Introduction

Keeping Data Consistent: How to Read This Module

This is the smallest module in the course: four patterns and the deepest water. Everything before it quietly relied on a miracle you've been using your whole career without noticing: the single-machine transaction. On one database, "subtract from checking, add to savings" either fully happens or fully doesn't: the database's write-ahead log manufactures that all-or-nothing property (the word for it is atomicity) so smoothly you've never had to think about it.

Then the course happened. The [sharding lesson](