Understanding Separation of Concerns (SoC) in NestJS
A guide to understanding Separation of Concerns in NestJS using modules, services, and controllers.
When building applications, one of the most important design principles to keep in mind is Separation of Concerns (SoC). NestJS, with its modular architecture, makes applying SoC almost effortless — but understanding why it matters and how to use it properly will help you write cleaner, testable, and future-proof code.
What is Separation of Concerns and Why it Matters?
The basic idea is:
A program should be divided into distinct sections, where each section addresses a single responsibility.
In simpler terms:
Every part of the code should have one clear job.
This makes the code easier to understand, test, and maintain.
Lets take an analogy of a coffee shop to better understand this concept. A coffee shop has multiple employees with distinct roles:
The cashier handles payments.
The barista makes coffee.
The inventory manager tracks beans and supplies.
If one person did everything, the system would collapse into chaos. The same goes for software. Each role has a single responsibility, and no one does everything.
Suppose you’re building an eCommerce platform. Without SoC, all the logic might end up in one massive file — authentication, product catalog, payments, inventory, order processing. This quickly becomes spaghetti code: hard to read, impossible to test, and dangerous to modify.
With Separation of Concerns, you break it down into modules:
Auth Module → Handles user registration, login, JWT tokens.
Products Module → Manages product catalog and search.
Checkout Module → Orchestrates payment and order creation.
Inventory Module → Keeps track of stock levels.
Each module does one thing. When you need to upgrade your payment provider or switch databases, you only touch the Checkout Module or Inventory Module — everything else continues to work.
This approach of programming enhances:
Readability: Developers can understand a piece of code without knowing the whole system.
Maintainability: Changing one part (like authentication) won’t break unrelated features.
Testability: Smaller, focused components are easier to test in isolation.
Flexibility: You can swap implementations (e.g., switch databases or auth providers) without rewriting the whole app.
Separation of Concerns in NestJS
One of the reasons developers love NestJS is that it bakes Separation of Concerns into its architecture. By default, NestJS applications are structured around three core building blocks: modules, services, and controllers. Each of these naturally maps to a specific concern.
Modules → Group related functionality
A module acts like a container for a specific domain or feature. Example: UsersModule, AuthModule, PostsModule.
A module owns everything related to its concern and can expose services for other modules to use.
Think of modules as departments in a company — each department specializes in one thing but can collaborate with others when needed.
Services → Handle business logic
Services handle the actual “work” of the application.
Example: “UsersService” deals only with user data.
Example: “AuthService” deals only with authentication and tokens.
Services should not depend on controllers or contain HTTP logic.
This makes them reusable. A service can be used in REST controllers, GraphQL resolvers, or even a CLI command without modification.
Controllers → Handle HTTP requests
Controllers act as the entry point for requests. Their job is to receive a request, delegate the work to a service, and return a response.
They should remain thin, containing no business logic. This ensures controllers are simple and services stay focused.
Example: Auth and Users in NestJS
Let’s look at a real-world scenario: Implementing authentication.
UserService in UsersModule
@Injectable()
export class UsersService {
constructor(private prisma: PrismaService) {}
findByEmail(email: string) {
return this.prisma.user.findUnique({ where: { email } });
}
create(data: any) {
return this.prisma.user.create({ data });
}
}UserService is only concerned with managing user data. It knows nothing about JWTs, passwords, or authentication.
AuthService in AuthModule
@Injectable()
export class AuthService {
constructor(
private usersService: UsersService,
private jwtService: JwtService,
) {}
async validateUser(email: string, password: string) {
const user = await this.usersService.findByEmail(email);
// validate password...
return user;
}
login(user: any) {
return { access_token: this.jwtService.sign({ sub: user.id }) };
}
}AuthService handles authentication logic. It uses UsersService, to work with user data (findByEmail) but doesn’t leak authentication concerns back into it.
A clear boundary is maintained: UsersService manages user data, AuthService manages auth.
Conclusion
Separation of Concerns isn’t just a design principle — it’s a mindset. By keeping each part of your application focused on a single responsibility, you reduce complexity, make testing easier, and keep your codebase flexible as your project grows.
NestJS makes this natural with its modules, services, and controllers. If you embrace SoC from the start, you’ll end up with applications that are not only scalable but also a joy to maintain.
(Proofread by ChatGPT)