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# Session Endings

The previous two examples introduced Twisted Web’s session APIs. This included accessing the session object, storing state on it, and retrieving it later, as well as the idea that the Session object has a lifetime which is tied to the notional session it represents. This example demonstrates how to 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 we’ll 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 . We could do this with another subclass, but we could 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.