Conflict serializability in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the type of schedule that ensures transactions appear to execute one after another.

DBMS Theory

A schedule is called [1] serializable if it is equivalent to some serial schedule.

Drag options to blanks, or click blank then click option'

Aconcurrent

Bconflict

Crecoverable

Dnon-serial

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing conflict serializability with recoverability.

Thinking all concurrent schedules are serializable.

A conflict serializable schedule is one that can be transformed into a serial schedule by swapping non-conflicting operations.

2fill in blank

medium

Complete the code to describe the condition for two operations to conflict.

DBMS Theory

Two operations conflict if they access the same data item and at least one of them is a [1] operation.

Drag options to blanks, or click blank then click option'

Aabort

Bcommit

Cread

Dwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming two reads conflict.

Thinking commits cause conflicts.

Two operations conflict if they access the same data item and at least one is a write operation, because writes can change data.

3fill in blank

hard

Fix the error in the statement about conflict serializability.

DBMS Theory

If a schedule has no cycle in its [1] graph, it is conflict serializable.

Drag options to blanks, or click blank then click option'

Aconflict

Bdependency

Cprecedence

Dserialization

Attempts:

3 left

💡 Hint

Common Mistakes

Believing any cycle means serializability.

Confusing the type of graph used.

A schedule is conflict serializable if and only if its precedence graph has no cycles. A cycle means conflicts that prevent serializability.

4fill in blank

hard

Fill both blanks to complete the code that builds a precedence graph for conflict serializability.

DBMS Theory

For each pair of conflicting operations, add an edge from transaction [1] to transaction [2] in the precedence graph.

Drag options to blanks, or click blank then click option'

Aperforming the first operation

Bperforming the second operation

Ccommitting first

Daborting second

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the direction of edges.

Using commit or abort events instead of operation order.

Edges in the precedence graph go from the transaction performing the first conflicting operation to the one performing the second.

5fill in blank

hard

Fill all three blanks to complete the code that checks conflict serializability using a precedence graph.

DBMS Theory

Build the graph with nodes as transactions, add edges for conflicts, then check if the graph has any [1]. If no cycles, the schedule is [2] serializable; otherwise, it is [3] serializable.

Drag options to blanks, or click blank then click option'

Acycles

Bconflict

Cnot

Drecoverable

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing recoverability with conflict serializability.

Thinking cycles mean serializability.

The presence of cycles in the precedence graph means the schedule is not conflict serializable. Without cycles, it is conflict serializable.

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