Two-Phase Locking Protocol
Two Phase Locking
A transaction is said to follow Two Phase Locking protocol if Locking and Unlocking can be done in two phases.
- Growing Phase: New locks on data items may be acquired but none can be released.
- Shrinking Phase: Existing locks may be released but no new locks can be acquired.
Note – If lock conversion is allowed, then upgrading of lock( from S(a) to X(a) ) is allowed in Growing Phase and downgrading of lock (from X(a) to S(a)) must be done in shrinking phase.
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.