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)

Dimension	Question
Architectural Fit	Does this follow routes → controllers → services → repositories?
Business Logic Complexity	How complex is the domain logic?
Data Risk	Does this affect critical data paths or transactions?
Operational Risk	Does this impact auth, billing, messaging, or infra?
Testability	Can this be reliably unit + integration tested?

Score Formula

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

Range: -10 → +10

Interpretation

BFRI	Meaning	Action
6–10	Safe	Proceed
3–5	Moderate	Add tests + monitoring
0–2	Risky	Refactor or isolate
** {
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`

```ts
export class UserController extends BaseController {
  async getUser(req: Request, res: Response): Promise {
    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)

Layer	Convention
Controller	PascalCaseController.ts
Service	camelCaseService.ts
Repository	PascalCaseRepository.ts
Routes	camelCaseRoutes.ts
Validators	camelCase.schema.ts

---

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
• No anti-patterns present

---

14. Skill Status

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