Handle Concurrent Increments Across Multiple Nodes - The Counter

      ARCHIVED from builddistributedsystem.com on 2026-04-28
      — URL: https://builddistributedsystem.com/tracks/counter/tasks/task-4-5-concurrent-increments
    

                      TASK
                    

Implementation

Test your G-Counter under heavy concurrent load. Multiple nodes will simultaneously increment and the final value must equal the sum of all increments.

Sample Test Cases

Local increments accumulate correctlyTimeout: 10000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1","n2","n3"]}}
{"src":"c1","dest":"n1","body":{"type":"add","msg_id":2,"delta":10}}
{"src":"c2","dest":"n1","body":{"type":"add","msg_id":3,"delta":20}}
{"src":"c3","dest":"n1","body":{"type":"add","msg_id":4,"delta":30}}
{"src":"c4","dest":"n1","body":{"type":"read","msg_id":5}}

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": "add_ok", "in_reply_to": 4, "msg_id": 3}}
{"src": "n1", "dest": "c4", "body": {"type": "read_ok", "value": 60, "in_reply_to": 5, "msg_id": 4}}

Hints

Hint 1▾

Periodically gossip your G-Counter state

Hint 2▾

Merge received states into yours

Hint 3▾

All nodes should converge to same value

                      OVERVIEW
                    

Theoretical Hub

Eventual Consistency

With proper CRDT implementation, all nodes will eventually converge to the same value. The G-Counter guarantees this even under network partitions and arbitrary message delays.

Key Concepts

concurrent operationsdistributed testingverification

main.py

python

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