Show 3PC Blocking Under Network Partition - The Coordinator

                      TASK
                    

Implementation

While 3PC improves on 2PC, it still has blocking scenarios. The key limitation: if a network partition occurs before PreCommit, participants may not be able to proceed.

Scenario 1: Partition before PreCommit:

Coordinator sends CanCommit to all participants
Some participants vote Yes, others are partitioned
Coordinator cannot collect all votes
No participant can proceed: non-partitioned participants don't know if all voted Yes
Result: blocking

Scenario 2: Partition during PreCommit:

Coordinator sends CanCommit, all participants vote Yes
Network partition occurs
Coordinator sends PreCommit to only some participants
Partitioned participants never receive PreCommit
Result: partitioned participants block, non-partitioned participants can timeout and commit

Example: Partition before PreCommit:

Request:  {"type": "txn_begin", "msg_id": 1, "participants": ["p1", "p2", "p3"], "partition_after": "can_commit", "partitioned_participants": ["p3"]}
Response: {"type": "txn_begin_ok", "in_reply_to": 1, "txn_id": "txn42"}

// p1 and p2 vote Yes
{"type": "can_commit_yes", "src": "p1", "txn_id": "txn42"}
{"type": "can_commit_yes", "src": "p2", "txn_id": "txn42"}

// p3 is partitioned, cannot vote
// Coordinator cannot proceed: doesn't know if p3 would vote Yes or No
// p1 and p2 block: they don't know if p3 voted No

Why this is unavoidable:

Before PreCommit, participants don't know if all participants voted Yes
If any participant voted No, the transaction must abort
Without knowing all votes, no participant can safely decide
This is a fundamental trade-off: 3PC reduces but doesn't eliminate blocking

Sample Test Cases

Partition before PreCommit blocksTimeout: 6000ms

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","p3"],"operations":[{"transfer":100,"from":"a","to":"b"}],"partition_after":"can_commit","partitioned_participants":["p3"],"timeout_ms":2000}}

Expected Output

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

Partition during PreCommit allows commitTimeout: 6000ms

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","p3"],"operations":[{"transfer":100,"from":"a","to":"b"}],"partition_after":"pre_commit","partitioned_participants":["p3"],"timeout_ms":2000}}

Expected Output

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

Hints

Hint 1▾

3PC still blocks if coordinator crashes before PreCommit

Hint 2▾

3PC blocks if network partition separates coordinator from some participants

Hint 3▾

If participants are in CAN_COMMIT state and can't reach coordinator, they must wait

Hint 4▾

Show: partition coordinator from participants after CanCommit phase

Hint 5▾

Result: non-partitioned participants block because they don't know if all voted Yes

Resources

CAP Theorem and Commit Protocols

How CAP theorem relates to distributed commit protocols

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

network partitionblocking scenariossplit brainsafety vs livenessCAP theorem

main.py

python