Implement Basic Counter with Lost Update Problem - The Counter

                      TASK
                    

Implementation

Implement a basic grow-only counter that handles add and read operations. Multiple nodes share counter state, but your initial implementation will lose updates under concurrency.

This task intentionally demonstrates the lost update problem. Your counter will work for sequential operations but fail verification under concurrent updates from multiple nodes.

Sample Test Cases

Basic counter addTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1"]}}
{"src":"c1","dest":"n1","body":{"type":"add","msg_id":2,"delta":5}}
{"src":"c1","dest":"n1","body":{"type":"read","msg_id":3}}

Expected Output

{"src":"n1","dest":"c0","body":{"type":"init_ok","in_reply_to":1,"msg_id":0}}
{"src":"n1","dest":"c1","body":{"type":"add_ok","in_reply_to":2,"msg_id":1}}
{"src":"n1","dest":"c1","body":{"type":"read_ok","in_reply_to":3,"msg_id":2,"value":5}}

Sequential increments workTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1"]}}
{"src":"c1","dest":"n1","body":{"type":"add","msg_id":2,"delta":3}}
{"src":"c2","dest":"n1","body":{"type":"add","msg_id":3,"delta":2}}
{"src":"c3","dest":"n1","body":{"type":"read","msg_id":4}}

Expected Output

{"src":"n1","dest":"c0","body":{"type":"init_ok","in_reply_to":1,"msg_id":0}}
{"src":"n1","dest":"c1","body":{"type":"add_ok","in_reply_to":2,"msg_id":1}}
{"src":"n1","dest":"c2","body":{"type":"add_ok","in_reply_to":3,"msg_id":2}}
{"src":"n1","dest":"c3","body":{"type":"read_ok","in_reply_to":4,"msg_id":3,"value":5}}

Hints

Hint 1▾

Start with a simple counter

Hint 2▾

Notice what happens with concurrent updates

Hint 3▾

This task is meant to fail

Resources

G-Counter Challenge

Fly.io grow-only counter challenge

                      OVERVIEW
                    

Theoretical Hub

The Lost Update Problem

When multiple nodes read, modify, and write state, updates can be lost. Node A reads 5, Node B reads 5, both increment to 6, both write 6. One increment is lost. This is why distributed counters need special handling.

Key Concepts

lost updatesrace conditionsnaive replication

main.py

python