Introduction

1. What This Module Covers

The last module was about the arrows on the diagram: how parts talk to each other. This one is about the heaviest box on that diagram: the database.

Here is why it gets its own module. Almost every other part of your system can be replaced at any moment. API servers hold no data. You can add ten more or lose three, and nobody notices.

The database is different. It holds the facts: who paid, who owns what, what happened. You cannot simply "add another one." Then there would be two sets of facts, and they would disagree.