Express Microservices Architecture

Express.js Microservices Architecture

A comprehensive skill for building production-ready microservices with Express.js. Master middleware patterns, routing strategies, error handling, scalability techniques, and deployment architectures for Node.js microservices at scale.

When to Use This Skill

Use this skill when:

• Building RESTful APIs and microservices with Node.js
• Designing scalable distributed systems with Express.js
• Implementing middleware-based architecture patterns
• Creating API gateways and service mesh architectures
• Developing production-ready Node.js applications
• Migrating monoliths to microservices architecture
• Building event-driven microservices
• Implementing authentication, authorization, and security layers
• Optimizing Express.js applications for high performance
• Setting up monitoring, logging, and observability
• Deploying Express.js apps with Docker and Kubernetes
• Implementing circuit breakers and resilience patterns

Core Concepts

Express.js Fundamentals

Express.js is a minimal and flexible Node.js web application framework that provides robust features for web and mobile applications. It's the de facto standard for building Node.js APIs and microservices.

Key Characteristics:

• Minimal: Unopinionated framework with essential web app features
• Middleware-based: Request/response pipeline architecture
• Routing: Powerful routing mechanism with parameter support
• Template Engines: Support for various view engines
• Performance: Built on top of Node.js for high performance
• Extensible: Rich ecosystem of middleware and plugins

Middleware Architecture

Middleware functions are the backbone of Express.js applications. They have access to the request object (req), response object (res), and the next middleware function (next).

Middleware Flow:

Request → Middleware 1 → Middleware 2 → ... → Route Handler → Response
                ↓              ↓                      ↓
           Error Handler  Error Handler         Error Handler

Middleware Types:

1. Application-level middleware: Bound to app instance
2. Router-level middleware: Bound to express.Router() instance
3. Error-handling middleware: Has 4 parameters (err, req, res, next)
4. Built-in middleware: Express built-in functions (static, json, urlencoded)
5. Third-party middleware: External packages (cors, helmet, morgan)

Routing Strategies

Express routing enables you to map HTTP methods and URLs to handler functions.

Routing Components:

• Route paths: String patterns, regex, or path parameters
• Route parameters: Named URL segments (:userId)
• Route handlers: Single or multiple callback functions
• Response methods: res.send(), res.json(), res.status(), etc.
• Router instances: Modular, mountable route handlers

Error Handling

Error handling in Express requires special middleware with 4 parameters: (err, req, res, next).

Error Handling Flow:

1. Synchronous errors are caught automatically
2. Asynchronous errors must be passed to next(err)
3. Error middleware processes errors centrally
4. Proper status codes and error formats returned

Microservices Principles

Characteristics of Microservices:

• Single Responsibility: Each service does one thing well
• Independence: Services can be deployed independently
• Decentralized: Each service owns its data
• Resilience: Failure in one service doesn't crash entire system
• Scalability: Scale services independently based on demand
• Technology Diversity: Different services can use different tech stacks

Microservices Patterns

Pattern 1: API Gateway Pattern

The API Gateway acts as a single entry point for all client requests, routing them to appropriate microservices.

Benefits:

• Single entry point for clients
• Request routing and composition
• Authentication and authorization
• Rate limiting and throttling
• Request/response transformation
• Protocol translation

Implementation Structure:

Client → API Gateway → Microservice 1 (Users)
                    → Microservice 2 (Orders)
                    → Microservice 3 (Products)
                    → Microservice 4 (Notifications)

Pattern 2: Service Discovery

Services register themselves and discover other services dynamically.

Approaches:

• Client-side discovery: Client queries service registry
• Server-side discovery: Load balancer queries registry
• DNS-based discovery: Using DNS for service location

Popular Tools:

• Consul
• Eureka
• etcd
• Kubernetes built-in discovery

Pattern 3: Circuit Breaker

Prevents cascading failures by stopping requests to failing services.

States:

• Closed: Normal operation, requests pass through
• Open: Service failing, requests fail immediately
• Half-Open: Testing if service recovered

Pattern 4: Event-Driven Architecture

Services communicate through events instead of direct calls.

Components:

• Event producers: Services that emit events
• Event consumers: Services that listen to events
• Message broker: RabbitMQ, Kafka, Redis
• Event store: Persist events for replay

Pattern 5: Database per Service

Each microservice owns its database, ensuring loose coupling.

Benefits:

• Service independence
• Technology diversity
• Easier scaling
• Clear boundaries

Challenges:

• Distributed transactions
• Data consistency
• Joins across services

Pattern 6: Saga Pattern

Manages distributed transactions across multiple services.

Types:

• Choreography: Services coordinate through events
• Orchestration: Central coordinator manages transaction

Pattern 7: CQRS (Command Query Responsibility Segregation)

Separate read and write operations into different models.

