Backend Service Patterns - Building Scalable Server Applications

When to use this skill

• Building API services and backend applications
• Implementing business logic in service layers
• Creating controller-service-repository architectures
• Setting up Express/Fastify middleware chains
• Implementing dependency injection containers
• Designing domain models and business entities
• Handling cross-cutting concerns (logging, auth, validation)
• Implementing CQRS (Command Query Responsibility Segregation)
• Creating background job processing services
• Designing event-driven service architectures
• Implementing API gateway patterns
• Building microservices with clear boundaries

When to use this skill

• Designing backend architecture, implementing APIs, handling business logic, managing microservices, or building serverless functions.
• When working on related tasks or features
• During development that requires this expertise

Use when: Designing backend architecture, implementing APIs, handling business logic, managing microservices, or building serverless functions.

Core Principles

1. Separation of Concerns - Clear boundaries between layers
2. Single Responsibility - Each service does one thing well
3. Stateless Design - Easy to scale horizontally
4. Idempotency - Safe to retry operations
5. Fail Fast, Recover Gracefully - Handle errors explicitly

Layered Architecture

1. Three-Tier Architecture

// ✅ Routes Layer - HTTP endpoints
// routes/users.ts
import express from 'express';
import { UserController } from '../controllers/UserController';

const router = express.Router();
const userController = new UserController();

router.get('/users', userController.list);
router.post('/users', userController.create);
router.get('/users/:id', userController.get);

export default router;

// ✅ Controller Layer - Request/response handling
// controllers/UserController.ts
import { Request, Response } from 'express';
import { UserService } from '../services/UserService';
import { CreateUserSchema } from '../schemas/user';

export class UserController {
  private userService = new UserService();

  list = async (req: Request, res: Response) => {
    try {
      const users = await this.userService.listUsers({
        page: parseInt(req.query.page as string) || 1,
        limit: parseInt(req.query.limit as string) || 20
      });
      res.json(users);
    } catch (error) {
      res.status(500).json({ error: 'Failed to fetch users' });
    }
  };

  create = async (req: Request, res: Response) => {
    try {
      const validated = CreateUserSchema.parse(req.body);
      const user = await this.userService.createUser(validated);
      res.status(201).json(user);
    } catch (error) {
      res.status(400).json({ error: error.message });
    }
  };
}

// ✅ Service Layer - Business logic
// services/UserService.ts
import { UserRepository } from '../repositories/UserRepository';
import { EmailService } from './EmailService';

export class UserService {
  private userRepo = new UserRepository();
  private emailService = new EmailService();

  async createUser(data: CreateUserInput) {
    // Business logic here
    const existingUser = await this.userRepo.findByEmail(data.email);
    if (existingUser) {
      throw new Error('Email already exists');
    }

    const hashedPassword = await bcrypt.hash(data.password, 10);
    const user = await this.userRepo.create({
      ...data,
      passwordHash: hashedPassword
    });

    // Send welcome email asynchronously
    await this.emailService.sendWelcome(user.email);

    return user;
  }

  async listUsers(params: { page: number; limit: number }) {
    return await this.userRepo.findMany({
      skip: (params.page - 1) * params.limit,
      take: params.limit
    });
  }
}

// ✅ Repository Layer - Data access
// repositories/UserRepository.ts
import { prisma } from '../db';

export class UserRepository {
  async create(data: CreateUserData) {
    return await prisma.user.create({ data });
  }

  async findByEmail(email: string) {
    return await prisma.user.findUnique({ where: { email } });
  }

  async findMany(params: { skip: number; take: number }) {
    return await prisma.user.findMany(params);
  }
}

2. Dependency Injection

// ✅ Constructor injection for testability
export class OrderService {
  constructor(
    private orderRepo: OrderRepository,
    private paymentService: PaymentService,
    private emailService: EmailService
  ) {}

  async processOrder(orderId: string) {
    const order = await this.orderRepo.findById(orderId);
    const payment = await this.paymentService.charge(order);
    await this.emailService.sendReceipt(order, payment);
    return payment;
  }
}

// Easy to mock in tests
const mockOrderRepo = {
  findById: jest.fn().mockResolvedValue({ id: '1', total: 100 })
};
const mockPaymentService = {
  charge: jest.fn().mockResolvedValue({ id: 'pay_123' })
};
const service = new OrderService(mockOrderRepo, mockPaymentService, mockEmailService);

