Two Phase Locking (2PL)

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

• In the first phase, when the transaction begins to execute, it requires permission for the locks it
  needs.
• The second part the transaction acquires 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

2PL protocol offers serializability. The 2PL algorithm offers Strict Serializability, which is the golden standard when it comes to data integrity. But it does not avoid cascading rollback and deadlocks.

Consider a schedule:

Where LP is lock point, Read(A) in T₂ and T₃ denotes Dirty Read because of Write(A) in T₁.

Because of Dirty Read in T₂ and T₃ in lines 8 and 12 respectively, when T₁ failed we have to rollback others also. Hence Cascading Rollbacks are possible in 2-PL.