Benefits:

• Optimized read/write models
• Scalability
• Performance
• Flexibility

Middleware Architecture Patterns

Custom Middleware Development

Middleware functions execute in the order they're defined.

Basic Middleware Structure:

const express = require('express');
const app = express();

// Basic middleware
const requestLogger = (req, res, next) => {
  console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
  next(); // Pass control to next middleware
};

app.use(requestLogger);

From Context7 - Saving Data in Request Object:

const express = require('express');
const app = express();
const port = 3000;

// Middleware to add user data to the request object
const addUserInfo = (req, res, next) => {
  req.user = {
    id: 123,
    username: 'testuser'
  };
  next();
};

// Middleware to add request timestamp
const addTimestamp = (req, res, next) => {
  req.requestTime = Date.now();
  next();
};

// Apply middleware globally
app.use(addUserInfo);
app.use(addTimestamp);

app.get('/', (req, res) => {
  const userId = req.user.id;
  const username = req.user.username;
  const timestamp = req.requestTime;

  res.send(`User ID: ${userId}, Username: ${username}, Request Time: ${new Date(timestamp).toISOString()}`);
});

app.listen(port, () => {
  console.log(`Request data sharing example listening at http://localhost:${port}`);
});

Error-Handling Middleware

Error middleware has 4 parameters and should be defined after all other middleware.

From Context7 - Error Handling Middleware:

const express = require('express');
const app = express();
const port = 3000;

// A regular middleware
app.use((req, res, next) => {
  console.log('Request received');
  next(); // Pass control to the next middleware
});

// A route that might throw an error
app.get('/throw-error', (req, res, next) => {
  // Simulate an error
  const error = new Error('This is a simulated error');
  error.status = 400;
  next(error);
});

// Error-handling middleware (must have 4 arguments)
app.use((err, req, res, next) => {
  console.error('Error caught:', err.message);
  res.status(err.status || 500).send(`An error occurred: ${err.message}`);
});

app.listen(port, () => {
  console.log(`Error middleware example listening at http://localhost:${port}`);
});

From Context7 - Global Error Handler:

app.use(express.bodyParser())
app.use(express.cookieParser())
app.use(express.session())
app.use(app.router) // the router itself (app.get(), app.put() etc)
app.use(function(err, req, res, next){
  // if an error occurs Connect will pass it down
  // through these "error-handling" middleware
  // allowing you to respond however you like
  res.send(500, { error: 'Sorry something bad happened!' });
})

Route-Specific Middleware

Apply middleware to specific routes for targeted functionality.

From Context7 - Route Middleware:

const express = require('express');
const app = express();
const port = 3000;

// Middleware function
const requestLogger = (req, res, next) => {
  console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
  next();
};

// Apply middleware to a specific route
app.get('/protected', requestLogger, (req, res) => {
  res.send('This route is protected by middleware!');
});

// Apply middleware to multiple routes
const adminMiddleware = (req, res, next) => {
  console.log('Admin access check...');
  // In a real app, you'd check user roles here
  next();
};

app.get('/admin/dashboard', adminMiddleware, (req, res) => {
  res.send('Welcome to the admin dashboard!');
});

// Middleware applied globally
app.use(requestLogger);

app.get('/', (req, res) => {
  res.send('Hello, world!');
});

app.listen(port, () => {
  console.log(`Route middleware example listening at http://localhost:${port}`);
});

Authentication Middleware

const jwt = require('jsonwebtoken');

// JWT Authentication Middleware
const authenticateToken = (req, res, next) => {
  const authHeader = req.headers['authorization'];
  const token = authHeader && authHeader.split(' ')[1];

  if (!token) {
    return res.status(401).json({ error: 'Access token required' });
  }

  jwt.verify(token, process.env.JWT_SECRET, (err, user) => {
    if (err) {
      return res.status(403).json({ error: 'Invalid or expired token' });
    }
    req.user = user;
    next();
  });
};

// Role-based Authorization Middleware
const authorize = (...roles) => {
  return (req, res, next) => {
    if (!req.user) {
      return res.status(401).json({ error: 'Unauthorized' });
    }

    if (!roles.includes(req.user.role)) {
      return res.status(403).json({ error: 'Insufficient permissions' });
    }

    next();
  };
};

// Usage
app.get('/admin/users', authenticateToken, authorize('admin'), (req, res) => {
  res.json({ users: [] });
});

Request Validation Middleware

const { body, param, query, validationResult } = require('express-validator');