Design Patterns

1. Repository Pattern

// ✅ Abstract data access behind interface
interface IUserRepository {
  create(data: CreateUserData): Promise<User>;
  findById(id: string): Promise<User | null>;
  update(id: string, data: UpdateUserData): Promise<User>;
  delete(id: string): Promise<void>;
}

// PostgreSQL implementation
export class PostgresUserRepository implements IUserRepository {
  async create(data: CreateUserData) {
    return await prisma.user.create({ data });
  }
  
  async findById(id: string) {
    return await prisma.user.findUnique({ where: { id } });
  }
  
  // ... other methods
}

// MongoDB/Cosmos DB implementation (easy to swap)
export class MongoUserRepository implements IUserRepository {
  async create(data: CreateUserData) {
    return await User.create(data);
  }
  
  // ... other methods
}

2. Service Layer Pattern

// ✅ Encapsulate business logic
export class OrderService {
  async placeOrder(userId: string, items: OrderItem[]) {
    // Validate inventory
    for (const item of items) {
      const product = await this.productRepo.findById(item.productId);
      if (product.stock < item.quantity) {
        throw new InsufficientStockError(item.productId);
      }
    }

    // Calculate total
    const total = items.reduce((sum, item) => sum + item.price * item.quantity, 0);

    // Apply discount
    const discount = await this.discountService.calculateDiscount(userId, total);

    // Create order
    const order = await this.orderRepo.create({
      userId,
      items,
      subtotal: total,
      discount,
      total: total - discount,
      status: 'pending'
    });

    // Reserve inventory
    for (const item of items) {
      await this.inventoryService.reserve(item.productId, item.quantity);
    }

    // Queue payment processing
    await this.paymentQueue.add({ orderId: order.id });

    return order;
  }
}

3. Factory Pattern

// ✅ Create objects based on conditions
interface PaymentProvider {
  charge(amount: number, currency: string): Promise<PaymentResult>;
}

class StripeProvider implements PaymentProvider {
  async charge(amount: number, currency: string) {
    return await stripe.charges.create({ amount, currency });
  }
}

class PayPalProvider implements PaymentProvider {
  async charge(amount: number, currency: string) {
    return await paypal.payment.create({ amount, currency });
  }
}

class PaymentProviderFactory {
  static create(provider: string): PaymentProvider {
    switch (provider) {
      case 'stripe':
        return new StripeProvider();
      case 'paypal':
        return new PayPalProvider();
      default:
        throw new Error(`Unknown provider: ${provider}`);
    }
  }
}

// Usage
const provider = PaymentProviderFactory.create(user.preferredPaymentMethod);
await provider.charge(100, 'USD');

4. Strategy Pattern

// ✅ Swap algorithms at runtime
interface ShippingStrategy {
  calculateCost(weight: number, distance: number): number;
}

class StandardShipping implements ShippingStrategy {
  calculateCost(weight: number, distance: number) {
    return weight * 0.5 + distance * 0.1;
  }
}

class ExpressShipping implements ShippingStrategy {
  calculateCost(weight: number, distance: number) {
    return weight * 1.0 + distance * 0.3;
  }
}

class OvernightShipping implements ShippingStrategy {
  calculateCost(weight: number, distance: number) {
    return weight * 2.0 + distance * 0.5;
  }
}

class ShippingCalculator {
  constructor(private strategy: ShippingStrategy) {}

  calculate(weight: number, distance: number) {
    return this.strategy.calculateCost(weight, distance);
  }
}

// Usage
const calculator = new ShippingCalculator(new ExpressShipping());
const cost = calculator.calculate(10, 500);

5. Observer Pattern (Event-Driven)

// ✅ Decouple components with events
import { EventEmitter } from 'events';

class OrderEvents extends EventEmitter {
  static CREATED = 'order.created';
  static PAID = 'order.paid';
  static SHIPPED = 'order.shipped';
  static CANCELLED = 'order.cancelled';
}

const orderEvents = new OrderEvents();

// Listeners
orderEvents.on(OrderEvents.CREATED, async (order) => {
  await emailService.sendOrderConfirmation(order);
  await analyticsService.trackOrderCreated(order);
});

orderEvents.on(OrderEvents.PAID, async (order) => {
  await inventoryService.deductStock(order.items);
  await shippingService.createShipment(order);
});

