# Các Pattern Middleware

Express middleware patterns and best practices for GoodGo microservices including middleware chain ordering, custom middleware creation, authentication, validation, logging, and error handling middleware.
> Các patterns và best practices về Express middleware cho GoodGo microservices bao gồm thứ tự middleware chain, tạo custom middleware, authentication, validation, logging, và error handling middleware.

## Tổng Quan

Middleware functions are the fundamental building blocks of Express.js applications. They provide a way to execute code, make changes to requests and responses, end the request-response cycle, and call the next middleware. This guide covers middleware patterns, ordering, common middleware implementations, and best practices for GoodGo microservices.

Middleware functions là các building blocks cơ bản của ứng dụng Express.js. Chúng cung cấp cách để thực thi code, thay đổi requests và responses, kết thúc request-response cycle, và gọi middleware tiếp theo. Hướng dẫn này bao gồm các middleware patterns, thứ tự, common middleware implementations, và best practices cho GoodGo microservices.

## Khi Nào Sử Dụng

Use middleware patterns when:
- Creating custom Express middleware
- Organizing middleware chains and ordering
- Implementing authentication/authorization middleware
- Creating request/response transformation middleware
- Handling cross-cutting concerns (logging, metrics, validation)
- Implementing async middleware patterns
- Testing middleware implementations

Sử dụng middleware patterns khi:
- Tạo custom Express middleware
- Tổ chức middleware chains và thứ tự
- Implement authentication/authorization middleware
- Tạo request/response transformation middleware
- Xử lý cross-cutting concerns (logging, metrics, validation)
- Implement async middleware patterns
- Testing middleware implementations

## Khái Niệm Chính

### Chữ Ký Function Middleware

Express middleware functions have a specific signature that receives request, response, and next function.

Express middleware functions có chữ ký cụ thể nhận request, response, và next function.

```typescript
(req: Request, res: Response, next: NextFunction) => void | Promise<void>
```

### Các Loại Middleware

1. **Application-level / Cấp Ứng Dụng**: Applied to all routes (`app.use()`)
2. **Router-level / Cấp Router**: Applied to specific routes (`router.use()`)
3. **Route-level / Cấp Route**: Applied to specific route handlers

### Thứ Tự Thực Thi Middleware

Middleware order is critical! Execution flows top-to-bottom in the order they are registered.

Thứ tự middleware rất quan trọng! Thực thi theo trình tự từ trên xuống dưới theo thứ tự đăng ký.

```
Request → Middleware 1 → Middleware 2 → ... → Route Handler → Response
```

Biểu đồ sau minh họa luồng middleware chain hoàn chỉnh trong GoodGo services:

The following diagram illustrates the complete middleware chain flow in GoodGo services:

```mermaid
flowchart TD
    Start([HTTP Request]) --> Security["Security Middleware<br/>Helmet, CORS"]
    Security --> RateLimit["Rate Limiting<br/>Middleware"]
    RateLimit --> Correlation["Correlation ID<br/>Middleware"]
    Correlation --> BodyParsing["Body Parsing<br/>JSON, URLEncoded, Cookies"]
    BodyParsing --> Logging["Request Logging<br/>Middleware"]
    Logging --> Metrics["Metrics Collection<br/>Middleware"]
    Metrics --> Routes["Route Handlers"]
    Routes -->|Success| Response([HTTP Response])
    Routes -->|Error| ErrorHandler["Error Handler<br/>Middleware"]
    Routes -->|Not Found| NotFound["Not Found<br/>Handler"]
    ErrorHandler --> Response
    NotFound --> Response
```

## Patterns

### Thứ Tự Middleware Chain

Standard middleware order in GoodGo services ensures proper request processing flow.

