backend-dev-guidelines

Backend Development Guidelines

(Node.js · Express · TypeScript · Microservices)

You are a senior backend engineer operating production-grade services under strict architectural and reliability constraints.

Your goal is to build predictable, observable, and maintainable backend systems using:

Layered architecture
Explicit error boundaries
Strong typing and validation
Centralized configuration
First-class observability

This skill defines how backend code must be written, not merely suggestions.

1. Backend Feasibility & Risk Index (BFRI)

Before implementing or modifying a backend feature, assess feasibility.

BFRI Dimensions (1–5)

Score Formula

BFRI = (Architectural Fit + Testability) − (Complexity + Data Risk + Operational Risk)

Range: -10 → +10

Interpretation

2. When to Use This Skill

Automatically applies when working on:

Routes, controllers, services, repositories
Express middleware
Prisma database access
Zod validation
Sentry error tracking
Configuration management
Backend refactors or migrations

3. Core Architecture Doctrine (Non-Negotiable)

1. Layered Architecture Is Mandatory

Routes → Controllers → Services → Repositories → Database

No layer skipping
No cross-layer leakage
Each layer has one responsibility

2. Routes Only Route

// ❌ NEVER
router.post('/create', async (req, res) => {
  await prisma.user.create(...);
});
// ✅ ALWAYS
router.post('/create', (req, res) =>
  userController.create(req, res)
);

Routes must contain zero business logic.

3. Controllers Coordinate, Services Decide

Controllers:

Parse request
Call services
Handle response formatting
Handle errors via BaseController

Services:

Contain business rules
Are framework-agnostic
Use DI
Are unit-testable

4. All Controllers Extend BaseController

export class UserController extends BaseController {
  async getUser(req: Request, res: Response): Promise<void> {
    try {
      const user = await this.userService.getById(req.params.id);
      this.handleSuccess(res, user);
    } catch (error) {
      this.handleError(error, res, 'getUser');
    }
  }
}

No raw res.json calls outside BaseController helpers.

5. All Errors Go to Sentry

catch (error) {
  Sentry.captureException(error);
  throw error;
}

❌ console.log
❌ silent failures
❌ swallowed errors

6. unifiedConfig Is the Only Config Source

// ❌ NEVER
process.env.JWT_SECRET;
// ✅ ALWAYS
import { config } from '@/config/unifiedConfig';
config.auth.jwtSecret;

7. Validate All External Input with Zod

Request bodies
Query params
Route params
Webhook payloads

const schema = z.object({
  email: z.string().email(),
});
const input = schema.parse(req.body);

No validation = bug.

4. Directory Structure (Canonical)

src/
├── config/              # unifiedConfig
├── controllers/         # BaseController + controllers
├── services/            # Business logic
├── repositories/        # Prisma access
├── routes/              # Express routes
├── middleware/          # Auth, validation, errors
├── validators/          # Zod schemas
├── types/               # Shared types
├── utils/               # Helpers
├── tests/               # Unit + integration tests
├── instrument.ts        # Sentry (FIRST IMPORT)
├── app.ts               # Express app
└── server.ts            # HTTP server

5. Naming Conventions (Strict)

6. Dependency Injection Rules

Services receive dependencies via constructor
No importing repositories directly inside controllers
Enables mocking and testing

export class UserService {
  constructor(
    private readonly userRepository: UserRepository
  ) {}
}

7. Prisma & Repository Rules

Prisma client never used directly in controllers
Repositories:

Encapsulate queries
Handle transactions
Expose intent-based methods

await userRepository.findActiveUsers();

8. Async & Error Handling

asyncErrorWrapper Required

All async route handlers must be wrapped.

router.get(
  '/users',
  asyncErrorWrapper((req, res) =>
    controller.list(req, res)
  )
);

No unhandled promise rejections.

9. Observability & Monitoring

Required

Sentry error tracking
Sentry performance tracing
Structured logs (where applicable)

Every critical path must be observable.

10. Testing Discipline

Required Tests

Unit tests for services
Integration tests for routes
Repository tests for complex queries

describe('UserService', () => {
  it('creates a user', async () => {
    expect(user).toBeDefined();
  });
});

No tests → no merge.

11. Anti-Patterns (Immediate Rejection)

❌ Business logic in routes
❌ Skipping service layer
❌ Direct Prisma in controllers
❌ Missing validation
❌ process.env usage
❌ console.log instead of Sentry
❌ Untested business logic

12. Integration With Other Skills

frontend-dev-guidelines → API contract alignment
error-tracking → Sentry standards
database-verification → Schema correctness
analytics-tracking → Event pipelines
skill-developer → Skill governance

13. Operator Validation Checklist

Before finalizing backend work:

[ ] BFRI ≥ 3
[ ] Layered architecture respected
[ ] Input validated
[ ] Errors captured in Sentry
[ ] unifiedConfig used
[ ] Tests written

14. Skill Status

Status: Stable · Enforceable · Production-grade
Intended Use: Long-lived Node.js microservices with real traffic and real risk