Recoverability and cascadeless schedules in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Recoverable Schedules

Which of the following best describes a recoverable schedule in database transactions?

A A schedule where transactions can commit in any order regardless of dependencies.

B A schedule where transactions never read data written by other transactions.

C A schedule that allows cascading rollbacks when a transaction fails.

D A schedule where transactions commit only after all transactions whose changes they read have committed.

💡 Hint

Think about when a transaction is allowed to commit if it reads data from another transaction.

A recoverable schedule ensures that if a transaction reads data written by another transaction, it commits only after that other transaction commits. This prevents inconsistencies if the first transaction aborts.

📋 Factual

intermediate

Identifying Cascadeless Schedules

What is a key property of a cascadeless schedule in transaction processing?

A Transactions can read uncommitted data but must abort if the data changes.

B Transactions only read data written by committed transactions, preventing cascading aborts.

C Transactions never write data to the database.

D Transactions commit immediately after writing data.

💡 Hint

Consider how cascadeless schedules avoid cascading rollbacks.

Cascadeless schedules prevent cascading aborts by ensuring that transactions only read data from transactions that have already committed.

🚀 Application

advanced

Analyzing Schedule Recoverability

Given the following schedule of two transactions T1 and T2:

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

Assuming T2 reads the value of A written by T1, which statement is true about this schedule?

A The schedule is recoverable and cascadeless.

B The schedule is cascadeless but not recoverable.

C The schedule is recoverable but not cascadeless.

D The schedule is not recoverable and not cascadeless.

💡 Hint

Check when T2 reads data and when T1 commits.

T2 reads data written by T1 before T1 commits, so the schedule is not cascadeless. However, since T2 commits after T1, the schedule is recoverable.

🔍 Analysis

advanced

Effects of Cascading Rollbacks

What is the main disadvantage of schedules that are not cascadeless?

A They can cause cascading rollbacks, where one transaction's failure causes multiple transactions to abort.

B They prevent any transaction from committing, causing deadlocks.

C They allow transactions to read only committed data, reducing concurrency.

D They guarantee serial execution of all transactions.

💡 Hint

Think about what happens when a transaction reads uncommitted data and the writing transaction aborts.

Non-cascadeless schedules allow transactions to read uncommitted data, so if the writing transaction aborts, all dependent transactions must also abort, causing cascading rollbacks.

❓ Reasoning

expert

Determining Schedule Properties from Transaction Logs

Consider a schedule with three transactions T1, T2, and T3. The following events occur in order:

1. T1 writes data item X
2. T2 reads X
3. T3 writes X
4. T1 commits
5. T2 commits
6. T3 commits

Which of the following statements is correct about this schedule?

A The schedule is recoverable but not cascadeless because T2 reads uncommitted data from T1.

B The schedule is cascadeless but not recoverable because T3 writes before T1 commits.

C The schedule is neither recoverable nor cascadeless due to T3's write before T1 commits.

D The schedule is both recoverable and cascadeless since all commits happen after reads.

💡 Hint

Focus on when T2 reads X and when T1 commits, and the effect of T3's write.

T2 reads X written by T1 before T1 commits, so the schedule is not cascadeless. However, T2 commits after T1, so it is recoverable. T3 writing X before T1 commits does not affect recoverability but may cause other concurrency issues.

Practice

1. What does a recoverable schedule in a database ensure?

easy

A. Transactions can be undone safely without causing inconsistency

B. All transactions run at the same time without waiting

C. Data is always encrypted during transactions

D. Transactions never read data from other transactions

Solution

1. Step 1: Understand recoverable schedule meaning

   A recoverable schedule ensures that if a transaction needs to be undone, it can be done safely without causing database inconsistency.

2. Step 2: Compare options with definition

   Transactions can be undone safely without causing inconsistency matches this definition exactly, while others describe unrelated concepts.

