~drizzle-trunk/drizzle/development

User Defined Barriers

=====================

SELECT create_barrier();

SELECT release_barrier();

SELECT wait();

SELECT wait_until();

SELECT signal();

A barrier is a synchronization objest which can be used to syncronize a

group of session to a specific rendezvous by calling wait(). At which point

any session of the user may call signal() and all sessions being held by wait() are allowed to proceed. 

Barriers can optional be created with a limit so that once a set number of sessions have called wait() that all "waiters" are then allowed to proceed. 

The session that creates the barrier via create_barrier() is not allowed to

call either wait() or wait_until().

The scope of barriers is to the given username.

Beyond waiters, you can also create observers by using the wait_until()

function. Observers are released not only when signal() or release_barrier()

is called, but also when their definitine predicate happens. You can use

wait_until() to have a session wait for a certain number of waiters to

occur, and then do some body of work before the waiters() are signalled to

continue.

All waiters and observers are released if release_barrier() is called by the

session which created the barrier. Also, if the session that created the

barrier disconnects, all waiters and observers are notified.

Information on all barriers can be found in the DATA_DICTIONARY.USER_BARRIERS

table;