How one URL serves millions
Type a popular site's address and millions of other people are doing the same thing right now. Yet no single computer on Earth could handle that, one server has finite CPU, memory, and network. So how does one URL serve a crowd that would crush any single machine? The answer is the most important pattern in scalable systems, and it's made of exactly two ideas.
Idea one: run many identical copies of your app and spread traffic across them, that's load balancing. Idea two: automatically add more copies when it's busy and remove them when it's quiet, that's auto-scaling. Together they turn "my one server fell over under load" into "the system grew to meet demand and shrank to save money." This article builds both from scratch.
Note: Who this is for: Anyone who's deployed an app to one server and wondered how real products handle traffic. No prior scaling experience needed. If you understand that a server can get overloaded, you're ready.
What a load balancer actually is
