Primary-Replica

1. The Incident

Your store has run on one PostgreSQL machine for years, handling every product view, search, cart update, and order.

Growth becomes a problem on a Tuesday, when traffic analysis shows 8,000 reads per second from product pages, search, and order history. It also shows 300 writes per second from orders and cart changes, while the machine can handle only 5,000 total queries per second.

Product pages take four seconds to load, and checkout slows because reads and writes compete for the same CPU, memory, and disk.