What backend architecture do you use for your applications?

The backend is where many applications quietly become complicated.

A simple request can turn into authentication, validation, database queries, caching, queues, file processing, permissions, third-party APIs, background jobs, logging, rate limiting, and error handling.

I'm curious how developers here structure their backend systems.

Do you prefer a traditional monolith? Modular monolith? Microservices? Serverless? Event-driven architecture? Something custom?

And how do you decide?

One of the things I find interesting is that architecture discussions often jump immediately to scale. People start designing systems for millions of users before they've built the first hundred. But the architecture that makes sense for a small product can be very different from the architecture needed by a massive platform.

There is a lot of value in understanding where simplicity stops being enough.

What backend language do you use? How do you structure services and modules? How do you handle authentication and permissions? What does your database layer look like? What happens when a third-party API fails? How do you process background jobs?

Share your architecture and explain why you built it that way.

If you have migrated from one backend architecture to another, tell us what forced the change. Those stories are often more useful than theoretical diagrams.