Conflict serializability in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Conflict Serializability

Which of the following best describes conflict serializability in database transactions?

A A schedule where transactions are executed one after another without overlapping.

B A schedule where all transactions are rolled back to maintain consistency.

C A schedule that allows transactions to execute in parallel without any restrictions.

D A schedule where non-conflicting operations can be swapped to produce a serial schedule.

💡 Hint

Think about how operations can be reordered without changing the final result.

Conflict serializability means that a schedule can be transformed into a serial schedule by swapping non-conflicting operations. This ensures the final database state is the same as if transactions ran one after another.

🔍 Analysis

intermediate

Detecting Conflict Serializability from a Schedule

Given the following schedule of two transactions T1 and T2:

T1: R(A), W(A), R(B), W(B)
T2: R(B), W(B), R(A), W(A)

Is this schedule conflict serializable?

A No, because the conflicting operations create a cycle in the precedence graph.

B Yes, because the schedule is already serial.

C Yes, because the operations on A and B do not conflict.

D No, because transactions are executed simultaneously.

💡 Hint

Try drawing the precedence graph to check for cycles.

The schedule has conflicting operations on A and B that form a cycle in the precedence graph, so it is not conflict serializable.

❓ Comparison

advanced

Conflict Serializability vs View Serializability

Which statement correctly compares conflict serializability and view serializability?

A All conflict serializable schedules are view serializable, but not all view serializable schedules are conflict serializable.

B All view serializable schedules are conflict serializable, but not all conflict serializable schedules are view serializable.

C Conflict serializability and view serializability are exactly the same concepts.

D Neither conflict serializability nor view serializability guarantees correctness of schedules.

💡 Hint

Think about which type of serializability is more general.

Conflict serializability is a stricter condition. Every conflict serializable schedule is view serializable, but some view serializable schedules are not conflict serializable.

❓ Reasoning

advanced

Effect of Conflicting Operations on Schedule Serializability

Consider two transactions T1 and T2 accessing data items X and Y. If T1 writes X before T2 reads X, and T2 writes Y before T1 reads Y, what does this imply about the schedule's conflict serializability?

A The schedule is conflict serializable because writes before reads do not cause conflicts.

B The schedule is not conflict serializable because these conflicting operations create a cycle in the precedence graph.

C The schedule is conflict serializable because the operations are on different data items.

D The schedule is not conflict serializable because reads always conflict with writes.

💡 Hint

Analyze the direction of conflicts and check for cycles.

Conflicting operations in opposite directions on different data items create a cycle in the precedence graph, making the schedule not conflict serializable.

🚀 Application

expert

Determining the Number of Conflict Serializable Schedules

Given two transactions T1 and T2, each with two operations: T1 performs R(A) then W(B), and T2 performs W(A) then R(B). How many possible schedules of these four operations are conflict serializable?

A 2

B 6

C 4

D 8

💡 Hint

List all possible interleavings and check which have no cycles in their precedence graphs.

Out of all possible schedules, only 4 maintain conflict serializability because others create cycles due to conflicting operations on A and B.

Practice

1. What does conflict serializability ensure in database transactions?

easy

A. Transactions produce the same result as if they were executed one after another

B. Transactions run faster by skipping some operations

C. Transactions always lock all data to avoid conflicts

D. Transactions are executed only by one user at a time

Solution

1. Step 1: Understand the goal of conflict serializability

   It aims to keep the database consistent by ensuring concurrent transactions behave like they ran sequentially.

2. Step 2: Compare options with the definition

   Only Transactions produce the same result as if they were executed one after another correctly states that the final result is as if transactions ran one after another.

3. Final Answer:

   Transactions produce the same result as if they were executed one after another -> Option A

4. Quick Check:

   Conflict serializability = Sequential result [OK]

Hint: Think: final result matches some order of transactions [OK]

Common Mistakes:

• Confusing conflict serializability with locking all data
• Thinking it speeds up transactions by skipping steps
• Assuming only one user can run transactions at a time

2. Which of the following is a correct way to check conflict serializability?

easy

A. Build a precedence graph and check for cycles

B. Count the number of transactions running simultaneously

C. Verify if all transactions use the same isolation level

D. Check if transactions use locks on all data items

Solution

1. Step 1: Recall the method to test conflict serializability

   It involves creating a precedence graph where nodes are transactions and edges show conflicts.