// Emit events
export class OrderService {
  async createOrder(data: CreateOrderData) {
    const order = await this.orderRepo.create(data);
    orderEvents.emit(OrderEvents.CREATED, order);
    return order;
  }

  async markAsPaid(orderId: string) {
    const order = await this.orderRepo.update(orderId, { status: 'paid' });
    orderEvents.emit(OrderEvents.PAID, order);
    return order;
  }
}

Microservices Patterns

1. API Gateway

// ✅ Single entry point for multiple services
import express from 'express';
import { createProxyMiddleware } from 'http-proxy-middleware';

const app = express();

// Route to different services
app.use('/api/users', createProxyMiddleware({ 
  target: 'http://user-service:3001',
  changeOrigin: true 
}));

app.use('/api/orders', createProxyMiddleware({ 
  target: 'http://order-service:3002',
  changeOrigin: true 
}));

app.use('/api/payments', createProxyMiddleware({ 
  target: 'http://payment-service:3003',
  changeOrigin: true 
}));

// Add authentication, rate limiting, logging here
app.use(authMiddleware);
app.use(rateLimiter);
app.use(logger);

2. Service-to-Service Communication

// ✅ HTTP/REST communication
export class UserService {
  private orderServiceUrl = process.env.ORDER_SERVICE_URL;

  async getUserWithOrders(userId: string) {
    const user = await this.userRepo.findById(userId);
    
    // Call order service
    const response = await fetch(`${this.orderServiceUrl}/orders?userId=${userId}`);
    const orders = await response.json();

    return { ...user, orders };
  }
}

// ✅ Message queue communication (async)
import { Queue } from 'bullmq';

const emailQueue = new Queue('emails', {
  connection: { host: 'redis', port: 6379 }
});

// Producer (User Service)
export class UserService {
  async createUser(data: CreateUserData) {
    const user = await this.userRepo.create(data);
    
    // Async notification
    await emailQueue.add('welcome', {
      userId: user.id,
      email: user.email
    });

    return user;
  }
}

// Consumer (Email Service)
import { Worker } from 'bullmq';

const worker = new Worker('emails', async (job) => {
  if (job.name === 'welcome') {
    await sendWelcomeEmail(job.data.email);
  }
});

3. Circuit Breaker Pattern

// ✅ Prevent cascading failures
import CircuitBreaker from 'opossum';

const options = {
  timeout: 3000,        // If function takes > 3s, trigger failure
  errorThresholdPercentage: 50,  // When 50% of requests fail...
  resetTimeout: 30000   // ...open circuit for 30s
};

const breaker = new CircuitBreaker(callExternalAPI, options);

breaker.fallback(() => {
  return { data: null, fromCache: true }; // Fallback response
});

breaker.on('open', () => {
  console.log('Circuit breaker opened - too many failures');
});

async function callExternalAPI() {
  const response = await fetch('https://external-api.com/data');
  return response.json();
}

// Usage
try {
  const data = await breaker.fire();
} catch (error) {
  // Circuit is open or request failed
}

Serverless Patterns

1. Function-as-a-Service

// ✅ AWS Lambda handler
import { APIGatewayProxyHandler } from 'aws-lambda';

export const handler: APIGatewayProxyHandler = async (event) => {
  try {
    const body = JSON.parse(event.body || '{}');
    
    // Business logic
    const result = await processOrder(body);

    return {
      statusCode: 200,
      headers: {
        'Content-Type': 'application/json',
        'Access-Control-Allow-Origin': '*'
      },
      body: JSON.stringify(result)
    };
  } catch (error) {
    return {
      statusCode: 500,
      body: JSON.stringify({ error: error.message })
    };
  }
};

// ✅ Vercel Edge Function
export const config = {
  runtime: 'edge'
};

export default async function handler(request: Request) {
  const data = await processData();
  
  return new Response(JSON.stringify(data), {
    headers: { 'content-type': 'application/json' }
  });
}

2. Cold Start Optimization

// ✅ Initialize outside handler (persists across invocations)
import { PrismaClient } from '@prisma/client';

// Created once, reused across warm invocations
const prisma = new PrismaClient();

export const handler = async (event) => {
  // This runs on every invocation
  const users = await prisma.user.findMany();
  return { statusCode: 200, body: JSON.stringify(users) };
};

// ✅ Lazy initialization
let connection: Connection | null = null;

async function getConnection() {
  if (!connection) {
    connection = await createConnection();
  }
  return connection;
}

export const handler = async (event) => {
  const conn = await getConnection();
  // Use connection
};

