HLC Handles Backward Clock Gracefully - The Identifier

                      TASK
                    

Implementation

NTP can adjust the system clock backward. Physical timestamps break. Lamport clocks keep working but lose time correlation. HLC handles it gracefully by keeping its physical component and incrementing the logical counter.

Implement a simulation that demonstrates this:

simulate_clock_backward sets the HLC's internal notion of "now" backward by a given amount
After the backward jump, HLC.tick() should still produce advancing timestamps
Report the drift between HLC.pt and actual physical time

Request:  {"type": "simulate_backward", "msg_id": 1, "offset_ms": -5000}
Response: {"type": "simulate_backward_ok", "in_reply_to": 1}

Then tick and observe HLC advances despite backward clock:

Request:  {"type": "hlc_tick", "msg_id": 2}
Response: {"type": "hlc_tick_ok", "in_reply_to": 2, "pt": 1234567, "lc": 1, "drift_ms": 5000}

The drift_ms field shows how far HLC.pt is ahead of the (simulated) physical clock.

Sample Test Cases

Simulate backward responds okTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1"]}}
{"src":"c1","dest":"n1","body":{"type":"simulate_backward","msg_id":2,"offset_ms":-5000}}

Expected Output

{"src": "n1", "dest": "c0", "body": {"type": "init_ok", "in_reply_to": 1, "msg_id": 0}}
{"src": "n1", "dest": "c1", "body": {"type": "simulate_backward_ok", "in_reply_to": 2, "msg_id": 1}}

Tick after backward still advancesTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1"]}}
{"src":"c1","dest":"n1","body":{"type":"hlc_tick","msg_id":2}}
{"src":"c1","dest":"n1","body":{"type":"simulate_backward","msg_id":3,"offset_ms":-10000}}
{"src":"c1","dest":"n1","body":{"type":"hlc_tick","msg_id":4}}

Expected Output

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

Hints

Hint 1▾

When system clock goes backward, HLC keeps its pt and increments lc

Hint 2▾

This means HLC never goes backward, preserving ordering

Hint 3▾

Lamport clocks also handle this, but lose physical time correlation

Hint 4▾

Physical clocks break entirely on backward adjustments

Hint 5▾

Track the drift between HLC.pt and actual physical time

Resources

NTP and Clock Synchronization

Overview of NTP and why clock backward jumps happen

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

NTP adjustmentclock backwardmonotonic guaranteeHLC resilience

main.py

python

#!/usr/bin/env python3
import sys
import json
import time
class HLC:
    def __init__(self):
        self.pt = 0
        self.lc = 0
        self.clock_offset = 0  # simulated offset in ms
    def now_ms(self):
        return int(time.time() * 1000) + self.clock_offset
    def tick(self):
        now = self.now_ms()
        if now > self.pt:
            self.pt = now
            self.lc = 0
        else:
            self.lc += 1
        drift = self.pt - now if self.pt > now else 0
        return {"pt": self.pt, "lc": self.lc, "drift_ms": drift}
    def simulate_backward(self, offset_ms):
        # TODO: Adjust clock_offset to simulate backward jump
        pass
    def get(self):
        return {"pt": self.pt, "lc": self.lc}
class Node:
    def __init__(self):
        self.node_id = None
        self.node_ids = []
        self.next_msg_id = 0
        self.hlc = HLC()
    def send(self, dest, body):
        body["msg_id"] = self.next_msg_id