The copy clerks (database replication)

The fourth episode of the System Design spine, and the one that closes the track’s oldest cliffhanger. The first episode left the chai stall with a single register and called it a future bottleneck. This is the future.

Before you start. You need the chai stall the first episode built, which grew from one counter into several, and the slip rail from The order slip rail (queues & the utilization cliff), which let the counter hand work to the kitchen without waiting for it. Both copied the people. Nothing has yet copied the register, the one book every stall writes its tabs and prices into, and that book is what this lesson is about.