Two Phase Commit (Blocking Commit Protocol)

distributedsystem distributedsystem/microservice

ACID Principles > Atomicity
Argus · Distributed Systems Engineering Lecture notes (MIT 6.824)

A blocking protocol to ensure atomic commit across distributed systems.

This protocol requires a coordinator node, which basically coordinates and oversees the transaction across different nodes. The coordinator tries to establish the consensus among a set of processes in two phases, hence the name.

1. Prepare Phase

• Coordinator sends PREPARE request to all participants.
• Each participant:
  • Checks if it can commit.
  • Writes to a log, then replies with YES or NO.

2. Commit Phase

• If all participants voted YES:
  • Coordinator sends COMMIT.
• If any voted NO:
  • Coordinator sends ABORT.

It Can lead to both the coordinator and the group members blocking, which may lead to the dreaded deadlock.

**Blocking issue**: if the coordinator crashes after participants vote `YES`, they get **stuck waiting** indefinitely.