ARCHIVED from builddistributedsystem.com on 2026-04-28 — URL: https://builddistributedsystem.com/tracks/counter/tasks/task-4-2-kv-integration
TASK

Implementation

Use Maelstrom's built-in seq-kv service to store your counter value. This provides sequential consistency but introduces new challenges around availability during network partitions.

Sample Test Cases

KV-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":10}}
{"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":10}}

Hints

Hint 1▾
Use Maelstrom seq-kv service
Hint 2▾
Store counter in external KV
Hint 3▾
This still has issues under partitions

Resources

Maelstrom Services
Documentation for Maelstrom built-in services
OVERVIEW

Theoretical Hub

Sequential Consistency

Sequential consistency guarantees that all operations appear to happen in some total order consistent with each process's local order. This is stronger than eventual consistency but weaker than linearizability.

Key Concepts

sequential consistencyexternal storagelinearizability
main.py
python
Integrate Sequentially Consistent Key-Value Store - The Counter | Build Distributed Systems