// Validation middleware factory
const validate = (validations) => {
  return async (req, res, next) => {
    await Promise.all(validations.map(validation => validation.run(req)));

    const errors = validationResult(req);
    if (errors.isEmpty()) {
      return next();
    }

    res.status(400).json({
      error: 'Validation failed',
      details: errors.array()
    });
  };
};

// Usage
app.post('/users', validate([
  body('email').isEmail().normalizeEmail(),
  body('password').isLength({ min: 8 }),
  body('name').trim().notEmpty()
]), (req, res) => {
  // Request is validated
  res.json({ success: true });
});

Rate Limiting Middleware

const rateLimit = require('express-rate-limit');
const RedisStore = require('rate-limit-redis');
const Redis = require('ioredis');

// In-memory rate limiter
const limiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100, // Limit each IP to 100 requests per windowMs
  message: 'Too many requests, please try again later',
  standardHeaders: true,
  legacyHeaders: false,
});

// Redis-based rate limiter for distributed systems
const redisClient = new Redis({
  host: process.env.REDIS_HOST,
  port: process.env.REDIS_PORT,
});

const distributedLimiter = rateLimit({
  store: new RedisStore({
    client: redisClient,
    prefix: 'rl:',
  }),
  windowMs: 15 * 60 * 1000,
  max: 100,
});

// Apply to all routes
app.use('/api/', distributedLimiter);

// Apply to specific routes
app.post('/api/login', rateLimit({
  windowMs: 15 * 60 * 1000,
  max: 5, // Only 5 login attempts per 15 minutes
}), loginHandler);

Logging Middleware

const morgan = require('morgan');
const winston = require('winston');
const { format } = winston;

// Create Winston logger
const logger = winston.createLogger({
  level: 'info',
  format: format.combine(
    format.timestamp(),
    format.errors({ stack: true }),
    format.json()
  ),
  defaultMeta: { service: 'user-service' },
  transports: [
    new winston.transports.File({ filename: 'error.log', level: 'error' }),
    new winston.transports.File({ filename: 'combined.log' }),
  ],
});

// Console logging in development
if (process.env.NODE_ENV !== 'production') {
  logger.add(new winston.transports.Console({
    format: format.combine(
      format.colorize(),
      format.simple()
    )
  }));
}

// HTTP request logging with Morgan
app.use(morgan('combined', {
  stream: {
    write: (message) => logger.info(message.trim())
  }
}));

// Custom logging middleware
const requestLogger = (req, res, next) => {
  const start = Date.now();

  res.on('finish', () => {
    const duration = Date.now() - start;
    logger.info({
      method: req.method,
      path: req.path,
      status: res.statusCode,
      duration: `${duration}ms`,
      ip: req.ip,
      userAgent: req.get('user-agent')
    });
  });

  next();
};

app.use(requestLogger);

CORS Middleware

const cors = require('cors');

// Basic CORS
app.use(cors());

// Configured CORS
const corsOptions = {
  origin: function (origin, callback) {
    const allowedOrigins = [
      'https://example.com',
      'https://app.example.com',
      process.env.FRONTEND_URL
    ];

    if (!origin || allowedOrigins.includes(origin)) {
      callback(null, true);
    } else {
      callback(new Error('Not allowed by CORS'));
    }
  },
  credentials: true,
  optionsSuccessStatus: 200,
  methods: ['GET', 'POST', 'PUT', 'DELETE', 'PATCH'],
  allowedHeaders: ['Content-Type', 'Authorization'],
  exposedHeaders: ['X-Total-Count', 'X-Page-Number'],
  maxAge: 86400, // 24 hours
};

app.use(cors(corsOptions));

// CORS for specific routes
app.options('/api/admin/*', cors(adminCorsOptions));
app.use('/api/admin/', cors(adminCorsOptions));

Security Middleware

const helmet = require('helmet');
const mongoSanitize = require('express-mongo-sanitize');
const xss = require('xss-clean');
const hpp = require('hpp');

// Helmet - Set security headers
app.use(helmet());

// Custom security headers
app.use((req, res, next) => {
  res.setHeader('X-Content-Type-Options', 'nosniff');
  res.setHeader('X-Frame-Options', 'DENY');
  res.setHeader('X-XSS-Protection', '1; mode=block');
  next();
});

// Sanitize data against NoSQL injection
app.use(mongoSanitize());

// Prevent XSS attacks
app.use(xss());

// Prevent HTTP Parameter Pollution
app.use(hpp({
  whitelist: ['sort', 'fields', 'page', 'limit']
}));

// Content Security Policy
app.use(helmet.contentSecurityPolicy({
  directives: {
    defaultSrc: ["'self'"],
    styleSrc: ["'self'", "'unsafe-inline'"],
    scriptSrc: ["'self'"],
    imgSrc: ["'self'", 'data:', 'https:'],
  },
}));

