Distributed transactions and 2PC in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Two-Phase Commit Protocol

What is the main purpose of the Two-Phase Commit (2PC) protocol in distributed transactions?

A To speed up transaction processing by allowing partial commits in distributed systems.

B To ensure all participating databases either commit or abort a transaction together, maintaining consistency.

C To allow transactions to commit independently on each node without coordination.

D To replicate data asynchronously across distributed databases.

💡 Hint

Think about how distributed systems keep data consistent when multiple databases are involved.

The 2PC protocol coordinates all participants to either commit or abort a transaction together, ensuring atomicity and consistency across distributed databases.

📋 Factual

intermediate

Phases of the Two-Phase Commit Protocol

Which two phases correctly describe the Two-Phase Commit protocol?

A Start phase and End phase

B Lock phase and Unlock phase

C Vote phase and Execute phase

D Prepare phase and Commit phase

💡 Hint

One phase asks participants if they are ready, the other finalizes the decision.

The Two-Phase Commit protocol consists of the Prepare phase, where participants vote to commit or abort, and the Commit phase, where the coordinator tells participants to finalize the transaction.

🔍 Analysis

advanced

Handling Coordinator Failure in 2PC

In a Two-Phase Commit protocol, what happens if the coordinator fails after sending the prepare message but before sending the commit or abort decision?

A Participants remain blocked, waiting indefinitely for the coordinator's decision.

B Participants automatically abort the transaction after a timeout.

C Participants commit the transaction immediately after prepare phase.

D Participants elect a new coordinator and proceed without delay.

💡 Hint

Consider what participants do when they don't receive the final decision after voting.

If the coordinator fails after the prepare phase, participants cannot decide to commit or abort and remain blocked, waiting for the coordinator's decision to avoid inconsistency.

❓ Comparison

advanced

Comparing 2PC and 3PC Protocols

Which statement correctly distinguishes the Three-Phase Commit (3PC) protocol from the Two-Phase Commit (2PC) protocol?

A 3PC adds an extra phase to avoid blocking participants during coordinator failure, unlike 2PC.

B 3PC is faster than 2PC because it skips the prepare phase.

C 2PC allows participants to commit independently, while 3PC requires strict coordination.

D 2PC uses asynchronous messaging, whereas 3PC uses synchronous messaging only.

💡 Hint

Think about how protocols handle failures and participant blocking.

3PC introduces a third phase to reduce the chance of participants being blocked if the coordinator fails, improving on 2PC's blocking problem.

❓ Reasoning

expert

Impact of Network Partition on 2PC

During a distributed transaction using 2PC, a network partition occurs isolating some participants from the coordinator. What is the most likely outcome for the isolated participants?

A They automatically abort the transaction after detecting the partition.

B They elect a new coordinator and proceed with the transaction.

C They remain in a prepared state, unable to commit or abort until the partition resolves.

D They commit the transaction independently to avoid delays.

💡 Hint

Consider what participants do when they cannot communicate with the coordinator after voting.

Participants isolated by a network partition remain blocked in the prepared state, waiting for the coordinator's commit or abort decision to maintain consistency.