and a seperate server for mysql. Now I am starting to use memcache in some areas to cut down on database load.

Now my sessions are stored on disk and I know some people use a database to store sessions data, for me it would seem that storing session data that I cached from mysql would kind of defeat the purpose if I were to switch to storingsessions in mysql. SO what am I missing here? Why do people choose to use a database for sessions?

Here are my ideas, using a database for sessions would make it easiar to store and access sessions across multiple servers, is this the main reason for using a database?

Also should I be using memcache to store temp variables instead of storing them into a session?

Answer 1

The single best reason to store sessions in the database is so you can load-balance your website. That way it doesn’t matter which server hands out the next page because they are all using the same database for storing their sessions.

Have a look at PHP’s set_save_handler() for how to install a custom session handler. It takes about 30 lines to set one up that puts the session in the database, though that doesn’t count the lines to make a decent database handler. 🙂 You will need to do:

ini_set('session.save_handler', 'user');
ini_set('session.auto_start', '0');

… although session.auto_start will need to be in your php.ini (and set to 0).

If the database access is going to be a bit expensive, there are some other things you can do to mitigate that. The obvious one is to have a DB server that is just for sessions. Another trick is to have it poke stuff into both memcache and the DB, so when it checks, if the memcache record is missing, it just falls back to the DB. You could get fancy with that, too, and split the session up so some of it is in memcache but the rest lives in the database. I think you’d need to put your own access functions on top of PHP’s session API, though.