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.