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Categories of Two Phase Locking (Strict, Rigorous & Conservative)

  • Difficulty Level : Easy
  • Last Updated : 02 Jun, 2021

Now that we are familiar with what is Two-Phase Locking (2-PL) and the basic rules which should be followed which ensures serializability. Moreover, we came across the problems with 2-PL, Cascading Aborts, and Deadlocks. Now, we turn towards the enhancements made on 2-PL which tries to make the protocol nearly error-free. Briefly, we allow some modifications to 2-PL to improve it. There are three categories: 

  1. Strict 2-PL
  2. Rigorous 2-PL
  3. Conservative 2-PL

Now recall the rules followed in Basic 2-PL, over that we make some extra modifications. Let’s now see what are the modifications and what drawbacks they solve. 

Strict 2-PL –

This requires that in addition to the lock being 2-Phase all Exclusive(X) locks held by the transaction be released until after the Transaction Commits. 
Following Strict 2-PL ensures that our schedule is:

  • Recoverable
  • Cascadeless

Hence, it gives us freedom from Cascading Abort which was still there in Basic 2-PL and moreover guarantee Strict Schedules but still, Deadlocks are possible

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. 
Following Rigorous 2-PL ensures that our schedule is:



  • Recoverable
  • Cascadeless

Hence, it gives us freedom from Cascading Abort which was still there in Basic 2-PL and moreover guarantee Strict Schedules but still, Deadlocks are possible

Note the difference between Strict 2-PL and Rigorous 2-PL is that Rigorous is more restrictive, it requires both Exclusive and Shared locks to be held until after the Transaction commits and this is what makes the implementation of Rigorous 2-PL easier.  

Conservative 2-PL –

A.K.A Static 2-PL, this protocol requires the transaction to lock all the items it access before the Transaction begins execution by predeclaring its read-set and write-set. If any of the predeclared 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. 

Conservative 2-PL is Deadlock free and but it does not ensure a Strict schedule(More about this here!). However, it is difficult to use in practice because of the need to predeclare the read-set and the write-set which is not possible in many situations. In practice, the most popular variation of 2-PL is Strict 2-PL. 

The Venn Diagram below shows the classification of schedules that are rigorous and strict. The universe represents the schedules which can be serialized as 2-PL. Now as the diagram suggests, and it can also be logically concluded, if a schedule is Rigorous then it is Strict. We can also think in another way, say we put a restriction on a schedule which makes it strict, adding another to the list of restrictions make it Rigorous. Take a moment to again analyze the diagram and you’ll definitely get it. 

 

33

Image – Venn Diagram showing categories of languages under 2-PL 



Now, let’s see the schedule below, tell me if this schedule can be locked using 2-PL, and if yes, show how and what class of 2-PL does your answer belongs to? 

 

 T1T2
1Read(A) 
2 Read(A)
3Read(B) 
4Write(B) 
5Commit 
6 Read(B)
7 Write(B)
6 Commit

I recommend you try before looking at the solution. 
Yes, the schedule is conflict serializable, so we can try implementing 2-PL. So, let’s try… 
 

Solution: 

 

 T1T2
1Lock-S(A) 
2Read(A) 
3 Lock-S(A)
4 Read(A)
5Lock-X(B) 
6Read(B) 
7Write(B) 
8Commit 
9Unlock(A) 
10Unlock(B) 
11 Lock-X(B)
12 Read(B)
13 Write(B)
14 Commit
15 Unlock(A)
16 Unlock(B)

Now, this is one way I choose to implement the locks on A and B. You may try a different sequence but remember to follow the 2-PL protocol. With that said, observe that our locks are released after Commit operation so this satisfies Rigorous 2-PL protocol. 

By now, I guess you must’ve got the idea of how to differentiate between types of 2-PL. Remember the theory as problems come in the examination sometimes just based on theoretical knowledge. Next, we’ll look at some examples of Conservative 2-PL and how does it differ from the above two types of 2-PL. What makes it Deadlock free and also so difficult to implement. Then we’ll conclude the topic of 2-PL. Shortly we’ll move on to another type of Lock-based Protocol- Graph-Based Protocols. They are also very interesting and provides a unique method to deal with the problem of Deadlocks! So we’ll learn a new type of locking protocol, that will conclude the topic of Lock-based Protocol for GATE, till then Happy Learning. 

GATE related question: 
GATE CS | IT 2004 | Question 77 

Attention reader! Don’t stop learning now.  Practice GATE exam well before the actual exam with the subject-wise and overall quizzes available in GATE Test Series Course.

Learn all GATE CS concepts with Free Live Classes on our youtube channel.

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