Background Jobs

1. Job Queue Pattern

// ✅ BullMQ job processing
import { Queue, Worker } from 'bullmq';

const videoQueue = new Queue('video-processing', {
  connection: { host: 'redis', port: 6379 }
});

// Add jobs
export class VideoService {
  async uploadVideo(file: File) {
    const video = await this.videoRepo.create({ 
      filename: file.name,
      status: 'pending'
    });

    // Queue processing job
    await videoQueue.add('transcode', {
      videoId: video.id,
      inputPath: file.path
    }, {
      attempts: 3,           // Retry up to 3 times
      backoff: {
        type: 'exponential',
        delay: 5000          // 5s, 10s, 20s
      }
    });

    return video;
  }
}

// Worker process
const worker = new Worker('video-processing', async (job) => {
  if (job.name === 'transcode') {
    await transcodeVideo(job.data.videoId, job.data.inputPath);
    
    // Update progress
    job.updateProgress(50);
    
    await generateThumbnail(job.data.videoId);
    
    job.updateProgress(100);
  }
}, {
  connection: { host: 'redis', port: 6379 },
  concurrency: 5  // Process 5 jobs concurrently
});

worker.on('completed', (job) => {
  console.log(`Job ${job.id} completed`);
});

worker.on('failed', (job, err) => {
  console.error(`Job ${job.id} failed:`, err);
});

2. Scheduled Tasks

// ✅ Cron jobs with node-cron
import cron from 'node-cron';

// Run every day at midnight
cron.schedule('0 0 * * *', async () => {
  await cleanupExpiredSessions();
  await generateDailyReport();
});

// Run every 5 minutes
cron.schedule('*/5 * * * *', async () => {
  await checkPaymentStatus();
});

// ✅ Or use BullMQ repeat
await queue.add('daily-report', {}, {
  repeat: {
    pattern: '0 0 * * *'  // Cron expression
  }
});

Backend Best Practices

1. Configuration Management

// ✅ Environment-based config
export const config = {
  port: parseInt(process.env.PORT || '3000'),
  database: {
    url: process.env.DATABASE_URL,
    poolSize: parseInt(process.env.DB_POOL_SIZE || '10')
  },
  redis: {
    host: process.env.REDIS_HOST || 'localhost',
    port: parseInt(process.env.REDIS_PORT || '6379')
  },
  jwt: {
    secret: process.env.JWT_SECRET,
    expiresIn: '1h'
  }
};

// Validate required vars on startup
const required = ['DATABASE_URL', 'JWT_SECRET'];
for (const key of required) {
  if (!process.env[key]) {
    throw new Error(`Missing required environment variable: ${key}`);
  }
}

2. Health Checks

// ✅ Health check endpoint
app.get('/health', async (req, res) => {
  const health = {
    uptime: process.uptime(),
    timestamp: Date.now(),
    status: 'ok',
    checks: {
      database: 'ok',
      redis: 'ok'
    }
  };

  try {
    await prisma.$queryRaw`SELECT 1`;
  } catch (error) {
    health.status = 'degraded';
    health.checks.database = 'error';
  }

  try {
    await redis.ping();
  } catch (error) {
    health.status = 'degraded';
    health.checks.redis = 'error';
  }

  const statusCode = health.status === 'ok' ? 200 : 503;
  res.status(statusCode).json(health);
});

Backend Service Checklist

Architecture:
□ Clear separation of concerns (routes/controllers/services/repos)
□ Dependency injection for testability
□ Business logic in service layer (not controllers)
□ Data access abstracted in repositories
□ No database queries in controllers

API Design:
□ RESTful endpoints
□ Proper HTTP methods and status codes
□ Input validation on all endpoints
□ Pagination for lists
□ API versioning strategy

Error Handling:
□ Try/catch in all async functions
□ Meaningful error messages
□ Proper error status codes
□ Error logging with context
□ No sensitive data in error responses

Performance:
□ Database queries optimized
□ Connection pooling configured
□ Caching strategy implemented
□ Background jobs for heavy operations
□ Rate limiting on public endpoints

Security:
□ Authentication required
□ Authorization checks
□ Input sanitization
□ SQL injection prevention
□ CORS configured properly

Monitoring:
□ Health check endpoint
□ Logging configured
□ Metrics collected
□ Error tracking (Sentry, etc.)
□ Performance monitoring

Resources

• Node.js Best Practices
• Microservices Patterns
• Clean Architecture

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Remember: Great backend services are maintainable, testable, scalable, and resilient. Design for failure, optimize for readability.