Service Layer

Where business logic lives, apart from HTTP and storage.

Also known as: service layer, application service, use-case layer

A service layer sits between the HTTP/API layer and the domain and data layers. It holds the application’s use cases: “register a user”, “place an order”, “apply a refund”. Each method orchestrates a workflow — load the relevant objects, invoke domain logic, persist the result, publish an event — without containing HTTP details or raw SQL.

controller (HTTP) → OrderService.place(cmd) → domain + repository → response

Why separate it: the HTTP layer should deal with requests and responses, not business workflows; the domain should hold rules, not transaction management or messaging. The service layer is where the workflow lives, so the same use case can be triggered from a REST endpoint, a CLI, a queue consumer or a test, unchanged.

The classic mistakes:

Business logic in the controller. Logic in the web layer can’t be reused or tested without a server. The service layer gives it a home (see keeping framework code at the edges).
A pass-through service. If the service only forwards to the repository, it adds indirection without value. Services earn their place when they orchestrate several steps or enforce workflow rules.
An anemic domain with fat services. If all the logic migrates into services and the domain objects become data holders, you’ve built an anemic domain model. Split rules (domain) from orchestration (service).
Managing transactions in the wrong place. Where a use case spans multiple writes, the service layer is a natural place to define the transaction boundary — one unit of work per use case.
Becoming a god service. One giant service with every operation for a domain is hard to navigate. Split by use case or cohesion.

How to size it: one method per meaningful use case, named in the domain’s language, with the workflow explicit. It’s the orchestrator that makes the layered architecture useful, wire dependencies through dependency injection, and keep the domain focused on rules. The request handler calls it; the service calls the domain and persistence.