Cascading Rollbacks in 2-PL
Let’s see the following Schedule
Cascading Roll-Back
Take a moment to analyze the schedule. Yes, you’re correct, because of Dirty Read in T2 and T3 in lines 8 and 12 respectively, when T1 failed we have to roll back others also. Hence, Cascading Rollbacks are possible in 2-PL. I have taken skeleton schedules as examples because it’s easy to understand when it’s kept simple. When explained with real-time transaction problems with many variables, it becomes very complex.
Deadlock in 2-PL
Consider this simple example, it will be easy to understand. Say we have two transactions T1 and T2.
Schedule: Lock-X1(A) Lock-X2(B) Lock-X1(B) Lock-X2(A)
Drawing the precedence graph, you may detect the loop. So Deadlock is also possible in 2-PL.
Two-phase locking may also limit the amount of concurrency that occurs in a schedule because a Transaction may not be able to release an item after it has used it. This may be because of the protocols and other restrictions we may put on the schedule to ensure serializability, deadlock freedom, and other factors. This is the price we have to pay to ensure serializability and other factors, hence it can be considered as a bargain between concurrency and maintaining the ACID properties.
The above-mentioned type of 2-PL is called Basic 2PL. To sum it up it ensures Conflict Serializability but does not prevent Cascading Rollback and Deadlock. Further, we will study three other types of 2PL, Strict 2PL, Conservative 2PL, and Rigorous 2PL.
Problem with Two-Phase Locking
- It does not insure recoverability which can be solved by strict two-phase locking and rigorous two-phase locking.
- It does not ensure a cascade-less schedule which can be solved by strict two-phase locking and rigorous two-phase locking.
- It may suffer from deadlock which can be solved by conservative two-phase locking.
GATE-Related Questions
Two Phase Locking Protocol
Pre-Requisite: Concurrency Control Protocol, Lock-based Protocol.
Now, recalling where we last left off, there are two types of Locks available Shared S(a) and Exclusive X(a). Implementing this lock system without any restrictions gives us the Simple Lock-based protocol (or Binary Locking), but it has its own disadvantages, they do not guarantee Serializability. Schedules may follow the preceding rules but a non-serializable schedule may result.
To guarantee serializability, we must follow some additional protocols concerning the positioning of locking and unlocking operations in every transaction. This is where the concept of Two-Phase Locking(2-PL) comes into the picture, 2-PL ensures serializability. Now, let’s dig deep!