Sending a notification is one HTTP call to Apple or Google. That is the entire mechanism, and you can demo it in five minutes.

The system design question is not about that call. It is about what happens when you make it sixteen million times a day, to devices that may have been wiped, through providers that fail in ways you do not control, for users who will uninstall your app if you get it wrong.

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. Building a complex system is no different.