Session Locking in NCache Session Store Provider

ASP.NET applications are inherently multithreaded. Because requests that arrive in parallel are processed on concurrent threads drawn from a thread pool, it's possible that two or more requests targeting the same session will execute at the same time. (The classic example is when a page contains two frames, each targeting a different ASPX in the same application, causing the browser to submit overlapping requests for the two pages.) To avoid data collisions and erratic behavior, the NCache Session Store Provider provides the functionality that exclusively locks the session-store item for a particular session when it begins processing the first request by calling GetItemExclusive method, causing other requests targeting the same session to wait for the lock to come free. When the request completes, the lock is released during the call to the SetAndReleaseItemExclusive method.

Because there's no harm in allowing concurrent requests to perform overlapping reads, the lock is typically implemented as a reader/writer lock-that is, one that allows any number of threads to read a session but that prevents overlapping reads and writes as well as overlapping writes.