Conflict serializability in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is conflict serializability in database systems?

Conflict serializability is a property of a schedule (sequence of operations) where the schedule can be transformed into a serial schedule by swapping non-conflicting operations. It ensures that concurrent transactions produce the same result as some serial execution.

beginner

What does it mean for two operations to be conflicting?

Two operations conflict if they belong to different transactions, access the same data item, and at least one of them is a write operation.

intermediate

Why is conflict serializability important in databases?

It ensures consistency by allowing concurrent transactions to execute without interfering in a way that causes incorrect results, making the schedule equivalent to some serial order.

intermediate

How can you check if a schedule is conflict serializable?

By constructing a precedence graph (or serialization graph) where nodes are transactions and edges represent conflicts. If the graph has no cycles, the schedule is conflict serializable.

intermediate

What is a precedence graph in the context of conflict serializability?

A directed graph where each node represents a transaction and edges represent conflicting operations that enforce order. It helps detect cycles to determine serializability.

Which of the following pairs of operations conflict?

A Write from T1 and Read from T1 on different data items

B Read from T1 and Write from T2 on the same data item

C Write from T1 and Write from T1 on the same data item

D Read from T1 and Read from T2 on the same data item

Operations conflict if they are from different transactions, access the same data item, and at least one is a write. Option B fits this.

What does a cycle in a precedence graph indicate?

A The schedule is conflict serializable

B The schedule is serial

C The transactions are independent

D The schedule is not conflict serializable

A cycle means there is no way to order transactions serially without conflicts, so the schedule is not conflict serializable.

Conflict serializability ensures that concurrent transactions produce results equivalent to:

A A random schedule

B A schedule with deadlocks

C A serial schedule

D A schedule with lost updates

Conflict serializability guarantees equivalence to some serial schedule, preserving correctness.

Which operation pair does NOT cause a conflict?

A Read T1 and Read T2 on same data

B Write T1 and Read T2 on same data

C Write T1 and Write T2 on same data

D Write T1 and Write T2 on different data

Two reads from different transactions on the same data do not conflict.

What is the first step to check conflict serializability?

A Build a precedence graph

B Run transactions serially

C Check for deadlocks

D Lock all data items

Building a precedence graph helps identify conflicts and cycles to determine serializability.

Explain conflict serializability and why it matters in managing database transactions.

Think about how transactions run together without causing errors.

Describe how to use a precedence graph to determine if a schedule is conflict serializable.

Imagine drawing arrows between transactions based on their conflicting operations.

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