Recoverability and cascadeless schedules in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a recoverable schedule in database systems?

A recoverable schedule is one where transactions commit only after all transactions whose changes they depend on have committed. This ensures the database can be restored to a consistent state after failures.

beginner

Define a cascadeless schedule.

A cascadeless schedule is a schedule where transactions only read data written by committed transactions. This prevents cascading rollbacks, making recovery simpler.

intermediate

Why are cascading rollbacks problematic in databases?

Cascading rollbacks occur when one transaction failure causes multiple dependent transactions to roll back, leading to complex recovery and potential data inconsistency.

intermediate

How does a cascadeless schedule improve database recovery?

By ensuring transactions read only committed data, cascadeless schedules avoid cascading rollbacks, making recovery faster and simpler.

advanced

What is the relationship between recoverable and cascadeless schedules?

All cascadeless schedules are recoverable, but not all recoverable schedules are cascadeless. Cascadeless schedules are a stricter form that prevent cascading rollbacks.

What does a recoverable schedule ensure?

A Transactions commit immediately

B Transactions read uncommitted data

C Transactions commit only after dependent transactions commit

D Transactions never roll back

Recoverable schedules ensure that a transaction commits only after all transactions it depends on have committed.

Which schedule type prevents cascading rollbacks?

A Cascadeless schedule

B Non-recoverable schedule

C Recoverable schedule

D Dirty schedule

Cascadeless schedules prevent cascading rollbacks by allowing transactions to read only committed data.

What happens if a transaction reads uncommitted data and the writing transaction aborts?

A No effect on other transactions

B Cascading rollback occurs

C The reading transaction commits immediately

D The database becomes faster

Reading uncommitted data can cause cascading rollbacks if the writing transaction aborts.

Are all recoverable schedules cascadeless?

A Only in single transaction systems

B Yes, always

C Only if transactions never abort

D No, cascadeless is stricter

Cascadeless schedules are a stricter subset of recoverable schedules.

Which of the following is true about cascadeless schedules?

A They prevent cascading rollbacks

B They allow reading uncommitted data

C They are non-recoverable

D They increase rollback complexity

Cascadeless schedules prevent cascading rollbacks by restricting reads to committed data.

Explain what makes a schedule recoverable and why it is important for database consistency.

Think about when transactions are allowed to commit in relation to others.

Describe how cascadeless schedules help avoid cascading rollbacks and improve recovery.

Focus on the data read by transactions and its commit status.

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