Conflict Serializability-

 

Before you go through this article, make sure that you have gone through the previous article on Conflict Serializability.

 

We have discussed-

• The concept of serializability helps to identify the correct non-serial schedules that will maintain the consistency of the database.
• There are two types of serializability-

 

 

In this article, we will discuss practice problems based on conflict serializability.

 

PRACTICE PROBLEMS BASED ON CONFLICT SERIALIZABILITY-

 

Problem-01:

 

Check whether the given schedule S is conflict serializable or not-

S : R₁(A) , R₂(A) , R₁(B) , R₂(B) , R₃(B) , W₁(A) , W₂(B)

 

Solution-

 

Step-01:

 

List all the conflicting operations and determine the dependency between the transactions-

• R₂(A) , W₁(A)              (T₂ → T₁)
• R₁(B) , W₂(B)              (T₁ → T₂)
• R₃(B) , W₂(B)              (T₃ → T₂)

 

Step-02:

 

Draw the precedence graph-

 

 

• Clearly, there exists a cycle in the precedence graph.
• Therefore, the given schedule S is not conflict serializable.

 

Problem-02:

 

Check whether the given schedule S is conflict serializable and recoverable or not-

 

 

Solution-

 

Checking Whether S is Conflict Serializable Or Not-

 

Step-01:

 

List all the conflicting operations and determine the dependency between the transactions-

• R₂(X) , W₃(X)              (T₂ → T₃)
• R₂(X) , W₁(X)              (T₂ → T₁)
• W₃(X) , W₁(X)             (T₃ → T₁)
• W₃(X) , R₄(X)              (T₃ → T₄)
• W₁(X) , R₄(X)              (T₁ → T₄)
• W₂(Y) , R₄(Y)              (T₂ → T₄)

 

Step-02:

 

Draw the precedence graph-

 

 

• Clearly, there exists no cycle in the precedence graph.
• Therefore, the given schedule S is conflict serializable.

 

Checking Whether S is Recoverable Or Not-

 

• Conflict serializable schedules are always recoverable.
• Therefore, the given schedule S is recoverable.

 

Alternatively,

• There exists no dirty read operation.
• This is because all the transactions which update the values commits immediately.
• Therefore, the given schedule S is recoverable.
• Also, S is a Cascadeless Schedule.

 

Problem-03:

 

Check whether the given schedule S is conflict serializable or not. If yes, then determine all the possible serialized schedules-

 

 

Solution-

 

Checking Whether S is Conflict Serializable Or Not-

 

Step-01:

 

List all the conflicting operations and determine the dependency between the transactions-

• R₄(A) , W₂(A)              (T₄ → T₂)
• R₃(A) , W₂(A)              (T₃ → T₂)
• W₁(B) , R₃(B)              (T₁ → T₃)
• W₁(B) , W₂(B)             (T₁ → T₂)
• R₃(B) , W₂(B)              (T₃ → T₂)

 

Step-02:

 

Draw the precedence graph-

 

 

• Clearly, there exists no cycle in the precedence graph.
• Therefore, the given schedule S is conflict serializable.

 

Finding the Serialized Schedules-

 

• All the possible topological orderings of the above precedence graph will be the possible serialized schedules.
• The topological orderings can be found by performing the Topological Sort of the above precedence graph.

 

After performing the topological sort, the possible serialized schedules are-

1. T₁ → T₃ → T₄ → T₂
2. T₁ → T₄ → T₃ → T₂
3. T₄ → T₁ → T₃ → T₂

 

Problem-04:

 

Determine all the possible serialized schedules for the given schedule-

 

 

Solution-

 

The given schedule S can be rewritten as-

 

 

This is because we are only concerned about the read and write operations taking place on the database.

