Welcome back to “Twisted Web in 60 seconds”. Over the previous two entries, I introduced Twisted Web’s session APIs. This included accessing the session object, storing state on it, and retrieving it later. I described how the Session object has a lifetime which is tied to the notional session it represents. In this installment, I’ll describe how you can exert some control over that lifetime and react when it expires.

The lifetime of a session is controlled by the sessionTimeout attribute of the Session class. This attribute gives the number of seconds a session may go without being accessed before it expires. The default is 15 minutes. In this example, I’ll show you change that to a different value.

One way to override the value is with a subclass:

  from twisted.web.server import Session

  class ShortSession(Session):
      sessionTimeout = 60

To have Twisted Web actually make use of this session class, rather than the default, it is also necessary to override the sessionFactory attribute of Site. I could do this with another subclass, but I can also do it to just one instance of Site:

  from twisted.web.server import Site

  factory = Site(rootResource)
  factory.sessionFactory = ShortSession

Sessions given out for requests served by this Site will use ShortSession and only last one minute without activity.

You can have arbitrary functions run when sessions expire, too. This can be useful for cleaning up external resources associated with the session, tracking usage statistics, and more. This functionality is provided via Session.notifyOnExpire. It accepts a single argument: a function to call when the session expires. Here’s a trivial example which prints a message whenever a session expires:

  from twisted.web.resource import Resource

  class ExpirationLogger(Resource):
      sessions = set()

      def render_GET(self, request):
          session = request.getSession()
          if session.uid not in self.sessions:
              self.sessions.add(session.uid)
              session.notifyOnExpire(lambda: self._expired(session.uid))
          return ""

      def _expired(self, uid):
          print "Session", uid, "has expired."
          self.sessions.remove(uid)

Keep in mind that using a method as the callback will keep the instance (in this case, the ExpirationLogger resource) in memory until the session expires.

With those pieces in hand, here’s an example that prints a message whenever a session expires, and uses sessions which last for 5 seconds:

from twisted.web.server import Site, Session
from twisted.web.resource import Resource
from twisted.internet import reactor

class ShortSession(Session):
    sessionTimeout = 5

class ExpirationLogger(Resource):
    sessions = set()

    def render_GET(self, request):
        session = request.getSession()
        if session.uid not in self.sessions:
            self.sessions.add(session.uid)
            session.notifyOnExpire(lambda: self._expired(session.uid))
        return ""

    def _expired(self, uid):
        print "Session", uid, "has expired."
        self.sessions.remove(uid)

rootResource = Resource()
rootResource.putChild("logme", ExpirationLogger())
factory = Site(rootResource)
factory.sessionFactory = ShortSession

reactor.listenTCP(8080, factory)
reactor.run()

Since Site customization is required, this example can’t be rpy-based, so it brings back the manual reactor.listenTCP and reactor.run calls. Run it and visit /logme to see it in action. Keep visiting it to keep your session active. Stop visiting it for five seconds to see your session expiration message.

That pretty much wraps things up for Twisted Web’s built in session support. Next time I’ll cover some of Twisted Web’s proxying features.