~justin-fathomdb/nova/justinsb-openstack-api-volumes

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<title>Twisted Documentation: Session Endings</title>

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<h1 class="title">Session Endings</h1>

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 <code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.web.server.Session.html" title="twisted.web.server.Session">Session</a></code> 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 <code>sessionTimeout</code>

attribute of the <code>Session</code> 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:

<pre class="python">1

from twisted.web.server import Session

class ShortSession(Session):

sessionTimeout = 60

</pre>

To have Twisted Web actually make use of this session class, rather than the

default, it is also necessary to override the <code>sessionFactory</code> attribute of

<code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.web.server.Site.html" title="twisted.web.server.Site">Site</a></code>. We could do this with

another subclass, but we could also do it to just one instance

of <code>Site</code>:

<pre class="python">1

factory = Site(rootResource)

factory.sessionFactory = ShortSession

</pre>

Sessions given out for requests served by this <code>Site</code> will

use <code>ShortSession</code> 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

<code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.web.server.Session.notifyOnExpire.html" title="twisted.web.server.Session.notifyOnExpire">Session.notifyOnExpire</a></code>. 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:

<pre class="python"> 1

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)

</pre>

Keep in mind that using a method as the callback will keep the instance (in

this case, the <code>ExpirationLogger</code> 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:

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<pre class="python"> 1

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from twisted.web.resource import Resource

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from twisted.internet import reactor

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class ShortSession(Session):

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sessionTimeout = 5

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class ExpirationLogger(Resource):

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sessions = set()

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def render_GET(self, request):

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session = request.getSession()

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return ""

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def _expired(self, uid):

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print "Session", uid, "has expired."

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self.sessions.remove(uid)

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rootResource = Resource()

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rootResource.putChild("logme", ExpirationLogger())

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factory = Site(rootResource)

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factory.sessionFactory = ShortSession

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reactor.listenTCP(8080, factory)

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reactor.run()

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</pre>

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Since <code>Site</code> customization is required, this example can't be

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rpy-based, so it brings back the manual <code>reactor.listenTCP</code>

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and <code>reactor.run</code> calls. Run it and visit <code>/logme</code> to see

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it in action. Keep visiting it to keep your session active. Stop visiting it for

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five seconds to see your session expiration message.

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<a href="../index.html">Index</a>

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Version: 10.0.0

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