Two-Phase Locking Protocol The use of locks has helped us to create neat and clean concurrent schedule. The Two Phase Locking Protocol defines the rules of how to acquire the locks on a data item and how to release the locks. Two phase locking (2PL) is a concurrency control method that guarantees serializability. The protocol utilizes locks, applied by a transaction on data, which may block (stop) other transactions from accessing the same data during the transaction's life. The Two Phase Locking Protocol assumes that a transaction can only be in one of two phases. Phase 1 - Growing Phase In this phase the transaction can only acquire locks, but cannot release any lock. The transaction enters the growing phase as soon as it acquires the first lock it wants. From now on it has no option but to keep acquiring all the locks it would need. It cannot release any lock at this phase even if it has finished working with a locked data item. Ultimately the transaction reaches a point where all the lock it may need has been acquired. This point is called Lock Point. Phase 2 - Shrinking Phase After Lock Point has been reached, the transaction enters the shrinking phase. In this phase the transaction can only release locks, but cannot acquire any new lock. The transaction enters the shrinking phase as soon as it releases the first lock after crossing the Lock Point. From now on it has no option but to keep releasing all the acquired locks. Initially the transaction is in growing phase, that is the transaction acquires locks as needed. Once the transaction releases lock, it enters the shrinking phase and no more lock request may be issued. Upgrading of lock is not possible in shrinking phase, but it is possible in growing phase. The two phase locking protocol ensures serializability. There are two different versions of the Two Phase Locking Protocol. Strict Two Phase Locking Protocol Rigorous Two Phase Locking Protocol.
