Case Study: An E-commerce Order System

1. The Question

"Design the order processing system for an e-commerce company: browsing is handled elsewhere; you own everything from 'add to cart' through payment, fulfillment, and customer notifications. Mid-size: about two million orders a month, 300 engineers across the company. 45 minutes."

This is the most common microservices question in the whole interview loop.

Here is the good part. You have been building this answer for six modules. Almost nothing below is new material. It is old pieces, put in order.