Implement Two-Phase Commit - The Coordinator

      ARCHIVED from builddistributedsystem.com on 2026-04-28
      — URL: https://builddistributedsystem.com/tracks/coordinator/tasks/task-9-1-two-phase-commit
    

                      TASK
                    

Implementation

Implement 2PC: Phase 1 sends PREPARE, collects votes. Phase 2 sends COMMIT if all YES, else ABORT.

Sample Test Cases

All participants vote yesTimeout: 5000ms

Input

{
  "src": "c0",
  "dest": "n1",
  "body": {
    "type": "init",
    "msg_id": 1,
    "node_id": "n1",
    "node_ids": [
      "n1"
    ]
  }
}

Expected Output

{"src":"n1","dest":"c0","body":{"type":"init_ok","in_reply_to":1,"msg_id":0}}

Hints

Hint 1▾

Phase 1: Prepare - ask all participants

Hint 2▾

Phase 2: Commit/Abort based on votes

Hint 3▾

Log decisions for recovery

Resources

DDIA Chapter 9

Consistency and Consensus

                      OVERVIEW
                    

Theoretical Hub

Two-Phase Commit

2PC ensures all-or-nothing across nodes. The blocking problem: if coordinator crashes after PREPARE, participants are stuck.

Key Concepts

2PCatomic commitprepare-commit

main.py

python

#!/usr/bin/env python3
class Coordinator:
    def __init__(self, participants):
        self.participants = participants
        self.tx_log = {}
    
    def prepare(self, tx_id, operations):
        votes = [p.prepare(tx_id, operations.get(p.id, [])) for p in self.
        participants]
        return votes
    
    def commit_or_abort(self, tx_id, votes):
        # TODO: If all YES, commit; else abort
        pass
class Participant:
    def __init__(self, id):
        self.id = id
        self.state = {}
        self.locks = {}
    
    def prepare(self, tx_id, ops):
        # TODO: Lock resources, vote YES or NO
        pass
    
    def commit(self, tx_id):
        # TODO: Apply changes, release locks
        pass

Implement Two-Phase Commit - The Coordinator | Build Distributed Systems