You can find the documentation on SQLServer2008R2 BOL.

This is one of the stupid design.

Imagine old days, you go to library asking for a book for a page you need. It is checked out (locked), so you cannot have it except wait.

Modern days, the librarian runs a digital replica of that page for you. It is called multi-versioning. (digital hash can be replicated) You got what you asked for as the moment it was.

Locking is not the solution (maybe it was back then at stone age)

Certain things are required to be placed under exclusive locks. That is why ACID was designed/discussed. The lock should be at a minimal unit. That is why data is at best under row level lock.

so what is the design background for "sp_getapplock" to be useful?

BOL--it says: Places a lock on an application resource.

Why do you want to do that? (just because there is no better way known doing it so chosen a bad way doing it)

Jason

http://dbace.us