ARCHIVED from builddistributedsystem.com on 2026-04-28 — URL: https://builddistributedsystem.com/tracks/counter/tasks/task-4-3-cas-operation
TASK

Implementation

Implement your counter using Compare-And-Swap (CAS) operations. CAS atomically updates a value only if it matches an expected value, preventing lost updates.

Sample Test Cases

CAS-based counterTimeout: 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":"c2","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":"c2","body":{"type":"read_ok","in_reply_to":3,"msg_id":2,"value":5}}

Hints

Hint 1▾
Read current value, compute new, CAS to update
Hint 2▾
Retry on CAS failure
Hint 3▾
Handle the race between read and CAS
OVERVIEW

Theoretical Hub

Compare-And-Swap

CAS is the foundation of lock-free algorithms. It atomically checks if a value equals an expected value and, if so, updates it. If the check fails, someone else modified the value and you must retry.

Key Concepts

CASoptimistic concurrencyatomic operations
main.py
python
Implement Compare-And-Swap (CAS) Operation - The Counter | Build Distributed Systems