What actually happens when a user sends a request?

You click a button.

What happens next is far more complicated than most engineers realize. Let's say you open a website. From a user's perspective, it feels instant. Behind the scenes, a lot is happening.

The browser first needs to find where the application lives.

• A DNS lookup translates a domain name into an IP address.
• The request then travels through multiple network hops before reaching a server.
• The server processes the request, may talk to other services, query a database, and eventually sends a response back.
• Only then does your browser render what you see on the screen.

All of this typically happens in a few hundred milliseconds. What's interesting is that every step introduces latency. And latency adds up.

• A slow DNS lookup.
• An overloaded API.
• A database query that's a little slower than expected.

Individually, they don't seem like a big deal. Together, they create the experience your users feel.

One lesson that changed how I think about systems: Users don't care where the delay comes from.

• They only experience the total delay.
• That's why great systems aren't built by optimizing one component.
• They're built by understanding the entire request journey.

Because before you can scale a system, you need to understand how it actually works.

Question: What's the most surprising performance bottleneck you've encountered in a real system?