New TypeScript service

Guide to implementing microservices using TypeScript at FlowMart

This guide details the recommended patterns, practices, and tools for implementing TypeScript-based microservices at FlowMart.

Why TypeScript?

At FlowMart, we recommend TypeScript for new microservices because it offers:

  • Type Safety: Catch errors during development instead of at runtime
  • Better IDE Support: Enhanced auto-completion, navigation, and refactoring
  • Self-Documenting Code: Types serve as documentation for your codebase
  • Enterprise-Ready: Better maintainability for large codebases and teams
  • Ecosystem Compatibility: Full access to the Node.js ecosystem

Prerequisites

Before you begin:

Scaffolding a TypeScript Service

Use our service generator to create a TypeScript service:

# Install the FlowMart service generator
npm install -g @flowmart/service-generator

# Create a new TypeScript service
flowmart-create-service my-service --type typescript

TypeScript Project Structure

The generated service follows our standard TypeScript structure:

my-service/
├── src/
│   ├── api/                 # API definition and controllers
│   ├── config/              # Configuration management
│   ├── domain/              # Domain models and business logic
│   │   ├── models/          # Domain entities and value objects
│   │   └── services/        # Domain services
│   ├── events/              # Event producers and consumers
│   ├── infrastructure/      # External dependencies and adapters
│   ├── repositories/        # Data access layer
│   ├── types/               # TypeScript type definitions
│   ├── utils/               # Utility functions
│   └── app.ts               # Application entry point
├── test/
│   ├── unit/                # Unit tests
│   ├── integration/         # Integration tests
│   └── contract/            # Contract tests
├── terraform/               # Infrastructure as code
├── .github/                 # GitHub Actions workflows
├── Dockerfile               # Container definition
├── docker-compose.yml       # Local development setup
├── tsconfig.json            # TypeScript configuration
├── package.json             # Dependencies and scripts
└── README.md                # Service documentation

TypeScript Configuration

Our template includes a pre-configured tsconfig.json:

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "lib": ["ES2020"],
    "outDir": "dist",
    "rootDir": "src",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "resolveJsonModule": true,
    "sourceMap": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "test"]
}

Domain Modeling with TypeScript

TypeScript enables us to model our domain with strong types:

Example Domain Model

// src/domain/models/product.ts
export enum ProductCategory {
  ELECTRONICS = 'electronics',
  CLOTHING = 'clothing',
  GROCERY = 'grocery',
  HOME = 'home',
  BEAUTY = 'beauty'
}

export interface ProductAttributes {
  [key: string]: string | number | boolean;
}

export class Product {
  constructor(
    public readonly id: string,
    public name: string,
    public summary: string,
    public price: number,
    public category: ProductCategory,
    public sku: string,
    public inventoryCount: number,
    public attributes: ProductAttributes = {},
    public isActive: boolean = true,
    public createdAt: Date = new Date(),
    public updatedAt: Date = new Date()
  ) {}

  updateInventory(count: number): void {
    if (count < 0) {
      throw new Error('Inventory count cannot be negative');
    }
    this.inventoryCount = count;
    this.updatedAt = new Date();
  }

  updatePrice(price: number): void {
    if (price < 0) {
      throw new Error('Price cannot be negative');
    }
    this.price = price;
    this.updatedAt = new Date();
  }

  deactivate(): void {
    this.isActive = false;
    this.updatedAt = new Date();
  }

  activate(): void {
    this.isActive = true;
    this.updatedAt = new Date();
  }

  isInStock(): boolean {
    return this.inventoryCount > 0;
  }
}

Repository Pattern with TypeScript

Use interfaces to define repositories:

// src/repositories/product-repository.ts
import { Product } from '../domain/models/product';

export interface ProductRepository {
  findById(id: string): Promise<Product | null>;
  findAll(limit?: number, offset?: number): Promise<Product[]>;
  findByCategory(category: string, limit?: number, offset?: number): Promise<Product[]>;
  save(product: Product): Promise<Product>;
  update(product: Product): Promise<Product>;
  delete(id: string): Promise<boolean>;
}

