Tricky issue with django sessions: sometimes session information is erased

Question

I have a weird bug with django sessions in my app: some times (about 10 times for ~20000 per day) session information for user is erased. I traced it via log files: at page A there is information for user's session, after it he submits the form and at the next page his session is empty. I tried two types of storage: memcached+db and db only and this problem is for both of them. I tried to reproduce these scenarios, but all works as expected, as I said, it happens very rare. I also checked that this problem exists for different users, and for them is doesn't reproduce each time. I don't have any ideas how to catch the root cause and I don't know what else post here as a description. If someone has any ideas, please let me know. If it is important, I'm running my app with django 1.2 + FastCGI. Thanks!

UPD: I checked and see that session key from uses is not changed during two sequential requests, at first request there is an actual session state, and at second session variables are relaced with empty.

Answer 1

As a way to debug this problem, I would subclass the standard Django session middleware (or whatever you're currently using):

django.contrib.sessions.middleware.SessionMiddleware

and wrap process_request and (probably more importantly) process_response in some extra logging. Then install your subclassed session middleware in the MIDDLEWARE_CLASSES, rather than the stock Django one.

You could also validate that session.save() has actually committed its changes by attempting to read it back. It could be that the problem lies in session-state serialisation, and it's failing on a particular key or value that you're attempting to store.

None of this will fix your problem, but it might help you to establish what's going on.

Answer 2

As @Steve Mayne mentioned, it would be good to do some logging on the sessions middleware and sessions model save method. That's something I'd start with.

In addition I'd like to say that this could be a database related issue, especially if you're using MySQL database backend for sessions. You can check the log for database locks and other concurrency issues. I had to deal with similar issues before and the solution is clear: optimization and additional performance.

If you have some specific application middleware, you can check for functionality that interferes with Django sessions. Such parallel operations can cause problems, if not implemented properly.

Another thing I would do is to upgrade to the latest stable release of Django and migrate to a mod_wsgi setup.