A transaction is made to wait until all compatible locks held on the item are released. This ensures that no other transaction is concurrently accessing the same item.

It is not necessarily desirable for a transaction to unlock a data item immediately after its final access because serializability may be violated due to this.

The protocol that indicates when a transaction may lock and unlock each of the data items is called as locking protocol. Locking protocols restrict the number of schedules.

The situation where no transaction can proceed with normal execution is known as a deadlock.

The two phased locking protocol consists of the growing phase and the shrinking phase.

If a transaction can be granted a lock on an item immediately in spite of the presence of another mode, then the two modes are said to be compatible.

If a transaction Ti has obtained a shared-mode lock (denoted by S) on item Q, then Ti can read, but cannot write, Q.

A transaction can proceed only after the concurrency control manager grants the lock to the transaction.

If a transaction Ti may never make progress, then the transaction is said to be starved.

If a transaction has obtained an exclusive mode lock, then it can both read and write on the item on which it in operating.