🛠️ Two-Phase Commit (2PC)

Definition:

Two-Phase Commit (2PC) is a distributed transaction protocol used to ensure all participants in a distributed system either commit or rollback a transaction, maintaining atomicity across multiple databases or services.

It’s commonly used in distributed databases, microservices, and financial systems.

1️⃣ Why 2PC is Needed

In a distributed system, a transaction may involve multiple databases/services.
Example: Transferring money from Bank A to Bank B:
- Debit from A
- Credit to B
Both operations must either succeed together or fail together.
2PC ensures consistency across all participants.

2️⃣ How 2PC Works

Phase 1: Prepare (Voting Phase)

Coordinator sends a “prepare” request to all participants.
Each participant checks if it can commit the transaction:
- If yes → responds “vote commit”
- If no → responds “vote abort”

Phase 2: Commit (Decision Phase)

Coordinator collects votes:
- If all vote commit → coordinator sends commit command to all.
- If any vote abort → coordinator sends rollback command to all.
Each participant executes commit or rollback as instructed.

3️⃣ Sequence Diagram (Text-Based)

Coordinator             Participant 1      Participant 2
     |                        |                  |
     |------Prepare----------->|                  |
     |                        |---Vote Commit-->|
     |                        |                  |
     |------Prepare----------->|                  |
     |                        |---Vote Commit-->|
     |                        |                  |
     |------Commit------------>|                  |
     |                        |---Commit-------->|

4️⃣ Pros and Cons

Pros	Cons
Ensures atomicity across systems	Blocking protocol – if coordinator crashes, participants may wait indefinitely
Maintains consistency	Slower due to multiple communication rounds
Simple to implement conceptually	No fault tolerance for coordinator crash

5️⃣ Analogy

Think of 2PC like group dinner ordering:

Phase 1 (Prepare): Everyone says “I can eat this dish?”

Phase 2 (Commit): If everyone agrees → order all dishes; if anyone disagrees → cancel the order.

💡 Key takeaway: 2PC is reliable for atomic transactions across multiple systems, but can be slow and blocking, so alternatives like 3PC or Saga pattern are used in high-availability systems.