The Application Layer

Separating the web tier from the application tier — and the application tier into services — lets each piece scale on its own axis. If image processing is CPU-bound and the API is IO-bound, running them together means you buy CPU for machines that mostly wait on the network.

It also lets teams deploy independently, which for a large organisation is often the real motivation. The architecture diagram is a picture of the org chart more often than anyone admits.

What it costs is that every call between services is now a network call. Local code can still be slow or throw, but it shares a process fate and cannot produce an ambiguous network timeout. Across a service boundary latency, partial failure, and uncertain outcomes are routine, so every caller needs a timeout and an explicit retry or fallback policy.