Two-phase locking (2PL) in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Phases of Two-Phase Locking

Which of the following best describes the two phases in the Two-Phase Locking (2PL) protocol?

A Growing phase where locks are acquired and shrinking phase where locks are released

B Shrinking phase where locks are acquired and growing phase where locks are released

C Phase one where transactions execute without locks, phase two where locks are applied

D Phase one where all locks are released, phase two where all locks are acquired

💡 Hint

Think about when locks can be obtained and when they must be released in 2PL.

In 2PL, the growing phase is when a transaction can acquire locks but cannot release any. The shrinking phase is when it releases locks but cannot acquire new ones.

📋 Factual

intermediate

Effect of 2PL on Transaction Serializability

What is the main guarantee provided by the Two-Phase Locking protocol regarding transaction execution?

A It guarantees transactions will never deadlock

B It guarantees serializability of concurrent transactions

C It guarantees transactions will execute faster than without locking

D It guarantees that transactions will always commit successfully

💡 Hint

Consider what serializability means for concurrent transactions.

2PL ensures that the schedule of transactions is serializable, meaning the outcome is equivalent to some serial order of execution.

🔍 Analysis

advanced

Deadlock Possibility in Two-Phase Locking

Which statement correctly describes deadlock behavior in strict Two-Phase Locking?

A Deadlocks can occur because transactions hold locks until commit, potentially waiting on each other

B Deadlocks cannot occur because locks are released immediately after use

C Deadlocks are prevented by releasing all locks before acquiring new ones

D Deadlocks are avoided by allowing transactions to acquire locks in any order

💡 Hint

Think about how holding locks until commit affects waiting between transactions.

Strict 2PL holds all locks until commit, so transactions may wait for each other indefinitely, causing deadlocks.

❓ Comparison

advanced

Difference Between Basic 2PL and Strict 2PL

What is the key difference between basic Two-Phase Locking and Strict Two-Phase Locking?

A Strict 2PL allows lock acquisition after releasing some locks, basic 2PL does not

B Basic 2PL holds all locks until commit, strict 2PL releases locks immediately after use

C Strict 2PL holds all exclusive locks until transaction commit, basic 2PL releases locks during shrinking phase

D Basic 2PL prevents deadlocks, strict 2PL does not

💡 Hint

Consider when exclusive locks are released in both protocols.

Strict 2PL requires exclusive locks to be held until commit to ensure recoverability, while basic 2PL releases locks in the shrinking phase.

❓ Reasoning

expert

Impact of Violating the Growing Phase in 2PL

What is the likely consequence if a transaction releases a lock and then later tries to acquire another lock in Two-Phase Locking?

A The transaction will commit successfully without any problems

B The transaction will automatically restart without any issues

C The transaction will deadlock immediately

D The transaction violates 2PL and may cause non-serializable schedules

💡 Hint

Recall the rule about acquiring and releasing locks in 2PL phases.

Acquiring locks after releasing any violates the growing phase rule, breaking 2PL and risking schedules that are not serializable.

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