Add Follower Reads with Bounded Staleness - The Store

      ARCHIVED from builddistributedsystem.com on 2026-04-28
      — URL: https://builddistributedsystem.com/tracks/store/tasks/task-8-2-3-follower-reads
    

                      TASK
                    

Implementation

Implement follower reads with bounded staleness. Clients can opt to read from any follower if they accept data up to T seconds stale. This scales read throughput linearly with cluster size.

Request:  {"type": "follower_read", "msg_id": 1, "key": "x", "max_staleness_ms": 5000}
Response: {"type": "follower_read_ok", "in_reply_to": 1, "value": "42", "actual_staleness_ms": 200, "served_by": "n2", "linearizable": false}

Request:  {"type": "follower_read", "msg_id": 2, "key": "x", "max_staleness_ms": 0}
Response: {"type": "follower_read_ok", "in_reply_to": 2, "value": "42", "served_by": "n1", "linearizable": true, "reason": "redirected_to_leader"}

Sample Test Cases

Follower read within staleness boundTimeout: 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":"follower_read","msg_id":2,"key":"x","max_staleness_ms":5000}}

Expected Output

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

Zero staleness 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":"follower_read","msg_id":2,"key":"x","max_staleness_ms":0}}

Expected Output

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

Hints

Hint 1▾

Clients opt in to reading from followers if they accept data up to T seconds stale

Hint 2▾

Followers track their applied index; reads are served if the data is fresh enough

Hint 3▾

This distributes read load across all replicas, not just the leader

Hint 4▾

The staleness bound is configurable per-request

Hint 5▾

Compare latency: follower reads avoid the leader bottleneck

Resources

CockroachDB Follower Reads

How CockroachDB implements follower reads for geo-distributed systems

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

follower readsbounded stalenessread scalabilityconsistency tradeoff

main.py

python

Add Follower Reads with Bounded Staleness - The Store | Build Distributed Systems