PSE ships with three difference session plugins, each with identical APIs. The default is a traditional dbm implementation using a bsddb back-end, which ships with most versions of Python. Another uses an SQLite back-end, which is slower but only uses a single file. Finally, there is a PostgreSQL back-end for stateless load balanced servers.
For most low traffic, single machine sites, the default dbm session back-end is fast and reliable. For sites that process an extremely high volume of requests (more than 100 per second) or that use load balanced servers, the PostgreSQL back-end is recommended.
The SQLite session plugin is interesting insomuch as it can be used as an example for implementing your own SQL session back-ends. Since it is fairly slow and unreliable for high volume web sites, its use is discouraged. The dbm implementation is much more reliable and efficient for low to middle traffic sites, and high volume sites should consider a real enterprise RDBMS implementation.
Please read the PSE API documentation for more details on using the session plugin in general.
In order to use the SQLite session plugin for PSE, you need to compile and install both at least version 3 of the SQLite libraries and at least version 2.0 of the Python bindings (pysqlite2, from http://initd.org/tracker/pysqlite). For the PostgreSQL session plugin, you need the psycopg module (from http://initd.org/software/initd/psycopg). Many Linux systems already have these modules available, either included with the distribution CD-ROM or available for download.
For configuration information of the session plugin, refer to session module documentation in The PSE API.