Introduction

1. What This Module Covers

The previous module examined the arrows on a system diagram, which show how its parts communicate; this module examines the diagram's most important box: the database.

The database needs separate attention because almost every other part of a system can be replaced immediately. API servers hold no durable data, so you can add ten or lose three without users noticing.

A database is different because it holds the system's facts: who paid, who owns each item, and what happened