Recoverability and cascadeless schedules in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the property of a schedule that ensures no transaction reads data from an uncommitted transaction.

DBMS Theory

A schedule is called [1] if it ensures no transaction reads data from an uncommitted transaction.

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

Arecoverable

Bcascadeless

Cserializable

Dconflict-serializable

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing recoverable with cascadeless schedules.

A cascadeless schedule prevents transactions from reading uncommitted data, avoiding cascading aborts.

2fill in blank

medium

Complete the code to define a schedule where transactions commit only after all transactions they depend on have committed.

DBMS Theory

A schedule is [1] if for every pair of transactions T_i and T_j, where T_j reads data written by T_i, T_i commits before T_j.

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

Aserializable

Bcascadeless

Crecoverable

Dstrict

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up cascadeless and recoverable definitions.

Recoverable schedules ensure that transactions commit only after the transactions they depend on have committed, preventing inconsistencies.

3fill in blank

hard

Fix the error in the statement about cascadeless schedules.

DBMS Theory

In a cascadeless schedule, transactions can read uncommitted data from other transactions, which may cause cascading aborts. This statement is [1].

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

Afalse

Btrue

Csometimes true

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming cascadeless schedules allow reading uncommitted data.

The statement is false because cascadeless schedules prevent reading uncommitted data, thus avoiding cascading aborts.

4fill in blank

hard

Fill both blanks to complete the definition of strict schedules.

DBMS Theory

A schedule is strict if transactions [1] any data item until they [2].

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Ado not read or write

Bcommit or abort

Cread only

Dwrite only

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing strict schedules with recoverable or cascadeless schedules.

Strict schedules prevent transactions from reading or writing data items until the transactions that last wrote them have committed or aborted, ensuring recoverability and cascadelessness.

5fill in blank

hard

Fill all three blanks to complete the explanation of schedule properties.

DBMS Theory

If a schedule is [1], it is also [2], but the reverse is not always true. Additionally, [3] schedules prevent cascading aborts by disallowing reads of uncommitted data.

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

Astrict

Brecoverable

Ccascadeless

Dserializable

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order and definitions of schedule properties.

Strict schedules are a subset of recoverable schedules. Cascadeless schedules specifically prevent cascading aborts by disallowing reads of uncommitted data.

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