 

Checking Whether S is Conflict Serializable Or Not-

 

Step-01:

 

List all the conflicting operations and determine the dependency between the transactions-

• R₁(A) , W₂(A)              (T₁ → T₂)
• R₂(A) , W₁(A)              (T₂ → T₁)
• W₂(A) , W₁(A)             (T₂ → T₁)
• R₂(B) , W₁(B)              (T₂ → T₁)
• R₁(B) , W₂(B)              (T₁ → T₂)
• W₁(B) , W₂(B)             (T₁ → T₂)

 

Step-02:

 

Draw the precedence graph-

 

 

• Clearly, there exists a cycle in the precedence graph.
• Therefore, the given schedule S is not conflict serializable.
• Thus, Number of possible serialized schedules = 0.

 

Next Article- View Serializability in DBMS

 

Get more notes and other study material of Database Management System (DBMS).

Watch video lectures by visiting our YouTube channel LearnVidFun.

Schedules in DBMS-

 

Before you go through this article, make sure that you have gone through the previous article on Schedules in DBMS.

 

We have discussed-

• A schedule is the order in which the operations of multiple transactions appear for execution.
• Serial schedules are always consistent.
• Non-serial schedules are not always consistent.

 

In DBMS, schedules may be classified as-

 

 

In this article, we will discuss about Serializability in DBMS.

 

Serializability in DBMS-

 

• Some non-serial schedules may lead to inconsistency of the database.
• Serializability is a concept that helps to identify which non-serial schedules are correct and will maintain the consistency of the database.

 

Serializable Schedules-

 

If a given non-serial schedule of ‘n’ transactions is equivalent to some serial schedule of ‘n’ transactions, then it is called as a serializable schedule.

 

Characteristics-

 

Serializable schedules behave exactly same as serial schedules.

Thus, serializable schedules are always-

• Consistent
• Recoverable
• Casacadeless
• Strict

 

Serial Schedules Vs Serializable Schedules-

 

Serial Schedules	Serializable Schedules
No concurrency is allowed. Thus, all the transactions necessarily execute serially one after the other.	Concurrency is allowed. Thus, multiple transactions can execute concurrently.
Serial schedules lead to less resource utilization and CPU throughput.	Serializable schedules improve both resource utilization and CPU throughput.
Serial Schedules are less efficient as compared to serializable schedules. (due to above reason)	Serializable Schedules are always better than serial schedules. (due to above reason)

 

Types of Serializability-

 

Serializability is mainly of two types-

 

 

1. Conflict Serializability
2. View Serializability

 

In this article, we will discuss about Conflict Serializability.

Learn about View Serializability.

 

Conflict Serializability-

 

If a given non-serial schedule can be converted into a serial schedule by swapping its non-conflicting operations, then it is called as a conflict serializable schedule.

 

Conflicting Operations-

 

Two operations are called as conflicting operations if all the following conditions hold true for them-

• Both the operations belong to different transactions
• Both the operations are on the same data item
• At least one of the two operations is a write operation

 

Example-

 

Consider the following schedule-

 

 

In this schedule,

• W1 (A) and R2 (A) are called as conflicting operations.
• This is because all the above conditions hold true for them.

 

Checking Whether a Schedule is Conflict Serializable Or Not-

 

Follow the following steps to check whether a given non-serial schedule is conflict serializable or not-

 

Step-01:

 

Find and list all the conflicting operations.

 

Step-02:

 

Start creating a precedence graph by drawing one node for each transaction.

 

Step-03:

 

• Draw an edge for each conflict pair such that if X_i (V) and Y_j (V) forms a conflict pair then draw an edge from T_i to T_j.
• This ensures that T_i gets executed before T_j.

 

Step-04:

 

• Check if there is any cycle formed in the graph.
• If there is no cycle found, then the schedule is conflict serializable otherwise not.

 

NOTE-

 

• By performing the Topological Sort of the Directed Acyclic Graph so obtained, the corresponding serial schedule(s) can be found.
• Such schedules can be more than 1.

 

Next Article- Practice Problems On Conflict Serializability

 

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Watch video lectures by visiting our YouTube channel LearnVidFun.