What is IoC

http://martinfowler.com/articles/injection.html#InversionOfControl

The Downsides of ASP.NET Session State

http://tech-journals.com/jonow/2011/10/22/the-downsides-of-asp-net-session-state

by default the ASP.NET pipeline will not process requests belonging to the same session concurrently. It serialises them, i.e. it queues them in the order that they were received so that they are processed serially rather than in parallel.