Guarantee Read-Your-Writes with Follower Reads - The Store

      ARCHIVED from builddistributedsystem.com on 2026-04-28
      — URL: https://builddistributedsystem.com/tracks/store/tasks/task-8-2-4-read-your-writes
    

                      TASK
                    

Implementation

Ensure read-your-writes consistency even when using follower reads. Clients send their last_write_index with each read. Followers only serve if they have applied that index.

Request:  {"type": "write", "msg_id": 1, "key": "x", "value": "new"}
Response: {"type": "write_ok", "in_reply_to": 1, "commit_index": 10}

Request:  {"type": "ryw_read", "msg_id": 2, "key": "x", "last_write_index": 10, "prefer_follower": true}
Response: {"type": "ryw_read_ok", "in_reply_to": 2, "value": "new", "served_by": "n2", "follower_applied_index": 10, "waited_ms": 50}

Request:  {"type": "ryw_read", "msg_id": 3, "key": "x", "last_write_index": 15, "prefer_follower": true}
Response: {"type": "ryw_read_ok", "in_reply_to": 3, "value": "new", "served_by": "n1", "reason": "follower_behind_redirected_to_leader"}

Sample Test Cases

Read-your-writes from followerTimeout: 5000ms

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":"ryw_read","msg_id":2,"key":"x","last_write_index":5,"prefer_follower":true}}

Expected Output

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

Follower behind redirects to leaderTimeout: 5000ms

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":"ryw_read","msg_id":2,"key":"x","last_write_index":999,"prefer_follower":true}}

Expected Output

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

Hints

Hint 1▾

Clients send their last-seen commit_index with each read request

Hint 2▾

Followers only serve the read if they have applied at least that index

Hint 3▾

If the follower is behind, it either waits or redirects to the leader

Hint 4▾

This combines the scalability of follower reads with read-your-writes guarantee

Hint 5▾

The client tracks the commit_index from write responses

Resources

Session Guarantees for Weakly Consistent Data

Formal definition of read-your-writes and other session guarantees

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

read-your-writessession consistencycommit index trackingclient token

main.py

python

Guarantee Read-Your-Writes with Follower Reads - The Store | Build Distributed Systems