Two-Phase Locking in Databases

What is Two-Phase Locking (2PL)?

• Two-Phase Locking (2PL) is a concurrency control method which divides the execution phase of a transaction into three parts.
• It ensures conflict serializable schedules.
• 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

• 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.

• 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.

• 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.