Routing Strategies

Basic Routing

From Context7 - Express Routing:

app.get('/', home);
app.use('/public', require('st')(process.cwd()));
app.get('/users', users.list);
app.post('/users', users.create);

Route Method Chaining

From Context7 - Route Chaining:

app.route('/users')
.get(function(req, res, next) {
  // Get all users
  res.json({ users: [] });
})
.post(function(req, res, next) {
  // Create new user
  res.status(201).json({ user: {} });
});

Router Modules

// routes/users.js
const express = require('express');
const router = express.Router();

// Middleware specific to this router
router.use((req, res, next) => {
  console.log('Time: ', Date.now());
  next();
});

// Define routes
router.get('/', (req, res) => {
  res.json({ users: [] });
});

router.get('/:id', (req, res) => {
  res.json({ user: { id: req.params.id } });
});

router.post('/', (req, res) => {
  res.status(201).json({ user: req.body });
});

router.put('/:id', (req, res) => {
  res.json({ user: { id: req.params.id, ...req.body } });
});

router.delete('/:id', (req, res) => {
  res.status(204).send();
});

module.exports = router;

// app.js
const usersRouter = require('./routes/users');
app.use('/api/users', usersRouter);

Route Parameters

// Named parameters
app.get('/users/:userId/posts/:postId', (req, res) => {
  const { userId, postId } = req.params;
  res.json({ userId, postId });
});

// Parameter middleware
app.param('userId', (req, res, next, userId) => {
  // Fetch user from database
  User.findById(userId)
    .then(user => {
      if (!user) {
        return res.status(404).json({ error: 'User not found' });
      }
      req.user = user;
      next();
    })
    .catch(next);
});

// Multiple callbacks
app.param('postId', [
  validatePostId,
  fetchPost,
  checkPermissions
]);

Query Parameters

// GET /api/users?role=admin&active=true&page=2&limit=10
app.get('/api/users', (req, res) => {
  const {
    role,
    active,
    page = 1,
    limit = 10,
    sort = '-createdAt'
  } = req.query;

  const query = {};
  if (role) query.role = role;
  if (active !== undefined) query.active = active === 'true';

  const skip = (page - 1) * limit;

  User.find(query)
    .sort(sort)
    .limit(parseInt(limit))
    .skip(skip)
    .then(users => res.json({ users, page, limit }))
    .catch(next);
});

API Versioning

// Version 1 routes
const v1Router = express.Router();
v1Router.get('/users', (req, res) => {
  res.json({ version: 'v1', users: [] });
});

// Version 2 routes
const v2Router = express.Router();
v2Router.get('/users', (req, res) => {
  res.json({ version: 'v2', users: [], meta: {} });
});

// Mount versioned routes
app.use('/api/v1', v1Router);
app.use('/api/v2', v2Router);

// Header-based versioning
app.use('/api/users', (req, res, next) => {
  const version = req.headers['api-version'] || 'v1';

  if (version === 'v2') {
    return v2UsersHandler(req, res, next);
  }
  return v1UsersHandler(req, res, next);
});

RESTful Route Organization

// controllers/users.controller.js
class UsersController {
  async list(req, res, next) {
    try {
      const users = await User.find();
      res.json({ users });
    } catch (error) {
      next(error);
    }
  }

  async get(req, res, next) {
    try {
      res.json({ user: req.user });
    } catch (error) {
      next(error);
    }
  }

  async create(req, res, next) {
    try {
      const user = await User.create(req.body);
      res.status(201).json({ user });
    } catch (error) {
      next(error);
    }
  }

  async update(req, res, next) {
    try {
      const user = await User.findByIdAndUpdate(
        req.params.id,
        req.body,
        { new: true, runValidators: true }
      );
      res.json({ user });
    } catch (error) {
      next(error);
    }
  }

  async delete(req, res, next) {
    try {
      await User.findByIdAndDelete(req.params.id);
      res.status(204).send();
    } catch (error) {
      next(error);
    }
  }
}

// routes/users.routes.js
const router = express.Router();
const controller = new UsersController();

router.get('/', controller.list);
router.get('/:id', controller.get);
router.post('/', controller.create);
router.put('/:id', controller.update);
router.delete('/:id', controller.delete);

module.exports = router;

Scalability Patterns

Horizontal Scaling

Deploy multiple instances of your service behind a load balancer.

