Introduction

1. What This Module Covers

Everything so far served requests. A user asks, the system answers, and milliseconds pass. This module serves questions about everything that ever happened. Revenue by hour. Fraud patterns across millions of orders. Features computed over a year of behavior. The input to these questions is not a request. It is a continuous flow: the clickstream, the order events, the change history of every table. Data engineers work here full time. Every other engineer visits weekly, usually by asking "where does the dashboard's number come from?"