Thứ tự middleware chuẩn trong GoodGo services đảm bảo luồng xử lý request đúng cách.

```typescript
// Bảo mật
app.use(helmet());
app.use(cors({ ... }));

// Giới hạn tốc độ
app.use('/api', rateLimitMiddleware);

// Correlation ID (sớm cho tracing)
app.use(correlationMiddleware());

// Phân tích body
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(cookieParser());

// Ghi log request
app.use(loggerMiddleware);

// Metrics
app.use(metricsMiddleware);

// Routes
app.use(createRouter());

// Xử lý lỗi (LUÔN CUỐI CÙNG)
app.use(notFoundHandler);
app.use(errorHandler);
```

**Tham Khảo**: [`services/iam-service/src/main.ts`](../../../services/iam-service/src/main.ts)

### Pattern Correlation Middleware

Adds correlation ID for request tracing across services.

Thêm correlation ID để tracing requests qua các services.

```typescript
export const correlationMiddleware = () => {
  return (req: Request, res: Response, next: NextFunction) => {
    const correlationId = req.headers['x-correlation-id'] || generateId();
    req.correlationId = correlationId;
    res.setHeader('x-correlation-id', correlationId);
    next();
  };
};
```

**Tham Khảo**: [`services/iam-service/src/middlewares/correlation.middleware.ts`](../../../services/iam-service/src/middlewares/correlation.middleware.ts)

### Pattern Authentication Middleware

Verifies JWT tokens and attaches user information to request.

Xác minh JWT tokens và gắn thông tin người dùng vào request.

```typescript
export const authenticate = () => {
  return async (req: Request, res: Response, next: NextFunction) => {
    try {
      const token = extractToken(req);
      if (!token) {
        return res.status(401).json({ error: 'Unauthorized' });
      }
      
      const payload = await jwtService.verify(token);
      req.user = payload;
      next();
    } catch (error) {
      return res.status(401).json({ error: 'Invalid token' });
    }
  };
};
```

Biểu đồ tuần tự sau minh họa luồng authentication middleware:

The following sequence diagram illustrates the authentication middleware flow:

```mermaid
sequenceDiagram
    participant Client
    participant AuthMW as Authentication Middleware
    participant JWTService as JWT Service
    participant RouteHandler as Route Handler
    
    Client->>AuthMW: HTTP Request with Token
    AuthMW->>AuthMW: Extract token from headers
    alt Token exists
        AuthMW->>JWTService: Verify token
        alt Token valid
            JWTService-->>AuthMW: Payload (user data)
            AuthMW->>AuthMW: Attach user to req.user
            AuthMW->>RouteHandler: next() - Continue
            RouteHandler->>Client: HTTP Response (200)
        else Token invalid
            JWTService-->>AuthMW: Verification error
            AuthMW->>Client: HTTP Response (401 Unauthorized)
        end
    else No token
        AuthMW->>Client: HTTP Response (401 Unauthorized)
    end
```

**Tham Khảo**: [`services/iam-service/src/middlewares/auth.middleware.ts`](../../../services/iam-service/src/middlewares/auth.middleware.ts)

### Pattern Validation Middleware

Validates request data using Zod schemas.

Validate dữ liệu request sử dụng Zod schemas.

```typescript
export const validateDto = (schema: AnyZodObject, property: 'body' | 'query' | 'params' = 'body') => {
  return (req: Request, res: Response, next: NextFunction) => {
    try {
      const validatedData = schema.parse(req[property]);
      (req as any)[property] = validatedData;
      next();
    } catch (error) {
      if (error instanceof ZodError) {
        return res.status(400).json({
          success: false,
          error: {
            code: 'VALIDATION_ERROR',
            details: error.errors,
          },
        });
      }
      next(error);
    }
  };
};
```

Biểu đồ tuần tự sau minh họa luồng validation middleware:

The following sequence diagram shows the validation middleware flow:

```mermaid
sequenceDiagram
    participant Client
    participant ValidateMW as Validation Middleware
    participant ZodSchema as Zod Schema
    participant RouteHandler as Route Handler
    
    Client->>ValidateMW: HTTP Request with Data
    ValidateMW->>ValidateMW: Extract data from req[property]
    ValidateMW->>ZodSchema: schema.parse(data)
    alt Validation successful
        ZodSchema-->>ValidateMW: Validated data
        ValidateMW->>ValidateMW: Replace req[property] with validated data
        ValidateMW->>RouteHandler: next() - Continue
        RouteHandler->>Client: HTTP Response (200)
    else Validation failed
        ZodSchema-->>ValidateMW: ZodError with details
        ValidateMW->>ValidateMW: Format error response
        ValidateMW->>Client: HTTP Response (400 Validation Error)
    end
```

**Tham Khảo**: [`services/iam-service/src/middlewares/validation.middleware.ts`](../../../services/iam-service/src/middlewares/validation.middleware.ts)

### Pattern Async Middleware

Handle async operations properly to catch promise rejections.