3. Final Answer:

   Transactions can be undone safely without causing inconsistency -> Option A

4. Quick Check:

   Recoverable schedule = safe undo [OK]

Hint: Recoverable means safe undo of transactions [OK]

Common Mistakes:

• Confusing recoverability with concurrency
• Thinking recoverable means no waiting
• Mixing encryption with recoverability

2. Which of the following is a correct property of a cascadeless schedule?

easy

A. Transactions never write data

B. Transactions can read uncommitted data from others

C. All transactions commit at the same time

D. No transaction reads data written by an uncommitted transaction

Solution

1. Step 1: Define cascadeless schedule property

   A cascadeless schedule prevents transactions from reading data that is not yet committed by other transactions.

2. Step 2: Match options to definition

   No transaction reads data written by an uncommitted transaction states no reading of uncommitted data, which is the key property of cascadeless schedules.

3. Final Answer:

   No transaction reads data written by an uncommitted transaction -> Option D

4. Quick Check:

   Cascadeless = no uncommitted reads [OK]

Hint: Cascadeless means no reading uncommitted data [OK]

Common Mistakes:

• Assuming cascadeless means all commit together
• Confusing cascadeless with recoverability
• Thinking transactions never write data

3. Consider the schedule:
T1: R(A), W(A), commit
T2: R(A), W(A), commit
Is this schedule recoverable and cascadeless?

medium

A. Both recoverable and cascadeless

B. Recoverable but not cascadeless

C. Cascadeless but not recoverable

D. Neither recoverable nor cascadeless

Solution

1. Step 1: Analyze reading and writing order

   T2 reads A before T1 commits. Since T1 has not committed before T2 reads, T2 reads uncommitted data.

2. Step 2: Check recoverability and cascadelessness

   Because T2 reads uncommitted data, the schedule is recoverable (since T1 commits before T2 commits) but not cascadeless.

3. Final Answer:

   Recoverable but not cascadeless -> Option B

4. Quick Check:

   Uncommitted read = not cascadeless, but commit order ensures recoverable [OK]

Hint: Check if reads happen after commit for cascadeless [OK]

Common Mistakes:

• Assuming reading after write means uncommitted read
• Confusing commit order with transaction order
• Ignoring commit before read

4. Identify the problem in this schedule:
T1: W(A), R(B), commit
T2: R(A), W(B), commit
Is this schedule cascadeless?

medium

A. Yes, because no transaction reads uncommitted data

B. No, because T1 reads B after T2 writes

C. No, because T2 reads A before T1 commits

D. Yes, because both commit at the end

Solution

1. Step 1: Check read of A by T2

   T2 reads A which was written by T1, but T1 has not committed before T2 reads A.

2. Step 2: Determine cascadeless property

   Since T2 reads uncommitted data from T1, the schedule is not cascadeless.

3. Final Answer:

   No, because T2 reads A before T1 commits -> Option C

4. Quick Check:

   Uncommitted read = not cascadeless [OK]

Hint: Check if reads happen before commit for cascadeless [OK]

Common Mistakes:

• Assuming commit order doesn't matter
• Thinking commit at end means cascadeless
• Ignoring read of uncommitted data

5. Given transactions:
T1: W(A), commit
T2: R(A), W(B), commit
T3: R(B), W(C), commit
Which schedule below is cascadeless and recoverable, ensuring T2 reads the value written by T1 and T3 reads the value written by T2?

A) T1 -> T2 -> T3
B) T2 -> T1 -> T3
C) T3 -> T2 -> T1
D) T1 -> T3 -> T2

hard

A. T1 -> T2 -> T3

B. T2 -> T1 -> T3

C. T3 -> T2 -> T1

D. T1 -> T3 -> T2

Solution

1. Step 1: Analyze schedule order T1 -> T2 -> T3

   T1 writes A and commits before T2 reads A. T2 writes B and commits before T3 reads B. So no transaction reads uncommitted data.

2. Step 2: Check recoverability and cascadelessness

   Since all reads happen after the respective writes commit, the schedule is both recoverable and cascadeless.

3. Final Answer:

   T1 -> T2 -> T3 -> Option A

4. Quick Check:

   Reads after commit = recoverable & cascadeless [OK]

Hint: Order transactions so reads follow commits [OK]

Common Mistakes:

• Ignoring commit order before reads
• Assuming any order is cascadeless
• Mixing up transaction dependencies