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.

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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 | | < 0 | Dangerous | Redesign before coding |

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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

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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

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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

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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.

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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

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5. Naming Conventions (Strict)

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

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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.

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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.

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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

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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

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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

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14. Skill Status

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