Database Recoverability Concepts

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Questions and Answers

What term describes the issue where one transaction reads data from another transaction that has not yet been committed?

Unrepeatable read

Cascading rollback

Dirty read (correct)

Incorrect summary

Which situation exemplifies the incorrect summary problem?

Transaction T reading updated item X after T1 fails.

Transaction T reading the same item multiple times successfully.

Transaction T calculating aggregate functions while others modify the data. (correct)

Transaction T conducting operations without any transaction interference.

What is the main risk associated with cascading rollback?

Transactions complete successfully without interdependencies.

Data integrity is improved across transactions.

Databases become permanently locked.

Multiple transactions become dependent on each other's failure. (correct)

When does the unrepeatable read problem occur?

When a transaction reads the same data item multiple times with different results. Signup and view all the answers

What is the outcome of a recoverable schedule in transaction management?

A transaction can only commit if all transactions it depends on have also committed. Signup and view all the answers

Which of the following is NOT a characteristic of a non-recoverable schedule?

It guarantees the integrity of the transactions. Signup and view all the answers

Which of the following protocols is designed to ensure that a transaction is either fully completed and committed or has no effect?

Two-phase commit protocol Signup and view all the answers

What is a potential problem when a transaction reads the same item multiple times under different states of updates?

Unrepeatable read Signup and view all the answers

What defines a recoverable schedule in a database system?

Every transaction in the schedule must read only items written by committed transactions. Signup and view all the answers

Which of the following can lead to a cascading rollback?

A transaction reading uncommitted data from another transaction. Signup and view all the answers

In a non-recoverable schedule, what happens if a transaction fails after being read by another transaction?

The read transaction must also be aborted. Signup and view all the answers

What does a cascadeless schedule ensure?

Transactions read only items that have been written by committed transactions. Signup and view all the answers

Which of the following statements about the practice question is true: 'Sc: r1(X); w1(X); r2(X); r1(Y); w2(X); c2; a1;'?

The schedule is not recoverable. Signup and view all the answers

What is a potential consequence of a cascading rollback?

Increased time and cost in recovering transactions. Signup and view all the answers

Which commit protocol is primarily a preventive measure against cascading rollbacks?

Cascadeless protocol. Signup and view all the answers

Which of the following best describes the role of the DBMS with regard to concurrent schedules?

It imposes an ordering on transaction commits to ensure recoverability. Signup and view all the answers

What characteristic disqualifies a schedule from being classified as cascadeless recoverable?

A transaction reads a value before the transaction that wrote it is committed. Signup and view all the answers

Why is a strict schedule generally considered safer for transaction management?

It only permits committed transactions to read and write. Signup and view all the answers

In the provided schedule, what is the primary reason it is deemed non-cascadeless recoverable?

One transaction reads a value updated by another non-committed transaction. Signup and view all the answers

What aspect of transaction committing dictates whether a schedule can be strict recoverable?

Transactions must commit in the sequence they are executed. Signup and view all the answers

Which of the following statements correctly describes a non-recoverable schedule?

It allows some transactions to write before they are committed. Signup and view all the answers

In the second analyzed schedule, why is it considered not strict recoverable?

One transaction writes data that another transaction subsequently overwrites. Signup and view all the answers

What consequence is inherent in a situation with cascading rollbacks?

Any single failure can require rolling back multiple transactions. Signup and view all the answers

Which of the following correctly refers to a schedule that executes operations in a way that no transaction reads uncommitted data?

Strict Schedule Signup and view all the answers

Study Notes

Recoverability

Every concurrent schedule must be recoverable by imposing a specific ordering on transaction commits
Recoverability means that if a transaction aborts, the database must be restored to a consistent state, which could require significant rework.
Cascading Rollback (or Cascading Abort) occurs when a failed transaction committed some changes, and other transactions read those changes before the failed transaction was rolled back
Cascadeless Schedule prevents cascading rollbacks by forbidding the reading of data from uncommitted transactions
Strict Schedule is even more restrictive: it only allows reading and writing from committed transactions.

Examples of Recoverable Schedules

Not recoverable
- Schedule: r1(X); w1(X); r2(X); r1(Y); w2(X); c2; a1;
Recoverable, not cascadeless recoverable
- Schedule: r1(x), r2(z), r1(z), r3(x), r3(y), w1(x), w3(y), r2(y), w2(z), c1, c3, c2
- This schedule is not cascadeless recoverable because T3 (uncommitted) writes to y, and the T2 read the value of y from T3 before T3 committed
Recoverable, cascadeless recoverable, not strict recoverable
- Schedule: r1(x), r2(z), r3(x), r1(z), r2(y), r3(y), w1(x), c1, w2(z), w3(y), w2(y), c3, c2
- This schedule suffers from dirty read problem, as T1 updates x and fails before completion, causing the system to rollback x, while T2 reads the temporary, non_committed value of x
Strict Recoverable
- Any schedule where transactions commit in the same order as they are completing their tasks.
- It avoids dirty reads and W-R issues, making it a good choice for ensuring data consistency.

Transaction States, Concepts

Incorrect Summary Problem may arise when a transaction calculates an aggregate summary based on data that is being modified by other transactions.
- This can lead to inconsistent sum, average, or count results.
Unrepeatable Read Problem occurs when a transaction reads the same item twice and in between the reads, another transaction changes the value.
- It produces inconsistent results for the same data points.
Recovery is Needed to ensure that either all the operations in a transaction are completed successfully and committed, or the system is rolled back to a consistent state.

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Description

This quiz covers essential concepts of recoverability in database transactions, including definitions and examples of recoverable schedules. You will explore terms like cascading rollback, cascadeless schedule, and strict schedule, as well as how they relate to transaction commits.