Two phase locking protocol in dbms

Two Phase Locking (2PL) Protocol

Two-Phase locking protocol which is also known as a 2PL protocol. It is also called P2L. In this type of locking protocol, the transaction should acquire a lock after it releases one of its locks.

This locking protocol divides the execution phase of a transaction into three different parts.

• In the first phase, when the transaction begins to execute, it requires permission for the locks it needs.
• The second part is where the transaction obtains all the locks. When a transaction releases its first lock, the third phase starts.
• In this third phase, the transaction cannot demand any new locks. Instead, it only releases the acquired locks.

The Two-Phase Locking protocol allows each transaction to make a lock or unlock request in two steps:

• Growing Phase: In this phase transaction may obtain locks but may not release any locks.
• Shrinking Phase: In this phase, a transaction may release locks but not obtain any new lock

example:-

              -- lock x(b);

              -- read(b);

              -- B=B-50;

              -- write(B);

              -- lock x(a);

             -- read(a);

             -- A=A-50;

            -- write(a);

            -- unlock(B);

            -- unlock(a);

ADVANTAGE:-

           1. the two phase locking protocol ensure conflict serializibility.

DISADVANTAGE;-

           1. The two phase locking protocol does not ensure freedom from deadlock.

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