1.Two-Phase Locking (2PL) is a concurrency control method which divides the execution phase of a transaction into three parts.

2.It ensures conflict serializable schedules.

3.If read and write operations introduce the first unlock operation in the transaction, then it is said to be Two-Phase Locking Protocol.

This protocol can be divided into two phases,

1. In Growing Phase, a transaction obtains locks, but may not release any lock.

2. In Shrinking Phase, a transaction may release locks, but may not obtain any lock.

   Two-Phase Locking does not ensure freedom from deadlocks.

Types of Two – Phase Locking Protocol Following are the types of two – phase locking protocol:

1. Strict Two – Phase Locking Protocol
2. Rigorous Two – Phase Locking Protocol
3. Conservative Two – Phase Locking Protocol

1.Strict Two-Phase Locking Protocol

Strict Two-Phase Locking Protocol avoids cascaded rollbacks. This protocol not only requires two-phase locking but also all exclusive-locks should be held until the transaction commits or aborts. It is not deadlock free. It ensures that if data is being modified by one transaction, then other transaction cannot read it until first transaction commits. Most of the database systems implement rigorous two – phase locking protocol.

2.Rigorous Two-Phase Locking

Rigorous Two – Phase Locking Protocol avoids cascading rollbacks. This protocol requires that all the share and exclusive locks to be held until the transaction commits.

3. Conservative Two-Phase Locking Protocol

Conservative Two – Phase Locking Protocol is also called as Static Two – Phase Locking Protocol. This protocol is almost free from deadlocks as all required items are listed in advanced. It requires locking of all data items to access before the transaction starts.