Xử lý async operations đúng cách để catch promise rejections.

```typescript
export const asyncHandler = (fn: Function) => {
  return (req: Request, res: Response, next: NextFunction) => {
    Promise.resolve(fn(req, res, next)).catch(next);
  };
};

// Usage
app.get('/users', asyncHandler(async (req, res) => {
  const users = await userService.findAll();
  res.json({ success: true, data: users });
}));
```

Biểu đồ tuần tự sau minh họa xử lý lỗi async middleware:

The following sequence diagram illustrates async middleware error handling:

```mermaid
sequenceDiagram
    participant Client
    participant AsyncMW as Async Middleware Wrapper
    participant AsyncHandler as Async Route Handler
    participant ErrorHandler as Error Handler
    
    Client->>AsyncMW: HTTP Request
    AsyncMW->>AsyncHandler: Execute async function
    alt Async operation succeeds
        AsyncHandler->>AsyncHandler: Process request
        AsyncHandler->>Client: HTTP Response (200)
    else Async operation fails
        AsyncHandler-->>AsyncMW: Promise rejection (Error)
        AsyncMW->>ErrorHandler: next(error)
        ErrorHandler->>ErrorHandler: Format error response
        ErrorHandler->>Client: HTTP Response (500 Error)
    end
```

### Pattern Logging Middleware

Log request details with timing and correlation tracking.

Ghi log chi tiết request với timing và correlation tracking.

```typescript
export const requestLogger = (req: Request, res: Response, next: NextFunction) => {
  const startTime = Date.now();
  
  res.on('finish', () => {
    const duration = Date.now() - startTime;
    logger.info('Request completed', {
      method: req.method,
      url: req.url,
      statusCode: res.statusCode,
      duration,
      correlationId: req.correlationId,
    });
  });
  
  next();
};
```

Biểu đồ tuần tự sau minh họa cách logging middleware theo dõi vòng đời request:

The following sequence diagram illustrates how logging middleware tracks request lifecycle:

```mermaid
sequenceDiagram
    participant Client
    participant LogMW as Logging Middleware
    participant RouteHandler as Route Handler
    participant Logger as Logger Service
    participant Response as Response Object
    
    Client->>LogMW: HTTP Request
    LogMW->>LogMW: Record startTime = Date.now()
    LogMW->>RouteHandler: next() - Continue chain
    
    RouteHandler->>RouteHandler: Process request
    RouteHandler->>Response: res.json(data) or res.send()
    
    Response->>Response: Set statusCode, send response
    Response->>Response: Emit 'finish' event
    
    Response->>LogMW: 'finish' event triggered
    LogMW->>LogMW: Calculate duration = Date.now() - startTime
    LogMW->>Logger: logger.info('Request completed', {<br/>method, url, statusCode,<br/>duration, correlationId})
    Logger->>Logger: Write structured log entry
    
    Response->>Client: HTTP Response
```

### Chuyển Đổi Request/Response

Transform request or response data in middleware.

