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.