// Enable cluster mode
const cluster = require('cluster');
const os = require('os');

if (cluster.isMaster) {
  const numCPUs = os.cpus().length;

  console.log(`Master process ${process.pid} is running`);
  console.log(`Forking ${numCPUs} workers...`);

  // Fork workers
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died. Restarting...`);
    cluster.fork();
  });
} else {
  // Workers share the TCP connection
  const app = require('./app');
  const port = process.env.PORT || 3000;

  app.listen(port, () => {
    console.log(`Worker ${process.pid} started on port ${port}`);
  });
}

Load Balancing

# nginx.conf
upstream backend {
    least_conn;
    server localhost:3001;
    server localhost:3002;
    server localhost:3003;
    server localhost:3004;
}

server {
    listen 80;

    location / {
        proxy_pass http://backend;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

Caching Strategies

const Redis = require('ioredis');
const redis = new Redis({
  host: process.env.REDIS_HOST,
  port: process.env.REDIS_PORT,
});

// Cache middleware
const cacheMiddleware = (duration = 300) => {
  return async (req, res, next) => {
    if (req.method !== 'GET') {
      return next();
    }

    const key = `cache:${req.originalUrl}`;

    try {
      const cached = await redis.get(key);

      if (cached) {
        return res.json(JSON.parse(cached));
      }

      // Store original res.json
      const originalJson = res.json.bind(res);

      // Override res.json
      res.json = (body) => {
        redis.setex(key, duration, JSON.stringify(body));
        return originalJson(body);
      };

      next();
    } catch (error) {
      console.error('Cache error:', error);
      next();
    }
  };
};

// Usage
app.get('/api/products', cacheMiddleware(600), async (req, res) => {
  const products = await Product.find();
  res.json({ products });
});

// Cache invalidation
const invalidateCache = async (pattern) => {
  const keys = await redis.keys(pattern);
  if (keys.length > 0) {
    await redis.del(...keys);
  }
};

// Invalidate on updates
app.post('/api/products', async (req, res) => {
  const product = await Product.create(req.body);
  await invalidateCache('cache:/api/products*');
  res.status(201).json({ product });
});

Database Connection Pooling

const mongoose = require('mongoose');

// MongoDB connection with pooling
mongoose.connect(process.env.MONGODB_URI, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
  poolSize: 10, // Maintain up to 10 socket connections
  socketTimeoutMS: 45000,
  family: 4,
});

// PostgreSQL with connection pooling
const { Pool } = require('pg');
const pool = new Pool({
  host: process.env.DB_HOST,
  port: process.env.DB_PORT,
  database: process.env.DB_NAME,
  user: process.env.DB_USER,
  password: process.env.DB_PASSWORD,
  max: 20, // Maximum number of clients
  idleTimeoutMillis: 30000,
  connectionTimeoutMillis: 2000,
});

// Query helper
const query = async (text, params) => {
  const start = Date.now();
  const res = await pool.query(text, params);
  const duration = Date.now() - start;
  console.log('Executed query', { text, duration, rows: res.rowCount });
  return res;
};

module.exports = { query, pool };

Response Compression

const compression = require('compression');

// Basic compression
app.use(compression());

// Custom compression settings
app.use(compression({
  filter: (req, res) => {
    if (req.headers['x-no-compression']) {
      return false;
    }
    return compression.filter(req, res);
  },
  level: 6, // Compression level (0-9)
  threshold: 1024, // Minimum size to compress (bytes)
}));

Request Throttling

const { Throttle } = require('stream-throttle');

// Throttle large responses
app.get('/api/large-dataset', (req, res) => {
  const dataStream = getLargeDataStream();

  // Throttle to 1MB/s
  const throttle = new Throttle({ rate: 1024 * 1024 });

  res.setHeader('Content-Type', 'application/json');
  dataStream.pipe(throttle).pipe(res);
});

Production Architecture

Docker Containerization

# Dockerfile
FROM node:18-alpine AS builder

WORKDIR /app

# Copy package files
COPY package*.json ./

# Install dependencies
RUN npm ci --only=production

# Copy source code
COPY . .

# Production image
FROM node:18-alpine

WORKDIR /app

# Create non-root user
RUN addgroup -g 1001 -S nodejs && \
    adduser -S nodejs -u 1001

# Copy from builder
COPY --from=builder --chown=nodejs:nodejs /app/node_modules ./node_modules
COPY --chown=nodejs:nodejs . .