Chuyển đổi dữ liệu request hoặc response trong middleware.

```typescript
export const transformResponse = () => {
  return (req: Request, res: Response, next: NextFunction) => {
    const originalJson = res.json.bind(res);
    
    res.json = function(data: any) {
      const transformed = {
        success: true,
        data,
        timestamp: new Date().toISOString(),
      };
      return originalJson(transformed);
    };
    
    next();
  };
};
```

Biểu đồ tuần tự sau minh họa cách middleware chuyển đổi request và response hoạt động:

The following sequence diagram shows how request and response transformation middleware works:

```mermaid
sequenceDiagram
    participant Client
    participant TransformMW as Transform Middleware
    participant RouteHandler as Route Handler
    participant Response as Response Object
    
    Client->>TransformMW: HTTP Request
    Note over TransformMW: Intercept res.json()
    TransformMW->>TransformMW: Store original res.json
    TransformMW->>TransformMW: Override res.json with transformation logic
    TransformMW->>RouteHandler: next() - Continue chain
    
    RouteHandler->>RouteHandler: Process request, Generate data
    RouteHandler->>Response: res.json(rawData)
    
    Note over Response: Transformed res.json executes
    Response->>Response: Wrap data: success, data, timestamp
    Response->>Client: HTTP Response (transformed)
```

## Thực Hành Tốt Nhất

1. **Order Matters / Thứ Tự Quan Trọng**: Đặt middleware đúng thứ tự (security → correlation → parsing → logging → routes → errors)
2. **Error Handling / Xử Lý Lỗi**: Luôn xử lý errors và gọi `next(error)` cho error middleware
3. **Async Support / Hỗ Trợ Async**: Wrap async middleware đúng cách để catch promise rejections
4. **Early Returns / Return Sớm**: Sử dụng early returns cho validation failures (không gọi `next()`)
5. **Request Extension / Mở Rộng Request**: Sử dụng TypeScript declaration merging để mở rộng Request type
6. **Conditional Logic / Logic Có Điều Kiện**: Sử dụng middleware factories cho conditional middleware
7. **Reusability / Tái Sử Dụng**: Tạo reusable middleware functions
8. **Performance / Hiệu Suất**: Giữ middleware lightweight, tránh heavy operations

## Lỗi Thường Gặp

1. **Wrong Order / Thứ Tự Sai**: Đặt middleware sai thứ tự (ví dụ: error handler trước routes)
2. **Not Calling Next / Không Gọi Next**: Quên gọi `next()` hoặc `next(error)`
3. **Async Errors / Lỗi Async**: Không xử lý promise rejections trong async middleware
4. **Early Return Issues / Vấn Đề Return Sớm**: Gọi `next()` sau khi đã send response
5. **Type Safety / An Toàn Kiểu**: Không mở rộng Express Request type đúng cách
6. **Performance / Hiệu Suất**: Thực hiện heavy operations trong middleware

## Xử Lý Sự Cố

### Middleware Không Thực Thi

**Problem / Vấn Đề**: Middleware không được gọi

**Solution / Giải Pháp**: 
- Kiểm tra thứ tự middleware, đảm bảo nó được thêm trước routes
- Verify `next()` được gọi
- Check middleware registration order

### Lỗi Async Không Được Bắt

**Problem / Vấn Đề**: Unhandled promise rejections trong async middleware

**Solution / Giải Pháp**: 
- Sử dụng `asyncHandler` wrapper
- Hoặc wrap async code trong try-catch với `next(error)`

**Example / Ví Dụ**:
```typescript
// ❌ Bad: Unhandled promise rejection
app.get('/users', async (req, res) => {
  const users = await userService.findAll(); // Error not caught!
  res.json(users);
});

// ✅ Good: Proper error handling
app.get('/users', asyncHandler(async (req, res) => {
  const users = await userService.findAll();
  res.json(users);
}));
```

## Tài Nguyên

- [Correlation Middleware](../../../services/iam-service/src/middlewares/correlation.middleware.ts) - Correlation ID implementation
- [Auth Middleware](../../../services/iam-service/src/middlewares/auth.middleware.ts) - Authentication middleware
- [Validation Middleware](../../../services/iam-service/src/middlewares/validation.middleware.ts) - Request validation
- [Error Middleware](../../../services/iam-service/src/middlewares/error.middleware.ts) - Error handling
- [Metrics Middleware](../../../services/iam-service/src/middlewares/metrics.middleware.ts) - Metrics collection
- [Error Handling](./error-handling-patterns.md) - Error middleware patterns
