Introduction

1. What This Module Covers

The storage module ended with an observation: a cache is a small, short-lived read model. This module takes that observation seriously. The cache is the most used and most misused piece of infrastructure in the industry. It is also the one interviewers ask about most.

The idea itself is simple: keep the things you use often somewhere fast. A cache is a small, fast copy of data whose real home is somewhere slower. One number decides whether yours is any good: the hit ratio, the fraction of lookups that find the data in the cache.

The math is strict