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Many developers start with a simple rule: if it works, put it in the controller.
At first, this works well. Your project stays small. Your controllers stay short.
Then, the application grows. Your 20-line controller becomes 100 lines. Then 300. Soon, it reaches 600 lines.
Large controllers create problems:
- Reading code becomes hard.
- Finding bugs takes more time.
- Testing becomes difficult.
- Making changes feels risky.
You need a Service Layer to fix this.
A Service Layer handles your business logic. Use this structure to separate duties:
- Routes decide which endpoint to call.
- Controllers handle HTTP requests and responses.
- Services handle the actual business logic.
Business logic includes:
- Database queries.
- Validation rules.
- Permission checks.
- User operations.
- Payment processing.
Think of a restaurant. A controller is a waiter. The waiter takes your order and brings your food. The waiter does not cook. The service layer is the kitchen. The kitchen does the actual work.
Compare these two approaches.
A bad controller does everything. It handles requests, checks the database, validates data, and manages errors.
A good controller stays small. It receives the request and passes it to a service. It then returns the result.
The controller should not know how to fetch a user. It only knows how to talk to the client. The service layer contains the logic to find the user.
Follow these rules for better code:
- Keep controllers focused on the request-response cycle.
- Move business logic to service files.
- Do not put HTTP logic like req, res, or status codes in your services.
- Let services throw errors.
- Let controllers catch errors and return responses.
Small projects might not need this architecture immediately. But as your app grows, separation becomes vital.
Keep your responsibilities separate. This makes your code easy to read, test, and scale.