Compare 2PC vs 3PC Protocols - The Coordinator

Implementation

Understanding the trade-offs between 2PC and 3PC helps choose the right protocol for your use case.

Message complexity:

2PC (happy path):
  Prepare → 2N messages (N requests, N replies)
  Commit  → 2N messages (N requests, N replies)
  Total: 4N messages

3PC (happy path):
  CanCommit → 2N messages
  PreCommit  → 2N messages
  DoCommit   → 2N messages
  Total: 6N messages

Blocking scenarios:

2PC blocks when:
  - Coordinator crashes after collecting all Yes votes
  - Participant crashes after voting Yes but before receiving decision

3PC blocks when:
  - Coordinator crashes before sending PreCommit
  - Network partition separates coordinator from participants before PreCommit
  - (Does NOT block if coordinator crashes after PreCommit)

Real-world usage:

2PC: Widely used (XA transactions, databases, message queues)
3PC: Rarely used due to complexity and remaining blocking scenarios
Consensus-based: Paxos/Raft are preferred for non-blocking commit

Performance comparison:

Request:  {"type": "benchmark", "msg_id": 1, "protocols": ["2pc", "3pc"], "participants": 5, "transactions": 100}
Response: {"type": "benchmark_ok", "in_reply_to": 1, "results": {"2pc": {"avg_latency_ms": 45, "throughput_tps": 2200}, "3pc": {"avg_latency_ms": 68, "throughput_tps": 1450}}}

When to use each:

Use 2PC: Simple, widely supported, acceptable blocking risk
Use 3PC: Need slightly better availability, can tolerate extra complexity
Use consensus: Need true non-blocking commit, can tolerate higher latency

Sample Test Cases

Benchmark 2PC vs 3PC latencyTimeout: 30000ms

Input

{"src":"c0","dest":"benchmarker","body":{"type":"init","msg_id":1,"participants":["p1","p2","p3"]}}
{"src":"c1","dest":"benchmarker","body":{"type":"benchmark","msg_id":2,"protocols":["2pc","3pc"],"participants":3,"transactions":100}}

Expected Output

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

Compare blocking scenariosTimeout: 5000ms

Input

{"src":"c0","dest":"comparator","body":{"type":"init","msg_id":1}}
{"src":"c1","dest":"comparator","body":{"type":"compare_blocking","msg_id":2,"protocols":["2pc","3pc"]}}

Expected Output

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

Hints

Hint 1▾

2PC: 2 rounds (Prepare + Commit/Abort), 3PC: 3 rounds (CanCommit + PreCommit + DoCommit)

Hint 2▾

2PC blocks if coordinator crashes after Prepare, 3PC blocks if coordinator crashes before PreCommit

Hint 3▾

3PC reduces but doesn't eliminate blocking

Hint 4▾

3PC is rarely used in practice due to complexity and remaining blocking scenarios

Hint 5▾

Most systems use 2PC or consensus-based approaches (Paxos/Raft)

Implementation

Sample Test Cases

Hints

Resources

Theoretical Hub

Key Concepts