Introduction

1. What This Module Covers

This is the course's smallest module, with four patterns, but it covers the most difficult material. Every earlier lesson relied on a guarantee that engineers often use without naming: the single-machine transaction.

On one database, "subtract from checking, add to savings" either completes fully or has no effect. The database's write-ahead log provides this all-or-nothing property, which is called atomicity. The database handles it so reliably that application code rarely needs to consider the mechanism