I gave this presentation in skills camp pune last year. I am sharing the slides here, in hope some one still find it useful even though joomla 1.5 is released last month.
In this presentation I talked about how Request flow in a joomla application. I also explained the entity relationship diagram (ERD) of joomla, that i created by reverse engineering the joomla database, reading the code and off course searching the internet.
The most difficult part to understand was user rights management part of the database. It is based on phpGACL library, reading their help files helped get clear idea about right management in joomla. Access Control chapter in joomla website also helped.
and here is the recorded session of same.
Joomla as a framework
Request flow in Joomla
Database and User Managment in Joomla