# Switch to non-root user
USER nodejs

# Expose port
EXPOSE 3000

# Health check
HEALTHCHECK --interval=30s --timeout=3s --start-period=40s \
  CMD node healthcheck.js

# Start application
CMD ["node", "server.js"]

# docker-compose.yml
version: '3.8'

services:
  api:
    build: .
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
      - REDIS_HOST=redis
      - MONGODB_URI=mongodb://mongo:27017/app
    depends_on:
      - redis
      - mongo
    restart: unless-stopped
    deploy:
      replicas: 3
      resources:
        limits:
          cpus: '0.5'
          memory: 512M

  redis:
    image: redis:7-alpine
    volumes:
      - redis-data:/data
    restart: unless-stopped

  mongo:
    image: mongo:6
    volumes:
      - mongo-data:/data/db
    restart: unless-stopped

  nginx:
    image: nginx:alpine
    ports:
      - "80:80"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
    depends_on:
      - api
    restart: unless-stopped

volumes:
  redis-data:
  mongo-data:

Process Management with PM2

// ecosystem.config.js
module.exports = {
  apps: [{
    name: 'api',
    script: './server.js',
    instances: 'max',
    exec_mode: 'cluster',
    autorestart: true,
    watch: false,
    max_memory_restart: '1G',
    env: {
      NODE_ENV: 'development',
      PORT: 3000
    },
    env_production: {
      NODE_ENV: 'production',
      PORT: 3000
    },
    error_file: './logs/err.log',
    out_file: './logs/out.log',
    log_file: './logs/combined.log',
    time: true,
    merge_logs: true,
  }]
};

Health Checks

// healthcheck.js
const http = require('http');

const options = {
  host: 'localhost',
  port: process.env.PORT || 3000,
  path: '/health',
  timeout: 2000
};

const healthCheck = http.request(options, (res) => {
  console.log(`HEALTHCHECK STATUS: ${res.statusCode}`);
  if (res.statusCode === 200) {
    process.exit(0);
  } else {
    process.exit(1);
  }
});

healthCheck.on('error', (err) => {
  console.error('ERROR:', err);
  process.exit(1);
});

healthCheck.end();

// Health endpoint
app.get('/health', async (req, res) => {
  const health = {
    uptime: process.uptime(),
    message: 'OK',
    timestamp: Date.now()
  };

  try {
    // Check database connection
    await mongoose.connection.db.admin().ping();
    health.database = 'connected';

    // Check Redis connection
    await redis.ping();
    health.cache = 'connected';

    res.status(200).json(health);
  } catch (error) {
    health.message = error.message;
    res.status(503).json(health);
  }
});

// Readiness check
app.get('/ready', (req, res) => {
  res.status(200).json({ ready: true });
});

// Liveness check
app.get('/live', (req, res) => {
  res.status(200).json({ alive: true });
});

Graceful Shutdown

// server.js
const gracefulShutdown = () => {
  console.log('Received shutdown signal, closing server gracefully...');

  server.close(async () => {
    console.log('HTTP server closed');

    try {
      // Close database connections
      await mongoose.connection.close();
      console.log('MongoDB connection closed');

      // Close Redis connection
      await redis.quit();
      console.log('Redis connection closed');

      // Close other resources
      // ...

      console.log('Graceful shutdown completed');
      process.exit(0);
    } catch (err) {
      console.error('Error during shutdown:', err);
      process.exit(1);
    }
  });

  // Force shutdown after 10 seconds
  setTimeout(() => {
    console.error('Forcing shutdown after timeout');
    process.exit(1);
  }, 10000);
};

process.on('SIGTERM', gracefulShutdown);
process.on('SIGINT', gracefulShutdown);

Monitoring and Observability

const promClient = require('prom-client');

// Create a Registry
const register = new promClient.Registry();

// Add default metrics
promClient.collectDefaultMetrics({ register });

// Custom metrics
const httpRequestDuration = new promClient.Histogram({
  name: 'http_request_duration_seconds',
  help: 'Duration of HTTP requests in seconds',
  labelNames: ['method', 'route', 'status_code'],
  buckets: [0.1, 0.5, 1, 2, 5]
});

const httpRequestTotal = new promClient.Counter({
  name: 'http_requests_total',
  help: 'Total number of HTTP requests',
  labelNames: ['method', 'route', 'status_code']
});

register.registerMetric(httpRequestDuration);
register.registerMetric(httpRequestTotal);

// Metrics middleware
app.use((req, res, next) => {
  const start = Date.now();

  res.on('finish', () => {
    const duration = (Date.now() - start) / 1000;
    const route = req.route ? req.route.path : req.path;

    httpRequestDuration.labels(req.method, route, res.statusCode).observe(duration);
    httpRequestTotal.labels(req.method, route, res.statusCode).inc();
  });

  next();
});

// Metrics endpoint
app.get('/metrics', async (req, res) => {
  res.set('Content-Type', register.contentType);
  res.end(await register.metrics());
});

Distributed Tracing

const { trace, context } = require('@opentelemetry/api');
const { NodeTracerProvider } = require('@opentelemetry/sdk-trace-node');
const { registerInstrumentations } = require('@opentelemetry/instrumentation');
const { HttpInstrumentation } = require('@opentelemetry/instrumentation-http');
const { ExpressInstrumentation } = require('@opentelemetry/instrumentation-express');

