Controller + Service Pattern

Why this pattern exists

NestJS applies the classic controller/service split so your code is easier to test, reason about, and grow. Controllers focus on how data enters or leaves the app (HTTP, WebSockets, etc.), while services focus on what the app needs to do (business logic, database access).

Responsibility chart

Concern	Controller	Service
Accepting HTTP requests	✅	❌
Parsing path/query/body data	✅	❌
Calling the database or external APIs	❌	✅
Handling business rules	❌	✅
Throwing HTTP-friendly errors	✅ (wrap service errors)	✅ (raise domain errors)

Keeping these roles separate means you can change one without breaking the other—swap out MongoDB, add caching, or expose the same service to a different transport (e.g., a CLI) with minimal code changes.

How dependency injection ties it together

You mark a class with @Injectable() to let Nest manage its lifecycle.
Controllers receive services via constructor injection, e.g., constructor(private tasksService: TasksService) {}.
Nest’s module system wires everything: the module provides the service, lists the controller, and the framework handles instantiation order.

Request ─► Controller ─► Service ─► MongoDB
            ▲             │
            └──── errors ─┘

Testing benefits

Service unit tests only focus on business logic. You can mock the Mongo model and verify methods like create or findOne directly.
Controller tests can mock the service to ensure the right status codes and DTO handling without touching the database.

When to add more layers

As projects grow you might introduce:

Repository layer to isolate database queries even further.
Use-case / application services that orchestrate multiple domain services.
Presenters or interceptors to shape responses.

For this tutorial, controller + service is enough—just keep their responsibilities clean.

Next steps

With the pattern in mind:

Review REST Basics if you need a refresher on routes and verbs.
Build the actual logic in Service.
Connect HTTP requests to that logic in Controller & Routes.