Implement the Apply Channel for State Machine - The Consensus

                      TASK
                    

Implementation

Implement an apply channel that feeds committed log entries to the state machine. The state machine is a simple key-value store that processes Put, Get, and Delete commands.

Request:  {"type": "apply_entries", "msg_id": 1, "entries": [
    {"index": 1, "term": 1, "command": {"op": "put", "key": "x", "value": "1"}},
    {"index": 2, "term": 1, "command": {"op": "put", "key": "y", "value": "2"}},
    {"index": 3, "term": 1, "command": {"op": "get", "key": "x"}}
]}
Response: {"type": "apply_entries_ok", "in_reply_to": 1, "results": [
    {"index": 1, "result": "ok"},
    {"index": 2, "result": "ok"},
    {"index": 3, "result": "1"}
], "last_applied": 3}

Request:  {"type": "get_state", "msg_id": 2}
Response: {"type": "get_state_ok", "in_reply_to": 2, "state": {"x": "1", "y": "2"}, "last_applied": 3}

Sample Test Cases

Apply put entries builds stateTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1"]}}
{"src":"c1","dest":"n1","body":{"type":"apply_entries","msg_id":2,"entries":[{"index":1,"term":1,"command":{"op":"put","key":"x","value":"hello"}}]}}
{"src":"c1","dest":"n1","body":{"type":"get_state","msg_id":3}}

Expected Output

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

Apply get returns current valueTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1"]}}
{"src":"c1","dest":"n1","body":{"type":"apply_entries","msg_id":2,"entries":[{"index":1,"term":1,"command":{"op":"put","key":"x","value":"42"}},{"index":2,"term":1,"command":{"op":"get","key":"x"}}]}}

Expected Output

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

Hints

Hint 1▾

Committed entries flow into an apply channel in order

Hint 2▾

The state machine reads from the channel and applies commands sequentially

Hint 3▾

Applied entries must never be re-applied (track lastApplied index)

Hint 4▾

The state machine must be deterministic: same commands = same state

Hint 5▾

Commands flow: client -> leader log -> replicated -> committed -> applied

Resources

State Machine Replication

How replicated state machines process deterministic commands from a log

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

apply channelstate machinecommitted entriesdeterministic replay

main.py

python