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)

| 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 |

---

## When to Use
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

---

## 2. 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

```ts
// ❌ 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`

```ts
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

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

❌ `console.log`
❌ silent failures
❌ swallowed errors

---

### 6. unifiedConfig Is the Only Config Source

```ts
// ❌ NEVER
process.env.JWT_SECRET;

// ✅ ALWAYS
import { config } from '@/config/unifiedConfig';
config.