Mission
Interactive Simulation
Before we explain anything — play. Push it until it breaks, then fix it.
Workload
Replication
Leader acks immediately. Fastest writes, but followers lag and a crash can lose recent writes.
Break it
What just happened?
Read replicas absorbed read traffic and gave you a failover target, so a primary failure no longer meant total outage.
The concept
Replication keeps copies of your data on multiple machines. A common setup is leader/follower: writes go to the leader and replicate to followers, which serve reads. This scales reads and provides redundancy for failover.
Trade-offs
Nothing is free. Here's what this solution costs you.
In the real world
Conceptually similar to managed Postgres/MySQL with read replicas and automated failover.
Mini quiz
Read replicas primarily scale…
Interview me
The app becomes your interviewer. One question, in your own words.
Boss challenge
The primary just died
9K reads/s and 1K writes/s. The primary crashes.
Goal: Keep errors under 1%, lose zero acknowledged writes, and keep stale reads under 5%.
Use the simulator above with no hints. These checks update live as you play.
Interview question
“Explain leader/follower, sync vs async replication, replication lag, and failover.”