Open In App
Related Articles

Conflict Serializability in DBMS

Improve
Improve
Improve
Like Article
Like
Save Article
Save
Report issue
Report

As discussed in Concurrency control, serial schedules have less resource utilization and low throughput. To improve it, two or more transactions are run concurrently. However, concurrency of transactions may lead to inconsistency in the database. To avoid this, we need to check whether these concurrent schedules are serializable or not.

Conflict Serializable

Concurrency serializability, also known as conflict serializability, is a type of concurrency control that guarantees that the outcome of concurrent transactions is the same as if the transactions were executed consecutively.

Conflict serializable schedules: A schedule is called conflict serializable if it can be transformed into a serial schedule by swapping non-conflicting operations.

Non-conflicting operations: When two operations operate on separate data items or the same data item but at least one of them is a read operation, they are said to be non-conflicting.

Conflicting Operations

Two operations are said to be conflicting if all conditions are satisfied: 

  • They belong to different transactions
  • They operate on the same data item
  • At Least one of them is a write operation

Example:

  • Conflicting operations pair (R1(A), W2(A)) because they belong to two different transactions on the same data item A and one of them is a write operation.
  • Similarly, (W1(A), W2(A)) and (W1(A), R2(A)) pairs are also conflicting.
  • On the other hand, the (R1(A), W2(B)) pair is non-conflicting because they operate on different data items.
  • Similarly, ((W1(A), W2(B)) pair is non-conflicting.

Consider the following schedule: 

S1: R1(A), W1(A), R2(A), W2(A), R1(B), W1(B), R2(B), W2(B)

If Oi and Oj are two operations in a transaction and Oi< Oj (Oi is executed before Oj), same order will follow in the schedule as well. Using this property, we can get two transactions of schedule S1: 

T1: R1(A), W1(A), R1(B), W1(B)
T2: R2(A), W2(A), R2(B), W2(B)

Possible Serial Schedules are: T1->T2 or T2->T1 

-> Swapping non-conflicting operations R2(A) and R1(B) in S1, the schedule becomes, 

S11: R1(A), W1(A), R1(B), W2(A), R2(A), W1(B), R2(B), W2(B)

-> Similarly, swapping non-conflicting operations W2(A) and W1(B) in S11, the schedule becomes, 

S12: R1(A), W1(A), R1(B), W1(B), R2(A), W2(A), R2(B), W2(B)

S12 is a serial schedule in which all operations of T1 are performed before starting any operation of T2. Since S has been transformed into a serial schedule S12 by swapping non-conflicting operations of S1, S1 is conflict serializable.

Let us take another Schedule: 

S2: R2(A), W2(A), R1(A), W1(A), R1(B), W1(B), R2(B), W2(B)

Two transactions will be:  

T1: R1(A), W1(A), R1(B), W1(B)
T2: R2(A), W2(A), R2(B), W2(B)

Possible Serial Schedules are: T1->T2 or T2->T1 

Original Schedule is as:  

S2: R2(A), W2(A), R1(A), W1(A), R1(B), W1(B), R2(B), W2(B)

Swapping non-conflicting operations R1(A) and R2(B) in S2, the schedule becomes, 

S21: R2(A), W2(A), R2(B), W1(A), R1(B), W1(B), R1(A), W2(B)

Similarly, swapping non-conflicting operations W1(A) and W2(B) in S21, the schedule becomes,  

S22: R2(A), W2(A), R2(B), W2(B), R1(B), W1(B), R1(A), W1(A)

In schedule S22, all operations of T2 are performed first, but operations of T1 are not in order (order should be R1(A), W1(A), R1(B), W1(B)). So S2 is not conflict serializable.

Conflict Equivalent

Two schedules are said to be conflict equivalent when one can be transformed to another by swapping non-conflicting operations. In the example discussed above, S11 is conflict equivalent to S1 (S1 can be converted to S11 by swapping non-conflicting operations). Similarly, S11 is conflict equivalent to S12, and so on.

Note 1: Although S2 is not conflict serializable, still it is conflict equivalent to S21 and S21 because S2 can be converted to S21 and S22 by swapping non-conflicting operations.

Note 2: The schedule which is conflict serializable is always conflict equivalent to one of the serial schedule. S1 schedule discussed above (which is conflict serializable) is equivalent to the serial schedule (T1->T2).

GATE Question

Question: Consider the following schedules involving two transactions. Which one of the following statements is true? [GATE 2007] 

S1: R1(X) R1(Y) R2(X) R2(Y) W2(Y) W1(X) 
S2: R1(X) R2(X) R2(Y) W2(Y) R1(Y) W1(X) 

  • Both S1 and S2 are conflict serializable
  • Only S1 is conflict serializable
  • Only S2 is conflict serializable
  • None

Solution: Two transactions of given schedules are: 

 T1: R1(X) R1(Y) W1(X)
T2: R2(X) R2(Y) W2(Y)

Let us first check serializability of S1: 

S1: R1(X) R1(Y) R2(X) R2(Y) W2(Y) W1(X)

To convert it to a serial schedule, we have to swap non-conflicting operations so that S1 becomes equivalent to serial schedule T1->T2 or T2->T1. In this case, to convert it to a serial schedule, we must have to swap R2(X) and W1(X) but they are conflicting. So S1 can’t be converted to a serial schedule. 

Now, let us check serializability of S2: 

S2: R1(X) R2(X) R2(Y) W2(Y) R1(Y) W1(X)

Swapping non conflicting operations R1(X) and R2(X) of S2, we get 

S2’: R2(X) R1(X) R2(Y) W2(Y) R1(Y) W1(X)

Again, swapping non conflicting operations R1(X) and R2(Y) of S2’, we get 

S2’’: R2(X) R2(Y) R1(X) W2(Y) R1(Y) W1(X)

Again, swapping non conflicting operations R1(X) and W2(Y) of S2’’, we get 

S2’’’: R2(X) R2(Y) W2(Y) R1(X) R1(Y) W1(X)

which is equivalent to a serial schedule T2->T1. 

So, correct option is C. Only S2 is conflict serializable. 

Advantages of Conflict Serializability

  • Consistency: Conflict serializability guarantees that the transactions’ outcomes correspond to the sequence in which they were carried out.
  • Correctness: Regardless of the order in which transactions were submitted, conflict serializability guarantees that transactions are executed correctly.
  • Decreased Overhead: By doing away with pointless locking and other conflict resolution techniques, conflict serializability lowers overhead.
  • Enhanced Concurrency: By enabling concurrent execution of operations without causing conflicts, conflict serializability enhances concurrency.

Disadvantages of Conflict Serializability

  • Complexity: Conflict serializability can be complex to implement, especially in large and complex databases.
  • Reduced Performance: Conflict serializability can reduce performance by introducing delays and overhead due to locking and other conflict resolution mechanisms.
  • Limited Concurrency: Conflict serializability can limit the degree of concurrency in the system because it may delay some transactions to avoid conflicts.
  • Increased Overhead: Conflict serializability requires additional overhead to maintain the order of the transactions and ensure that they do not conflict with each other.

Conclusion

  • The consistency of the database is guaranteed via conflict serializability, even in situations when several transactions are running simultaneously.
  • Schedules that are conflict serializable are always view serializable.
  • Compared to view serializability, conflict serializability is simpler to accomplish.


Last Updated : 12 Nov, 2023
Like Article
Save Article
Previous
Next
Share your thoughts in the comments
Similar Reads