Enqueues are another type of locking mechanism used in Oracle.
An enqueue is a more sophisticated mechanism which permits several concurrent processes to have varying degree of sharing of “known” resources.
Any object which can be concurrently used, can be protected with enqueues.
A good example is of locks on tables. We allow varying levels of sharing on tables e.g. two processes can lock a table in share mode or in share update mode
One difference is that the enqueue is obtained using an OS specific locking mechanism.
An enqueue allows the user to store a value in the lock, i.e the mode in which we are requesting it.
The OS lock manager keeps track of the resources locked. If a process cannot be granted the lock because it is incompatible with the mode requested and the lock is requested
with wait, the OS puts the requesting process on a wait queue which is serviced in FIFO.
Another difference between latches and enqueues is that in latches there is no ordered queue of waiters like in enqueues.
Latch waiters may either use timers to wakeup and retry or spin (only in multiprocessors). Since all waiters are concurrently retrying (depending on the scheduler), anyone might get the latch and conceivably the first one to try might be the last one to get.