2. Step 2: Understand the graph's role

   If the graph has no cycles, the schedule is conflict serializable.

3. Final Answer:

   Build a precedence graph and check for cycles -> Option A

4. Quick Check:

   Precedence graph + no cycles = conflict serializable [OK]

Hint: Use graph cycles to detect conflicts fast [OK]

Common Mistakes:

• Thinking counting transactions shows serializability
• Confusing isolation levels with conflict serializability
• Assuming locking all data guarantees serializability

3. Consider two transactions:
T1: Read(A), Write(A)
T2: Read(A), Write(A)
Which schedule is conflict serializable?

medium

A. None of the above

B. Read(A) by T1, Read(A) by T2, Write(A) by T1, Write(A) by T2

C. Read(A) by T2, Read(A) by T1, Write(A) by T2, Write(A) by T1

D. T1 followed by T2

Solution

1. Step 1: Analyze schedules for conflicts

   Schedules where one transaction runs fully before the other have no conflicts and are serializable.

2. Step 2: Check interleaved schedule

   Interleaving reads and writes on the same data causes conflicts and cycles in the precedence graph.

3. Final Answer:

   T1 followed by T2 -> Option D

4. Quick Check:

   Non-interleaved = serializable [OK]

Hint: Serial schedules are always conflict serializable [OK]

Common Mistakes:

• Assuming interleaved schedules are always serializable
• Ignoring write-write conflicts on same data
• Thinking any order of reads is conflict free

4. Given a schedule with transactions T1 and T2:
T1: Read(X), Write(X)
T2: Read(X), Write(X)
The schedule is:
Read(X) by T1, Read(X) by T2, Write(X) by T2, Write(X) by T1.
What is the problem with this schedule?

medium

A. It is a serial schedule with no conflicts

B. It causes a deadlock between T1 and T2

C. It is not conflict serializable due to a cycle in the precedence graph

D. It is conflict serializable because reads happen before writes

Solution

1. Step 1: Build the precedence graph

   T1 reads X before T2 writes X, so edge T1 -> T2; T2 writes X before T1 writes X, so edge T2 -> T1.

2. Step 2: Detect cycle in graph

   Edges T1 -> T2 and T2 -> T1 form a cycle, meaning schedule is not conflict serializable.

3. Final Answer:

   It is not conflict serializable due to a cycle in the precedence graph -> Option C

4. Quick Check:

   Cycle in graph = not conflict serializable [OK]

Hint: Look for cycles in transaction dependency graph [OK]

Common Mistakes:

• Confusing deadlock with conflict serializability
• Assuming reads before writes always mean serializable
• Thinking schedule is serial when it is interleaved

5. You have three transactions:
T1: Read(A), Write(A)
T2: Read(B), Write(B)
T3: Read(A), Write(B)
If the schedule is:
Read(A) by T1, Read(B) by T2, Read(A) by T3, Write(A) by T1, Write(B) by T3, Write(B) by T2,
Is this schedule conflict serializable? If yes, what is a possible serial order?

hard

A. Yes, serial order: T2 -> T1 -> T3

B. No, schedule is not conflict serializable due to cycles

C. Yes, serial order: T1 -> T3 -> T2

D. Yes, serial order: T3 -> T1 -> T2

Solution

1. Step 1: Identify conflicts and build precedence graph

   T3 reads A before T1 writes A -> T3 -> T1.
   T2 reads B before T3 writes B -> T2 -> T3.
   T3 writes B before T2 writes B -> T3 -> T2.

2. Step 2: Check for cycles and find serial order

   Cycle T2 -> T3 -> T2 exists, so the schedule is not conflict serializable.

3. Final Answer:

   No, schedule is not conflict serializable due to cycles -> Option B

4. Quick Check:

   Precedence graph has cycle = not serializable [OK]

Hint: Build graph, check for cycles: cycle = not serializable [OK]

Common Mistakes:

• Missing edges like T2 read B before T3 write B
• Missing edges in the precedence graph
• Confusing read-write order causing edges