// src/repositories/mongodb-product-repository.ts
import { Collection, MongoClient, ObjectId } from 'mongodb';
import { Product, ProductCategory } from '../domain/models/product';
import { ProductRepository } from './product-repository';
import { logger } from '../infrastructure/observability';

export class MongoDBProductRepository implements ProductRepository {
  private collection: Collection;

  constructor(client: MongoClient) {
    this.collection = client.db('ecommerce').collection('products');
  }

  async findById(id: string): Promise<Product | null> {
    try {
      const result = await this.collection.findOne({ _id: new ObjectId(id) });
      if (!result) return null;
      return this.mapToProduct(result);
    } catch (error) {
      logger.error('Error finding product by id', { error, id });
      throw error;
    }
  }

  async findAll(limit = 100, offset = 0): Promise<Product[]> {
    try {
      const results = await this.collection
        .find({})
        .skip(offset)
        .limit(limit)
        .toArray();
      return results.map(this.mapToProduct);
    } catch (error) {
      logger.error('Error finding all products', { error });
      throw error;
    }
  }

  async findByCategory(category: string, limit = 100, offset = 0): Promise<Product[]> {
    try {
      const results = await this.collection
        .find({ category })
        .skip(offset)
        .limit(limit)
        .toArray();
      return results.map(this.mapToProduct);
    } catch (error) {
      logger.error('Error finding products by category', { error, category });
      throw error;
    }
  }

  async save(product: Product): Promise<Product> {
    try {
      const productDoc = {
        name: product.name,
        summary: product.description,
        price: product.price,
        category: product.category,
        sku: product.sku,
        inventoryCount: product.inventoryCount,
        attributes: product.attributes,
        isActive: product.isActive,
        createdAt: product.createdAt,
        updatedAt: product.updatedAt
      };
      
      const result = await this.collection.insertOne(productDoc);
      return {
        ...product,
        id: result.insertedId.toString()
      };
    } catch (error) {
      logger.error('Error saving product', { error, product });
      throw error;
    }
  }

  async update(product: Product): Promise<Product> {
    try {
      const result = await this.collection.updateOne(
        { _id: new ObjectId(product.id) },
        {
          $set: {
            name: product.name,
            summary: product.description,
            price: product.price,
            category: product.category,
            sku: product.sku,
            inventoryCount: product.inventoryCount,
            attributes: product.attributes,
            isActive: product.isActive,
            updatedAt: product.updatedAt
          }
        }
      );
      
      if (result.matchedCount === 0) {
        throw new Error(`Product with id ${product.id} not found`);
      }
      
      return product;
    } catch (error) {
      logger.error('Error updating product', { error, productId: product.id });
      throw error;
    }
  }

  async delete(id: string): Promise<boolean> {
    try {
      const result = await this.collection.deleteOne({ _id: new ObjectId(id) });
      return result.deletedCount === 1;
    } catch (error) {
      logger.error('Error deleting product', { error, id });
      throw error;
    }
  }

  private mapToProduct(doc: any): Product {
    return new Product(
      doc._id.toString(),
      doc.name,
      doc.description,
      doc.price,
      doc.category as ProductCategory,
      doc.sku,
      doc.inventoryCount,
      doc.attributes,
      doc.isActive,
      new Date(doc.createdAt),
      new Date(doc.updatedAt)
    );
  }
}

Type-safe API Controllers

TypeScript enables type-safe API controllers with Express:

// src/api/controllers/product-controller.ts
import { Router, Request, Response, NextFunction } from 'express';
import { ProductService } from '../../domain/services/product-service';
import { validateProduct } from '../middleware/product-validator';
import { tracing } from '../../infrastructure/observability';
import { Product, ProductCategory } from '../../domain/models/product';

export interface CreateProductRequest {
  name: string;
  summary: string;
  price: number;
  category: ProductCategory;
  sku: string;
  inventoryCount: number;
  attributes?: { [key: string]: string | number | boolean };
}

export interface UpdateProductRequest {
  name?: string;
  description?: string;
  price?: number;
  category?: ProductCategory;
  sku?: string;
  inventoryCount?: number;
  attributes?: { [key: string]: string | number | boolean };
  isActive?: boolean;
}

export class ProductController {
  private router: Router;
  
  constructor(private productService: ProductService) {
    this.router = Router();
    this.setupRoutes();
  }
  
  private setupRoutes(): void {
    this.router.get('/', tracing.middleware('get-all-products'), this.getAllProducts.bind(this));
    this.router.get('/:id', tracing.middleware('get-product'), this.getProductById.bind(this));
    this.router.post('/', tracing.middleware('create-product'), validateProduct, this.createProduct.bind(this));
    this.router.put('/:id', tracing.middleware('update-product'), this.updateProduct.bind(this));
    this.router.delete('/:id', tracing.middleware('delete-product'), this.deleteProduct.bind(this));
  }
  
