Difference between locks, latches, enqueues and semaphores

Difference between locks, latches, enqueues and semaphores.

Latches are used to protect Oracle data structures to be modified or run by more than one process. They are more restrictive than locks. Locking has a similar concept. When a transaction being modified is locked, it cannot be modified until committed. However, locks allow some user interaction. Enqueues are associated with a transaction or a session that serialize access to database resources. When an enqueue is associated with a session, it cannot be held by another session. Semaphores are mainly used for communication between processes. They control the processes by allowing them stop, wait and resume their processing.
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