// Create tracer provider
const provider = new NodeTracerProvider();
provider.register();

// Register instrumentations
registerInstrumentations({
  instrumentations: [
    new HttpInstrumentation(),
    new ExpressInstrumentation(),
  ],
});

const tracer = trace.getTracer('user-service');

// Custom tracing middleware
const tracingMiddleware = (req, res, next) => {
  const span = tracer.startSpan(`HTTP ${req.method} ${req.path}`);

  span.setAttributes({
    'http.method': req.method,
    'http.url': req.url,
    'http.user_agent': req.get('user-agent'),
  });

  res.on('finish', () => {
    span.setAttributes({
      'http.status_code': res.statusCode,
    });
    span.end();
  });

  next();
};

app.use(tracingMiddleware);

Best Practices

Project Structure

express-microservice/
├── src/
│   ├── config/
│   │   ├── database.js
│   │   ├── redis.js
│   │   └── logger.js
│   ├── controllers/
│   │   ├── users.controller.js
│   │   └── auth.controller.js
│   ├── middleware/
│   │   ├── auth.js
│   │   ├── validation.js
│   │   ├── errorHandler.js
│   │   └── requestLogger.js
│   ├── models/
│   │   └── user.model.js
│   ├── routes/
│   │   ├── index.js
│   │   ├── users.routes.js
│   │   └── auth.routes.js
│   ├── services/
│   │   ├── users.service.js
│   │   ├── auth.service.js
│   │   └── email.service.js
│   ├── utils/
│   │   ├── apiError.js
│   │   ├── catchAsync.js
│   │   └── validators.js
│   ├── app.js
│   └── server.js
├── tests/
│   ├── unit/
│   ├── integration/
│   └── e2e/
├── .env
├── .env.example
├── .dockerignore
├── Dockerfile
├── docker-compose.yml
├── ecosystem.config.js
├── package.json
└── README.md

Environment Configuration

// config/env.js
const dotenv = require('dotenv');
const Joi = require('joi');

dotenv.config();

const envSchema = Joi.object({
  NODE_ENV: Joi.string()
    .valid('development', 'production', 'test')
    .default('development'),
  PORT: Joi.number().default(3000),
  MONGODB_URI: Joi.string().required(),
  REDIS_HOST: Joi.string().required(),
  REDIS_PORT: Joi.number().default(6379),
  JWT_SECRET: Joi.string().required(),
  JWT_EXPIRES_IN: Joi.string().default('7d'),
  LOG_LEVEL: Joi.string()
    .valid('error', 'warn', 'info', 'debug')
    .default('info'),
}).unknown();

const { value: env, error } = envSchema.validate(process.env);

if (error) {
  throw new Error(`Config validation error: ${error.message}`);
}

module.exports = {
  env: env.NODE_ENV,
  port: env.PORT,
  mongodb: {
    uri: env.MONGODB_URI,
  },
  redis: {
    host: env.REDIS_HOST,
    port: env.REDIS_PORT,
  },
  jwt: {
    secret: env.JWT_SECRET,
    expiresIn: env.JWT_EXPIRES_IN,
  },
  logging: {
    level: env.LOG_LEVEL,
  },
};

Error Handling Best Practices

// utils/apiError.js
class ApiError extends Error {
  constructor(statusCode, message, isOperational = true, stack = '') {
    super(message);
    this.statusCode = statusCode;
    this.isOperational = isOperational;
    if (stack) {
      this.stack = stack;
    } else {
      Error.captureStackTrace(this, this.constructor);
    }
  }
}

module.exports = ApiError;

// utils/catchAsync.js
const catchAsync = (fn) => {
  return (req, res, next) => {
    Promise.resolve(fn(req, res, next)).catch(next);
  };
};

module.exports = catchAsync;

// middleware/errorHandler.js
const config = require('../config/env');
const logger = require('../config/logger');
const ApiError = require('../utils/apiError');

const errorConverter = (err, req, res, next) => {
  let error = err;
  if (!(error instanceof ApiError)) {
    const statusCode = error.statusCode || 500;
    const message = error.message || 'Internal Server Error';
    error = new ApiError(statusCode, message, false, err.stack);
  }
  next(error);
};

const errorHandler = (err, req, res, next) => {
  let { statusCode, message } = err;

  if (config.env === 'production' && !err.isOperational) {
    statusCode = 500;
    message = 'Internal Server Error';
  }

  res.locals.errorMessage = err.message;

  const response = {
    code: statusCode,
    message,
    ...(config.env === 'development' && { stack: err.stack }),
  };

  if (config.env === 'development') {
    logger.error(err);
  }

  res.status(statusCode).json(response);
};

module.exports = {
  errorConverter,
  errorHandler,
};

