Serializability in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the type of schedule that preserves consistency in transactions.

DBMS Theory

A schedule is considered [1] if it results in the same state as some serial execution of transactions.

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

Arecoverable

Bconcurrent

Cnon-serializable

Dserializable

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing serializable with concurrent schedules.

Thinking any interleaved schedule is serializable.

A serializable schedule ensures that the outcome is equivalent to some serial order of transactions, preserving consistency.

2fill in blank

medium

Complete the sentence to describe conflict serializability.

DBMS Theory

A schedule is conflict serializable if it can be transformed into a serial schedule by swapping [1] operations.

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Anon-conflicting

Bconflicting

Cread-only

Dwrite-only

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping conflicting operations which changes the schedule's effect.

Assuming all operations can be swapped.

Conflict serializability allows swapping only non-conflicting operations to achieve a serial schedule.

3fill in blank

hard

Fix the error in the statement about view serializability.

DBMS Theory

View serializability considers schedules equivalent if they have the same [1] and final writes.

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Aread-write dependencies

Bconflict order

Cview

Dtransaction order

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing conflict order with read-write dependencies.

Ignoring the importance of final writes.

View serializability depends on preserving view equivalence, which involves the same reads-from relations and final writes, not just conflict order.

4fill in blank

hard

Fill both blanks to complete the definition of recoverable schedules.

DBMS Theory

A schedule is recoverable if a transaction commits only after all transactions whose changes it [1] have [2].

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Areads from

Bcommitted

Cwrites to

Daborted

Attempts:

3 left

💡 Hint

Common Mistakes

Allowing commits before dependent transactions commit.

Confusing writes with reads in dependencies.

Recoverable schedules ensure that a transaction commits only after the transactions it reads from have committed, preventing cascading aborts.

5fill in blank

hard

Fill all three blanks to complete the explanation of strict schedules.

DBMS Theory

In a strict schedule, a transaction cannot [1] or [2] a data item until the transaction that last [3] it has committed or aborted.

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

Aread

Bwrite

Cwrote

Dlocked

Attempts:

3 left

💡 Hint

Common Mistakes

Allowing reads or writes before the last writer commits.

Confusing locking with writing.

Strict schedules prevent transactions from reading or writing data items until the transaction that last wrote them has finished, ensuring recoverability and avoiding cascading aborts.

Practice

1. What does serializability in a database ensure?

easy

A. Transactions appear to run one after another without overlap

B. All transactions run at the same time without any order

C. Data is deleted after each transaction

D. Only one transaction is allowed in the database

Solution

1. Step 1: Understand the meaning of serializability

   Serializability means the database behaves as if transactions are executed one by one, even if they run at the same time.

2. Step 2: Match the definition to the options

   Transactions appear to run one after another without overlap correctly states that transactions appear to run sequentially, which is the core idea of serializability.

3. Final Answer:

   Transactions appear to run one after another without overlap -> Option A

4. Quick Check:

   Serializability = Transactions run one by one [OK]

Hint: Serializability means no overlapping effects visible [OK]

Common Mistakes:

• Thinking transactions must run one at a time physically
• Confusing serializability with locking mechanisms
• Believing serializability deletes data
• Assuming only one transaction can exist

2. Which of the following schedules is not considered serializable?

easy

A. A schedule where transactions run one after another without overlap

B. A schedule where transactions are executed in a serial order

C. A schedule where transactions do not share any data

D. A schedule where transactions read and write the same data in conflicting order

Solution

1. Step 1: Identify what breaks serializability

   Conflicting operations like reading and writing the same data in overlapping order can cause non-serializable schedules.

2. Step 2: Analyze the options

   A schedule where transactions read and write the same data in conflicting order describes conflicting operations that break serializability. Options A, B, and C describe serial or conflict-free schedules.

3. Final Answer:

   A schedule where transactions read and write the same data in conflicting order -> Option D

4. Quick Check:

   Conflicting operations = Not serializable [OK]

Hint: Conflicting read/write means not serializable [OK]

Common Mistakes:

• Assuming all overlapping schedules are serializable
• Ignoring conflicts on shared data
• Thinking non-overlapping means non-serializable
• Confusing serial and serializable

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

medium

A. T2 writes A first, then T1 reads and writes A

B. T1 and T2 read A simultaneously, then both write A simultaneously

C. T1 reads and writes A, then T2 reads and writes A

D. T1 reads A, T2 writes A, then T1 writes A

Solution

1. Step 1: Understand conflict serializability

   A schedule is conflict serializable if it can be rearranged to a serial order without conflicts.

2. Step 2: Analyze each schedule

   T1 reads and writes A, then T2 reads and writes A runs T1 fully before T2, so no conflicts. T1 and T2 read A simultaneously, then both write A simultaneously has simultaneous writes causing conflict. T2 writes A first, then T1 reads and writes A runs T2 before T1. T1 reads A, T2 writes A, then T1 writes A has overlapping writes causing conflict.

3. Final Answer:

   T1 reads and writes A, then T2 reads and writes A -> Option C

4. Quick Check:

   Serial order without conflicts = T1 reads and writes A, then T2 reads and writes A [OK]

Hint: Serial order with no overlapping writes is serializable [OK]

Common Mistakes:

• Thinking simultaneous writes are allowed
• Ignoring order of writes
• Confusing conflict serializability with view serializability
• Assuming any schedule with reads is serializable

4. Identify the error in this schedule:
T1: Read(X), Write(X)
T2: Write(X), Read(X)
Schedule order: T1 Read(X), T2 Write(X), T1 Write(X), T2 Read(X)
What is the problem?

medium

A. This schedule is not conflict serializable due to write-read conflict

B. This schedule is serializable because writes happen in order

C. No error, schedule is correct

D. The schedule has a deadlock

Solution

1. Step 1: Check for conflicts in the schedule

   T1 reads X, then T2 writes X before T1 writes X, causing a write-read conflict.

2. Step 2: Determine serializability

   Because of the conflicting order of operations, the schedule cannot be rearranged into a serial order without conflicts.

3. Final Answer:

   This schedule is not conflict serializable due to write-read conflict -> Option A

4. Quick Check:

   Write-read conflict breaks serializability [OK]

Hint: Write-read conflicts break serializability [OK]

Common Mistakes:

• Assuming writes always serialize correctly
• Ignoring read after write conflicts
• Confusing deadlock with serializability issues
• Thinking schedule order doesn't matter

5. You have three transactions:
T1: Read(A), Write(B)
T2: Write(A), Read(B)
T3: Read(B), Write(A)
Which schedule is conflict serializable?

hard

A. T1 -> T2 -> T3

B. T2 -> T3 -> T1

C. T3 -> T1 -> T2

D. T1 -> T3 -> T2

Solution

1. Step 1: Analyze conflicts between transactions

   T1 and T2 conflict on A and B; T2 and T3 conflict on A and B; T1 and T3 conflict on B and A.

2. Step 2: Check which order avoids cycles in conflict graph

   Order T2 -> T3 -> T1 avoids cycles, making the schedule conflict serializable.

3. Final Answer:

   T2 -> T3 -> T1 -> Option B

4. Quick Check:

   Conflict graph acyclic = T2 -> T3 -> T1 [OK]

Hint: Check conflict graph cycles to find serializable order [OK]

Common Mistakes:

• Ignoring conflicts between transactions
• Assuming any order is serializable
• Not checking for cycles in conflict graph
• Mixing read-write and write-write conflicts