Two-phase commit (and why to avoid it) in Microservices - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the coordinator's first step in two-phase commit.

Microservices

The coordinator sends a [1] request to all participants to prepare for commit.

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

Aabort

Backnowledge

Cprepare

Dcommit

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'commit' instead of 'prepare' as the first step.

The coordinator first sends a prepare request to ask participants if they can commit.

2fill in blank

medium

Complete the code to describe what participants send back after prepare.

Microservices

Participants respond with a [1] message indicating if they can commit or not.

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

Aready

Bvote

Cacknowledge

Dcommit

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'acknowledge' with 'vote'.

Participants send a vote message: 'yes' if ready, 'no' if not.

3fill in blank

hard

Fix the error in the statement about two-phase commit's blocking problem.

Microservices

If the coordinator crashes after sending commit, participants may [1] indefinitely waiting for the final decision.

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

Aabort

Brollback

Ctimeout

Dblock

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'abort' which is an action, not the problem.

Participants block waiting for the coordinator's decision, causing delays.

4fill in blank

hard

Fill both blanks to describe why two-phase commit is often avoided in microservices.

Microservices

Two-phase commit is avoided because it [1] system availability and [2] complexity in distributed systems.

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

Areduces

Bincreases

Csimplifies

Dignores

Attempts:

3 left

💡 Hint

Common Mistakes

Saying it 'simplifies' complexity, which is incorrect.

Two-phase commit reduces availability and increases complexity, making it less suitable for microservices.

5fill in blank

hard

Fill all three blanks to complete the alternative approach to two-phase commit.

Microservices

Instead of two-phase commit, microservices often use [1] which relies on [2] and [3] to maintain data consistency.

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

Aeventual consistency

Bsagas

Casynchronous messaging

Dtwo-phase locking

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing two-phase locking with two-phase commit.

Microservices use sagas with asynchronous messaging to achieve eventual consistency instead of two-phase commit.

Practice

1. What is the main purpose of the two-phase commit protocol in microservices?

easy

A. To automatically retry failed requests

B. To speed up communication between services

C. To allow services to work independently without coordination

D. To ensure all services agree on a transaction before committing

Solution

1. Step 1: Understand the role of two-phase commit

   Two-phase commit is designed to make sure all parts of a distributed transaction agree to commit or abort together.

2. Step 2: Identify the main goal in microservices

   Its main goal is to keep data consistent across multiple services by coordinating their commit decisions.

3. Final Answer:

   To ensure all services agree on a transaction before committing -> Option D

4. Quick Check:

   Two-phase commit = agreement before commit [OK]

Hint: Two-phase commit means all must say yes before commit [OK]

Common Mistakes:

• Thinking it speeds up communication
• Believing services act independently
• Assuming it retries failed requests automatically

2. Which of the following correctly describes the two phases in the two-phase commit protocol?

easy

A. Abort phase where coordinator asks, Prepare phase where services finalize

B. Prepare phase where coordinator asks, Commit phase where services finalize

C. Commit phase where coordinator asks, Prepare phase where services finalize

D. Prepare phase where services finalize, Commit phase where coordinator asks

Solution

1. Step 1: Recall the two phases names and order

   The first phase is the prepare phase where the coordinator asks all services if they can commit.

2. Step 2: Understand the commit phase

   If all agree, the coordinator sends a commit command to finalize the transaction.

3. Final Answer:

   Prepare phase where coordinator asks, Commit phase where services finalize -> Option B

4. Quick Check:

   Prepare then commit = correct phase order [OK]

Hint: Prepare asks, commit finalizes transaction [OK]

Common Mistakes:

• Mixing up the order of prepare and commit phases
• Confusing abort with prepare phase
• Thinking services finalize before coordinator asks

3. Consider a microservices system using two-phase commit. If one service fails to respond during the prepare phase, what is the expected outcome?

medium

A. The coordinator ignores the failure and proceeds

B. The coordinator commits the transaction anyway

C. The coordinator aborts the transaction and tells all services to rollback

D. The coordinator retries the prepare phase indefinitely

Solution

1. Step 1: Analyze failure during prepare phase

   If any service fails to respond or votes no during prepare, the coordinator must abort to keep consistency.

2. Step 2: Understand coordinator's action

   The coordinator sends abort commands to all services to rollback any partial changes.

3. Final Answer:

   The coordinator aborts the transaction and tells all services to rollback -> Option C

4. Quick Check:

   Failure in prepare = abort transaction [OK]

Hint: Any no or failure in prepare means abort [OK]

Common Mistakes:

• Assuming commit happens despite failure
• Thinking coordinator retries forever
• Ignoring failure and proceeding anyway

4. A developer notices that their two-phase commit implementation causes long delays and system hangs when a service crashes. What is the most likely cause?

medium

A. The coordinator is waiting indefinitely for responses from crashed services

B. The services are committing too quickly without coordination

C. The coordinator is skipping the prepare phase

D. The services are not logging their transactions

Solution

1. Step 1: Identify cause of delays and hangs

   In two-phase commit, the coordinator waits for all services to respond during prepare phase.

2. Step 2: Understand impact of crashed services

   If a service crashes, the coordinator may wait indefinitely, causing delays and system hangs.

3. Final Answer:

   The coordinator is waiting indefinitely for responses from crashed services -> Option A

4. Quick Check:

   Waiting on crashed service = system hang [OK]

Hint: Coordinator waits forever if service crashes [OK]

Common Mistakes:

• Thinking services commit too fast causes hangs
• Believing skipping prepare phase causes delays
• Assuming missing logs cause system hangs

5. Why is two-phase commit often avoided in modern microservices architectures despite ensuring consistency?

hard

A. Because it causes blocking, reduces availability, and hurts scalability

B. Because it does not guarantee data consistency

C. Because it requires no coordination between services

D. Because it is too simple and lacks fault tolerance

Solution

1. Step 1: Understand drawbacks of two-phase commit

   Two-phase commit blocks resources while waiting, reducing system availability and scalability.

2. Step 2: Recognize why modern systems avoid it

   Modern microservices prefer eventual consistency and non-blocking patterns to improve performance and fault tolerance.

3. Final Answer:

   Because it causes blocking, reduces availability, and hurts scalability -> Option A

4. Quick Check:

   Blocking and low availability = avoid two-phase commit [OK]

Hint: Two-phase commit blocks and limits scalability [OK]

Common Mistakes:

• Thinking it does not guarantee consistency
• Believing it requires no coordination
• Assuming it is too simple