Testing Strategies

// tests/integration/users.test.js
const request = require('supertest');
const app = require('../../src/app');
const { User } = require('../../src/models');

describe('User API', () => {
  beforeEach(async () => {
    await User.deleteMany({});
  });

  describe('POST /api/users', () => {
    it('should create a new user', async () => {
      const userData = {
        name: 'John Doe',
        email: 'john@example.com',
        password: 'password123',
      };

      const res = await request(app)
        .post('/api/users')
        .send(userData)
        .expect(201);

      expect(res.body).toHaveProperty('user');
      expect(res.body.user).toHaveProperty('id');
      expect(res.body.user.email).toBe(userData.email);
      expect(res.body.user).not.toHaveProperty('password');
    });

    it('should return 400 for invalid email', async () => {
      const userData = {
        name: 'John Doe',
        email: 'invalid-email',
        password: 'password123',
      };

      const res = await request(app)
        .post('/api/users')
        .send(userData)
        .expect(400);

      expect(res.body).toHaveProperty('error');
    });
  });

  describe('GET /api/users/:id', () => {
    it('should return user by id', async () => {
      const user = await User.create({
        name: 'John Doe',
        email: 'john@example.com',
        password: 'hashedpassword',
      });

      const res = await request(app)
        .get(`/api/users/${user.id}`)
        .expect(200);

      expect(res.body.user.id).toBe(user.id);
    });

    it('should return 404 for non-existent user', async () => {
      await request(app)
        .get('/api/users/507f1f77bcf86cd799439011')
        .expect(404);
    });
  });
});

Performance Optimization

// Enable gzip compression
const compression = require('compression');
app.use(compression());

// Use efficient JSON parsing
app.use(express.json({ limit: '10mb' }));

// Database query optimization
const getUsers = async (filters) => {
  return User.find(filters)
    .select('name email role') // Select only needed fields
    .lean() // Return plain objects instead of Mongoose documents
    .limit(100);
};

// Implement pagination
const paginateResults = async (model, page = 1, limit = 10) => {
  const skip = (page - 1) * limit;

  const [results, total] = await Promise.all([
    model.find().skip(skip).limit(limit).lean(),
    model.countDocuments(),
  ]);

  return {
    results,
    pagination: {
      page,
      limit,
      total,
      pages: Math.ceil(total / limit),
    },
  };
};

// Use indexes
userSchema.index({ email: 1 });
userSchema.index({ role: 1, createdAt: -1 });

// Connection pooling and keep-alive
const agent = new http.Agent({
  keepAlive: true,
  maxSockets: 50,
});

Security Best Practices

// Validate and sanitize inputs
const { body } = require('express-validator');

const userValidation = [
  body('email').isEmail().normalizeEmail(),
  body('password').isLength({ min: 8 }).trim(),
  body('name').trim().escape(),
];

// Implement rate limiting
const loginLimiter = rateLimit({
  windowMs: 15 * 60 * 1000,
  max: 5,
  skipSuccessfulRequests: true,
});

app.post('/api/auth/login', loginLimiter, loginHandler);

// Use parameterized queries
const getUserByEmail = async (email) => {
  return User.findOne({ email }); // Protected against NoSQL injection
};

// Implement CSRF protection
const csrf = require('csurf');
app.use(csrf({ cookie: true }));

// Set secure cookies
res.cookie('token', token, {
  httpOnly: true,
  secure: process.env.NODE_ENV === 'production',
  sameSite: 'strict',
  maxAge: 7 * 24 * 60 * 60 * 1000, // 7 days
});

// Hash passwords
const bcrypt = require('bcrypt');
const hashPassword = async (password) => {
  return bcrypt.hash(password, 12);
};

Examples

Example 1: Basic Express Microservice

const express = require('express');
const helmet = require('helmet');
const cors = require('cors');
const morgan = require('morgan');

const app = express();

// Middleware
app.use(helmet());
app.use(cors());
app.use(morgan('combined'));
app.use(express.json());

// Routes
app.get('/health', (req, res) => {
  res.json({ status: 'healthy' });
});

app.get('/api/users', (req, res) => {
  res.json({ users: [] });
});

// Error handling
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ error: 'Something went wrong!' });
});

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

Example 2: Authentication Service

const express = require('express');
const jwt = require('jsonwebtoken');
const bcrypt = require('bcrypt');
const { body, validationResult } = require('express-validator');

const router = express.Router();

// Register
router.post('/register',
  body('email').isEmail(),
  body('password').isLength({ min: 8 }),
  

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