Two-phase locking (2PL) in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the first phase of Two-phase locking (2PL).

DBMS Theory

In Two-phase locking, the first phase is called the [1] phase where all locks are acquired.

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

Agrowing

Bshrinking

Ccommitting

Dwaiting

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the growing phase with the shrinking phase.

The first phase in 2PL is the growing phase where transactions acquire all the locks they need.

2fill in blank

medium

Complete the code to identify the second phase of Two-phase locking (2PL).

DBMS Theory

In Two-phase locking, the second phase is called the [1] phase where all locks are released.

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

Alocking

Bshrinking

Cwaiting

Dgrowing

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the names of the two phases.

The second phase in 2PL is the shrinking phase where transactions release locks and cannot acquire new ones.

3fill in blank

hard

Fix the error in the statement about Two-phase locking (2PL).

DBMS Theory

In 2PL, a transaction cannot acquire new locks during the [1] phase after it has started releasing locks.

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

Ashrinking

Bwaiting

Ccommitting

Dgrowing

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming locks can be acquired after releasing some locks.

In 2PL, once a transaction starts releasing locks (shrinking phase), it cannot acquire new locks. New locks can only be acquired during the growing phase.

4fill in blank

hard

Fill both blanks to complete the definition of strict Two-phase locking.

DBMS Theory

Strict 2PL requires that all [1] locks are held until the [2] of the transaction.

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Aexclusive

Bshared

Ccommit

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing shared locks with exclusive locks.

Releasing locks before commit.

Strict 2PL means all exclusive locks are held until the commit point of the transaction to ensure serializability and recoverability.

5fill in blank

hard

Fill all three blanks to complete the explanation of deadlock in 2PL.

DBMS Theory

Deadlock occurs when two transactions [1] locks that the other [2] and neither can [3].

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Ahold

Brequests

Cproceed

Drelease

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking deadlock means locks are released.

Confusing 'proceed' with 'release'.

Deadlock happens when two transactions hold locks that the other requests and neither can proceed, causing a standstill.

Practice

1. What is the main purpose of Two-phase locking (2PL) in a database system?

easy

A. To allow multiple users to write to the same data simultaneously

B. To speed up database queries by skipping locks

C. To ensure transactions are executed without interfering with each other

D. To delete old data automatically after transactions

Solution

1. Step 1: Understand the role of 2PL

   Two-phase locking controls how transactions lock data to avoid conflicts.

2. Step 2: Identify the main goal

   The goal is to keep data consistent by preventing transactions from interfering with each other.

3. Final Answer:

   To ensure transactions are executed without interfering with each other -> Option C

4. Quick Check:

   2PL ensures safe transaction execution = To ensure transactions are executed without interfering with each other [OK]

Hint: 2PL controls locks to keep data safe from conflicts [OK]

Common Mistakes:

• Thinking 2PL speeds up queries by skipping locks
• Believing 2PL allows simultaneous writes on same data
• Confusing 2PL with automatic data deletion

2. Which of the following correctly describes the two phases in Two-phase locking (2PL)?

easy

A. Lock acquisition phase followed by lock release phase

B. Lock release phase followed by lock acquisition phase

C. Transaction commit phase followed by rollback phase

D. Data read phase followed by data write phase

Solution

1. Step 1: Recall the two phases of 2PL

   2PL has a phase where locks are obtained first, then a phase where locks are released.

2. Step 2: Match the correct order

   The correct order is acquiring locks first, then releasing them after work is done.

3. Final Answer:

   Lock acquisition phase followed by lock release phase -> Option A

4. Quick Check:

   2PL phases = acquire locks then release locks [OK]

Hint: Locks must be acquired before releasing in 2PL [OK]

Common Mistakes:

• Reversing the order of lock release and acquisition
• Confusing 2PL phases with commit and rollback
• Mixing data read/write phases with locking phases

3. Consider two transactions T1 and T2 using strict 2PL. If T1 locks data item X and then T2 requests a lock on X, what will happen?

medium

A. Both T1 and T2 will share the lock on X

B. T2 will immediately get the lock on X

C. T1 will release the lock on X immediately

D. T2 will wait until T1 releases the lock on X

Solution

1. Step 1: Understand strict 2PL locking rules

   In strict 2PL, a transaction holds exclusive locks until it commits or aborts.

2. Step 2: Analyze the lock request by T2

   T2 must wait because T1 still holds the lock on X and has not released it yet.

3. Final Answer:

   T2 will wait until T1 releases the lock on X -> Option D

4. Quick Check:

   Strict 2PL means waiting for lock release [OK]

Hint: New lock requests wait if current lock is held [OK]

Common Mistakes:

• Assuming locks are shared simultaneously
• Thinking locks are released immediately after use
• Believing T2 can force lock release

4. A transaction T1 acquires a lock on data item A and then releases it before completing all its operations. What problem can this cause in Two-phase locking?

medium

A. It can cause a deadlock

B. It can violate serializability and cause inconsistent data

C. It speeds up transaction processing safely

D. It prevents other transactions from accessing data

Solution

1. Step 1: Understand the 2PL rule about lock release

   In 2PL, locks should be held until the transaction finishes to ensure consistency.

2. Step 2: Analyze early lock release effect

   Releasing locks early can allow other transactions to access data prematurely, causing inconsistent results.

3. Final Answer:

   It can violate serializability and cause inconsistent data -> Option B

4. Quick Check:

   Early lock release breaks serializability = It can violate serializability and cause inconsistent data [OK]

Hint: Releasing locks too soon breaks data consistency [OK]

Common Mistakes:

• Confusing deadlock with early lock release
• Thinking early release speeds up safely
• Assuming early release blocks others

5. In a database using strict 2PL, two transactions T1 and T2 both want to update data items X and Y. T1 locks X then Y, while T2 locks Y then X. What problem can arise and how can it be resolved?

hard

A. Deadlock can occur; resolved by ordering lock acquisition consistently

B. Data corruption can occur; resolved by disabling locks

C. No problem occurs; transactions proceed normally

D. Transactions will merge automatically; resolved by splitting data

Solution

1. Step 1: Identify the problem from lock order

   T1 locks X then Y, T2 locks Y then X, creating a circular wait condition.

2. Step 2: Recognize deadlock and resolution

   This circular wait causes deadlock; it can be resolved by enforcing a consistent lock order for all transactions.

3. Final Answer:

   Deadlock can occur; resolved by ordering lock acquisition consistently -> Option A

4. Quick Check:

   Conflicting lock order causes deadlock; fix by consistent order [OK]

Hint: Consistent lock order prevents deadlocks in 2PL [OK]

Common Mistakes:

• Ignoring deadlock possibility
• Thinking disabling locks solves corruption
• Assuming transactions merge automatically