# Backend Middleware

Middleware components provide cross-cutting concerns for authentication, authorization, validation, rate limiting, and error handling across all API endpoints.

## Middleware Architecture

Express middleware functions are composed in the request pipeline to:

- Authenticate users via JWT tokens
- Authorize access based on user roles
- Validate request bodies against Zod schemas
- Apply rate limits to prevent abuse
- Handle errors consistently
- Log requests and responses

## Core Middleware

### Authentication

**authenticate** (`middleware/auth.ts`)

- Verifies JWT access tokens from Authorization header
- Attaches `req.user` object with user ID, email, and role
- Returns 401 Unauthorized for missing/invalid tokens
- Supports both admin and public route protection

```typescript
router.get('/profile', authenticate, async (req, res) => {
  // req.user is guaranteed to exist
  const userId = req.user.id;
});
```

### Authorization

**requireRole** (`middleware/auth.ts`)

- Checks if authenticated user has one of the required roles
- Returns 403 Forbidden if role check fails
- Supports multiple roles (OR logic)

```typescript
router.post(
  '/campaigns',
  authenticate,
  requireRole('SUPER_ADMIN', 'INFLUENCE_ADMIN'),
  async (req, res) => {
    // Only admins can create campaigns
  }
);
```

**requireNonTemp** (`middleware/auth.ts`)

- Blocks TEMP users from accessing endpoints
- Used for routes that require full user accounts
- TEMP users are created during shift signups

```typescript
router.post(
  '/shifts/:id/signup',
  authenticate,
  requireNonTemp,
  async (req, res) => {
    // TEMP users cannot sign up for shifts
  }
);
```

### Validation

**validate** (`middleware/validate.ts`)

- Validates request body against Zod schemas
- Returns 400 Bad Request with sanitized error messages
- Supports nested validation and type coercion
- Prevents injection attacks by sanitizing output

```typescript
import { validate } from '../middleware/validate';
import { createCampaignSchema } from './campaigns.schemas';

router.post(
  '/campaigns',
  authenticate,
  validate(createCampaignSchema),
  async (req, res) => {
    // req.body is type-safe and validated
  }
);
```

### Rate Limiting

**rateLimit** (`middleware/rate-limit.ts`)

- Uses Redis for distributed rate limiting
- Configurable window size and max requests
- Returns 429 Too Many Requests when limit exceeded
- Per-IP tracking with X-RateLimit headers

**Common Configurations:**

```typescript
// Auth endpoints: 10 requests per minute
rateLimitRedis({
  windowMs: 60 * 1000,
  max: 10,
  standardHeaders: true,
  legacyHeaders: false,
  keyPrefix: 'rl:auth:',
})

// Canvass visits: 30 requests per minute
rateLimitRedis({
  windowMs: 60 * 1000,
  max: 30,
  keyPrefix: 'rl:canvass-visit:',
})
```

### Error Handling

**errorHandler** (`middleware/error-handler.ts`)

- Catches all unhandled errors in routes
- Formats errors consistently as JSON
- Logs errors with Winston
- Sanitizes error messages in production
- Returns appropriate HTTP status codes

### Request Logging

**requestLogger** (`middleware/logger.ts`)

- Logs all incoming requests with Morgan
- Tracks response times
- Formats logs with Winston
- Separates error logs from access logs

## Middleware Composition

Middleware is applied in order and can be composed:

```typescript
// Global middleware (applies to all routes)
app.use(helmet());
app.use(cors());
app.use(requestLogger);

// Route-specific middleware
router.post(
  '/api/campaigns',
  rateLimit({ max: 100 }),
  authenticate,
  requireRole('SUPER_ADMIN', 'INFLUENCE_ADMIN'),
  validate(createCampaignSchema),
  campaignController.create
);

// Global error handler (last middleware)
app.use(errorHandler);
```

## Security Features

### Password Policy

- Enforced at schema validation level
- 12+ characters required
- Must include uppercase, lowercase, and digit
- Validated in `auth.schemas.ts`

### User Enumeration Prevention

- Returns 401 instead of 404 for missing users
- Consistent response times for invalid credentials
- No detailed error messages about which field failed

### Refresh Token Rotation

- Atomic transaction to prevent race conditions
- Invalidates old refresh token on rotation
- Tracks token family for security
- Automatic cleanup of expired tokens

### Redis Authentication

- All Redis connections require password
- Configured via `REDIS_PASSWORD` environment variable
- Prevents unauthorized cache/queue access

## Middleware Chain

Typical middleware chain for protected routes:

1. **CORS** - Handle cross-origin requests
2. **Helmet** - Security headers
3. **Request Logger** - Log incoming request
4. **Body Parser** - Parse JSON body
5. **Rate Limit** - Check rate limits
6. **Authenticate** - Verify JWT token
7. **Authorize** - Check user role
8. **Validate** - Validate request schema
9. **Route Handler** - Execute business logic
10. **Error Handler** - Catch and format errors

## Related Documentation

- [Backend Overview](../index.md)
- [Authentication Module](../modules/auth.md)
- [Security Audit](../../deployment/security.md)
- [Environment Variables](../../deployment/environment-variables.md)
- [Rate Limiting](../../deployment/rate-limiting.md)