Implement Three-Phase Commit Protocol - The Coordinator

                      TASK
                    

Implementation

Three-Phase Commit (3PC) adds an extra phase to 2PC to reduce blocking scenarios. The third phase (PreCommit) ensures participants know a commit is imminent.

Protocol phases:

CanCommit: Coordinator asks "Can you commit?" Participants acquire locks and write to WAL, then vote Yes/No
PreCommit: If all voted Yes, coordinator sends PreCommit. Participants acknowledge and enter "pre-committed" state
DoCommit: Coordinator sends DoCommit. Participants apply changes and send HaveCommitted

State transitions:

Participant states:
  → INITIAL → CAN_COMMIT? → PRE_COMMITTED → COMMITTED
                  ↓              ↓
                ABORTED        ABORTED

Example 3PC execution:

Request:  {"type": "txn_begin", "msg_id": 1, "participants": ["p1", "p2", "p3"], "operations": [{"transfer": 100, "from": "a", "to": "b"}]}
Response: {"type": "txn_begin_ok", "in_reply_to": 1, "txn_id": "txn42"}

// Phase 1: CanCommit
{"type": "can_commit", "msg_id": 2, "txn_id": "txn42"}
{"type": "can_commit_yes", "in_reply_to": 2, "txn_id": "txn42", "participant": "p1"}
{"type": "can_commit_yes", "in_reply_to": 2, "txn_id": "txn42", "participant": "p2"}
{"type": "can_commit_yes", "in_reply_to": 2, "txn_id": "txn42", "participant": "p3"}

// Phase 2: PreCommit
{"type": "pre_commit", "msg_id": 3, "txn_id": "txn42"}
{"type": "pre_commit_ack", "in_reply_to": 3, "txn_id": "txn42", "participant": "p1"}
{"type": "pre_commit_ack", "in_reply_to": 3, "txn_id": "txn42", "participant": "p2"}
{"type": "pre_commit_ack", "in_reply_to": 3, "txn_id": "txn42", "participant": "p3"}

// Phase 3: DoCommit
{"type": "do_commit", "msg_id": 4, "txn_id": "txn42"}
{"type": "have_committed", "in_reply_to": 4, "txn_id": "txn42", "participant": "p1"}
{"type": "have_committed", "in_reply_to": 4, "txn_id": "txn42", "participant": "p2"}
{"type": "have_committed", "in_reply_to": 4, "txn_id": "txn42", "participant": "p3"}

Why 3PC helps:
If coordinator crashes after PreCommit, participants know:

All participants voted Yes
A commit is imminent
They can safely commit without waiting for recovery

Sample Test Cases

Successful 3PC transactionTimeout: 5000ms

Input

{"src":"c0","dest":"coord","body":{"type":"init","msg_id":1,"participants":["p1","p2","p3"]}}
{"src":"c1","dest":"coord","body":{"type":"txn_begin","msg_id":2,"participants":["p1","p2"],"operations":[{"transfer":100,"from":"a","to":"b"}]}}

Expected Output

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

Participant votes No in CanCommitTimeout: 5000ms

Input

{"src":"c0","dest":"coord","body":{"type":"init","msg_id":1,"participants":["p1","p2"]}}
{"src":"c1","dest":"coord","body":{"type":"txn_begin","msg_id":2,"participants":["p1","p2"],"operations":[{"transfer":999999,"from":"a","to":"b"}]}}

Expected Output

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

Hints

Hint 1▾

Phase 1 (CanCommit): coordinator asks "Can you commit?" Participants vote Yes/No

Hint 2▾

Phase 2 (PreCommit): coordinator sends PreCommit if all voted Yes. Participants acknowledge

Hint 3▾

Phase 3 (DoCommit): coordinator sends DoCommit. Participants commit and acknowledge

Hint 4▾

The key: PreCommit lets participants know a commit is coming, enabling recovery

Hint 5▾

If coordinator crashes after PreCommit, participants can commit without waiting

Resources

Three-Phase Commit Protocol

Original paper on 3PC

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

three-phase commitCanCommitPreCommitDoCommitnon-blocking commitcoordinator recovery

main.py

python