Notes on mod_python.Session
---------------------------

    Author: Tom Conway
    Date: 12/12/2007

``mod_python`` provides some automagic for cookie based sessions. It
carefully separates most of the session logic from how the session is
stored. The base class `BaseSession` contains most of the logic, and
``mod_python`` itself has three derived classes for storing session
objects in-memory, dbm, and on the filesystem. In each case, the
implementations use apache's locking mechanism to serialize updates to
the store of cookies. This mechanism takes care of mutual exclusion
between the multiple processes of an apache instance, but does not
provide any facility to provide any kind locking for multiple servers
sharing the filesystem for file bases session storage.  There is code
for storing sessions in MySQL (and SQLLite) floating round on the net,
though none has made it in to any distributions. This code uses the
underlying database to take care of the locking.

In the case of IVLE, we wish to be able to share the session objects not
merely between the separate processes of an apache instance, but between
the multiple servers in a load balancing cluster.  There are three high
level strategies we could use to deal with this:

1. Use a static load balancing strategy such as hashing the client's IP
   address to determine which node in the cluster should serve the
   request.

2. Use a SQL backend to store sessions, or create a filesystem based
   storage mechanism that does the necessary locking.

3. Work around the problem by using session objects in a way that avoids
   the locking problems.

Strategy 1 has the advantage that we could use in-memory or dbm session
storage without having to worry about race conditions between servers.
On the other hand, it can run into serious problems if the distribution
of IP addresses is such that load is not balanced. This can be the case
if an ISP uses NAT firewalling (some do!), since all the requests from
that ISP will aparently be coming from a single IP address and will therefore
be routed to the same node in the cluster. As well as the potential for
failing to balance the load, such a scheme, if it works routes an equal
proportion of requests to each node in the cluster. At times when overall
load is light, this may mean that we lose the opportunity to put nodes into
a powersaving mode, when they are superfluous.

Strategy 2, while having the advantage of avoiding race conditions, is likely
to be expensive. The use of a SQL backend is likely to be quite slow, and the
SQL backend itself will be subject to significant load (i.e. at least one op
per request). A filesystem based solution is likely to be quite slow too.
It has to work on a shared filesystem, for which locking is a general issue
(generally, you end up using `mkdir` as the mechanism for creating a lock).
If we want mutable session information, then we will *have* to do something in
this vein.

Strategy 3 is fragile because we need to be careful about how we use
session objects, but if the constraints are simple enough to be practicle
then avoiding the locking issue is highly desirable. A simple constraint
that may be workable is to require that once created, a session object is
treated as read-only until it is deleted. It is possible (though unlikely)
we could create session objects that immediately become orphaned, but we
will not ever create a situation in which the application does anything bad.
If we can make strategy 3 work, then it is easily the best strategy to use.

The main use for session objects in IVLE will be to *cache* authentication and
authorization information. This means that when a user logs in, we authenticate
(the authentication mechanism is not important to our current discussion),
then retrieve the authorization information for that user, and store it in
the session object. For each page access until the user logs out, we can then
use the information from the session object.