Utilizing advisory locks

PostgreSQL has a highly efficient and sophisticated transaction machinery that is capable of handling locks in a really fine grained and efficient way. Some years ago, some people came up with the idea of using this code to synchronize applications with each other. Thus, advisory locks were born.

When using advisory locks, it is important to mention that they won't go away on COMMIT as normal locks do. Therefore, it is really important to make sure that unlocking is done properly and in a totally reliable way.

If you decide to use an advisory lock, what you really lock is a number. So this is not about rows or data: it is really just a number. Here is how it works:

Session 1	Session 2
BEGIN;
SELECT pg_advisory_lock(15);
	SELECT pg_advisory_lock(15);
	It has to wait
COMMIT;	It still has to wait
SELECT pg_advisory_unlock(15);	It still waiting
	Lock is taken

 

 

The first transaction will lock 15. The second transaction has to wait until this number has been unlocked again. The second...