Question

I’m running Django 1.3, using Sessions Middleware and Auth Middleware:

# settings.py

SESSION_ENGINE = django.contrib.sessions.backends.db   # Persist sessions to DB
SESSION_COOKIE_AGE = 1209600                           # Cookies last 2 weeks

Each time a user logs in from a different location (different computer/browser), a new Session() is created and saved with a unique session_id. This can result in multiple database entries for the same user. Their login persists on that node until the cookie is deleted or session expires.

When a user changes their password, I want to delete all unexpired sessions for that user from the DB. That way after a password change, they’re forced to re-login. This is for security purposes, such as if your computer got stolen, or you accidentally left yourself logged-in on a public terminal.

I want to know the best way to optimize this. Here’s how I’ve done it:

# sessions_helpers.py

from django.contrib.sessions.models import Session
import datetime

def all_unexpired_sessions_for_user(user):
    user_sessions = []
    all_sessions  = Session.objects.filter(expire_date__gte=datetime.datetime.now())
    for session in all_sessions:
        session_data = session.get_decoded()
        if user.pk == session_data.get('_auth_user_id'):
            user_sessions.append(session)
    return user_sessions

def delete_all_unexpired_sessions_for_user(user, session_to_omit=None):
    for session in all_unexpired_sessions_for_user(user):
        if session is not session_to_omit:
            session.delete()

A very simplified view:

# views.py

from django.http import HttpResponse
from django.shortcuts import render_to_response
from myapp.forms import ChangePasswordForm
from sessions_helpers import delete_all_unexpired_sessions_for_user

@never_cache
@login_required
def change_password(request):
    user = request.user

    if request.method == 'POST':
        form = ChangePasswordForm(data=request)

        if form.is_valid():
            user.set_password(form.get('password'))
            user.save()
            request.session.cycle_key()         # Flushes and replaces old key. Prevents replay attacks.
            delete_all_unexpired_sessions_for_user(user=user, session_to_omit=request.session)
            return HttpResponse('Success!')

    else:
        form = ChangePasswordForm()

    return render_to_response('change_password.html', {'form':form}, context_instance=RequestContext(request))

As you can see in sessions_helpers.py, I have to pull every unexpired session out of the DB, Session.objects.filter(expire_date__gte=datetime.datetime.now()), decode all of them, and then check to see if it matches a user or not. This will be extremely costly to the database if there are, say, 100,000+ sessions stored in there.

Is there a more-database-friendly way to do this? Is there a Sessions/Auth Middleware setting that’ll let you store the username as a column in the Sessions table so I can run SQL against that, or will I have to modify Sessions to do that? Out-of-the-box it only has session_key, session_data, and expire_date columns.

Thanks for any insight or help you can offer. 🙂

Answer 1

If you return a QuerySet from your all_unexpired_sessions_for_user function, you could limit your database hits to two:

def all_unexpired_sessions_for_user(user):
    user_sessions = []
    all_sessions  = Session.objects.filter(expire_date__gte=datetime.datetime.now())
    for session in all_sessions:
        session_data = session.get_decoded()
        if user.pk == session_data.get('_auth_user_id'):
            user_sessions.append(session.pk)
    return Session.objects.filter(pk__in=user_sessions)

def delete_all_unexpired_sessions_for_user(user, session_to_omit=None):
    session_list = all_unexpired_sessions_for_user(user)
    if session_to_omit is not None:
        session_list.exclude(session_key=session_to_omit.session_key)
    session_list.delete()

This gives you a total of two hits to the database. Once to loop over all of the Session objects, and once to delete all of the sessions. Unfortunately, I don’t know of a more direct way to filter through the sessions themselves.