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Notes on mod_python.Session
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``mod_python`` provides some automagic for cookie based sessions. It
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carefully separates most of the session logic from how the session is
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stored. The base class `BaseSession` contains most of the logic, and
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``mod_python`` itself has three derived classes for storing session
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objects in-memory, dbm, and on the filesystem. In each case, the
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implementations use apache's locking mechanism to serialize updates to
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the store of cookies. This mechanism takes care of mutual exclusion
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between the multiple processes of an apache instance, but does not
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provide any facility to provide any kind locking for multiple servers
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sharing the filesystem for file bases session storage. There is code
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for storing sessions in MySQL (and SQLLite) floating round on the net,
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though none has made it in to any distributions. This code uses the
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underlying database to take care of the locking.
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In the case of IVLE, we wish to be able to share the session objects not
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merely between the separate processes of an apache instance, but between
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the multiple servers in a load balancing cluster. There are three high
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level strategies we could use to deal with this:
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1. Use a static load balancing strategy such as hashing the client's IP
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address to determine which node in the cluster should serve the
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2. Use a SQL backend to store sessions, or create a filesystem based
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storage mechanism that does the necessary locking.
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3. Work around the problem by using session objects in a way that avoids
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Strategy 1 has the advantage that we could use in-memory or dbm session
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storage without having to worry about race conditions between servers.
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On the other hand, it can run into serious problems if the distribution
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of IP addresses is such that load is not balanced. This can be the case
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if an ISP uses NAT firewalling (some do!), since all the requests from
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that ISP will aparently be coming from a single IP address and will therefore
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be routed to the same node in the cluster. As well as the potential for
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failing to balance the load, such a scheme, if it works routes an equal
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proportion of requests to each node in the cluster. At times when overall
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load is light, this may mean that we lose the opportunity to put nodes into
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a powersaving mode, when they are superfluous.
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Strategy 2, while having the advantage of avoiding race conditions, is likely
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to be expensive. The use of a SQL backend is likely to be quite slow, and the
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SQL backend itself will be subject to significant load (i.e. at least one op
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per request). A filesystem based solution is likely to be quite slow too.
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It has to work on a shared filesystem, for which locking is a general issue
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(generally, you end up using `mkdir` as the mechanism for creating a lock).
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If we want mutable session information, then we will *have* to do something in
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Strategy 3 is fragile because we need to be careful about how we use
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session objects, but if the constraints are simple enough to be practicle
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then avoiding the locking issue is highly desirable. A simple constraint
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that may be workable is to require that once created, a session object is
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treated as read-only until it is deleted. It is possible (though unlikely)
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we could create session objects that immediately become orphaned, but we
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will not ever create a situation in which the application does anything bad.
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If we can make strategy 3 work, then it is easily the best strategy to use.
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The main use for session objects in IVLE will be to *cache* authentication and
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authorization information. This means that when a user logs in, we authenticate
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(the authentication mechanism is not important to our current discussion),
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then retrieve the authorization information for that user, and store it in
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the session object. For each page access until the user logs out, we can then
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use the information from the session object.
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doc/notes/session.txt
