Difference between Conservative and Rigorous 2-PL

Prerequisites – Two Phase Locking Protocol, and Categories of Two Phase Locking

1. Conservative 2-PL :

• It is also called as Static 2-PL.
• This protocol requires the transaction to lock all the items it access before the Transaction begins execution by pre-declaring its read-set and write-set.
• If any of the pre-declared items needed cannot be locked, the transaction does not lock any of the items, instead, it waits until all the items are available for locking.

2. Rigorous 2-PL :

• This requires that in addition to the lock being 2-Phase all Exclusive(X) and Shared(S) Locks held by the transaction be released until after the Transaction Commits.
• Rigorous is more restrictive than Strict 2-PL.
• The implementation of Rigorous 2-PL is easy.

Difference between Conservative and Rigorous 2-PL :

S.No.	Conservative 2-PL	Rigorous 2-PL
1.	A transaction has to acquire locks on all the data items it requires before the transaction begins it execution.	A transaction can acquire locks on data items whenever it requires (only in growing phase) during its execution.
2.	It does not have a growing phase.	It has a growing phase.
3.	It has a shrinking phase.	It does not have a shrinking phase.
4.	It ensures that the schedule generated would be Serializable and Deadlock-Free.	It ensures that the schedule generated would be Serializable, Recoverable and Cascadeless.
5.	It does not ensures Recoverable and Cascadeless schedule.	It does not ensures Deadlock-Free schedule.
6.	It does not ensure Strict Schedule.	It ensures that the schedule generated would be Strict.
7.	It is not used in practise as its difficult to implement.	It is easy to implement but a lighter version of it (i.e Strict 2-PL) is used in practise.
8.	In Conservative 2-PL, a transaction can read a value of uncommitted transaction.	In Rigorous 2-PL, a transaction only reads value of committed transaction.