Imagine you are the infrastructure engineer at a hot social media platform. Your system is humming along with 4 cache servers, each holding about 25% of your data. Life is good. Then your platform goes viral overnight. You urgently add a 5th server. You restart everything. And suddenly, your database is on fire — every single cache server is getting a tsunami of cache misses. Users experience 10× slower page loads.

Designing a system that supports millions of users is challenging — it is a journey that requires continuous refinement and endless improvement. In this post, we build a system that supports a single user and gradually scale it up to serve millions of users. After reading this, you will master a handful of techniques that will help you crack system design interview questions. A journey of a thousand miles begins with a single step.