  getRouter(): Router {
    return this.router;
  }
  
  private async getAllProducts(req: Request, res: Response, next: NextFunction): Promise<void> {
    try {
      const limit = req.query.limit ? parseInt(req.query.limit as string, 10) : 100;
      const offset = req.query.offset ? parseInt(req.query.offset as string, 10) : 0;
      const products = await this.productService.getAllProducts(limit, offset);
      res.json(products);
    } catch (error) {
      next(error);
    }
  }
  
  private async getProductById(req: Request, res: Response, next: NextFunction): Promise<void> {
    try {
      const product = await this.productService.getProductById(req.params.id);
      if (!product) {
        res.status(404).json({ error: 'Product not found' });
        return;
      }
      res.json(product);
    } catch (error) {
      next(error);
    }
  }
  
  private async createProduct(req: Request, res: Response, next: NextFunction): Promise<void> {
    try {
      const productData = req.body as CreateProductRequest;
      const product = await this.productService.createProduct(productData);
      res.status(201).json(product);
    } catch (error) {
      next(error);
    }
  }
  
  private async updateProduct(req: Request, res: Response, next: NextFunction): Promise<void> {
    try {
      const productData = req.body as UpdateProductRequest;
      const product = await this.productService.updateProduct(req.params.id, productData);
      if (!product) {
        res.status(404).json({ error: 'Product not found' });
        return;
      }
      res.json(product);
    } catch (error) {
      next(error);
    }
  }
  
  private async deleteProduct(req: Request, res: Response, next: NextFunction): Promise<void> {
    try {
      const success = await this.productService.deleteProduct(req.params.id);
      if (!success) {
        res.status(404).json({ error: 'Product not found' });
        return;
      }
      res.status(204).send();
    } catch (error) {
      next(error);
    }
  }
}

Strongly Typed Event Handling

Create strongly typed event producers and consumers:

// src/events/types/event-types.ts
export interface EventMetadata {
  eventId: string;
  timestamp: string;
  service: string;
  correlationId?: string;
  causationId?: string;
}

export interface Event<T> {
  type: string;
  data: T;
  metadata: EventMetadata;
}

export interface ProductCreatedEvent extends Event<{
  id: string;
  name: string;
  price: number;
  category: string;
  sku: string;
  inventoryCount: number;
}> {
  type: 'PRODUCT_CREATED';
}

export interface ProductUpdatedEvent extends Event<{
  id: string;
  changes: {
    name?: string;
    price?: number;
    category?: string;
    inventoryCount?: number;
  };
}> {
  type: 'PRODUCT_UPDATED';
}

export interface InventoryUpdatedEvent extends Event<{
  productId: string;
  quantity: number;
  warehouseId: string;
}> {
  type: 'INVENTORY_UPDATED';
}

// src/events/producers/product-event-producer.ts
import { v4 as uuidv4 } from 'uuid';
import { KafkaClient } from '../../infrastructure/kafka';
import { Product } from '../../domain/models/product';
import { ProductCreatedEvent, ProductUpdatedEvent } from '../types/event-types';

export class ProductEventProducer {
  private readonly topic = 'product-events';
  
  constructor(private kafkaClient: KafkaClient) {}

  async productCreated(product: Product): Promise<void> {
    const event: ProductCreatedEvent = {
      type: 'PRODUCT_CREATED',
      data: {
        id: product.id,
        name: product.name,
        price: product.price,
        category: product.category,
        sku: product.sku,
        inventoryCount: product.inventoryCount
      },
      metadata: {
        eventId: uuidv4(),
        timestamp: new Date().toISOString(),
        service: 'product-service'
      }
    };

    await this.kafkaClient.produce({
      topic: this.topic,
      key: product.id,
      value: JSON.stringify(event)
    });
  }

  async productUpdated(product: Product, changes: Partial<Product>): Promise<void> {
    const event: ProductUpdatedEvent = {
      type: 'PRODUCT_UPDATED',
      data: {
        id: product.id,
        changes: {
          name: changes.name,
          price: changes.price,
          category: changes.category,
          inventoryCount: changes.inventoryCount
        }
      },
      metadata: {
        eventId: uuidv4(),
        timestamp: new Date().toISOString(),
        service: 'product-service'
      }
    };

    await this.kafkaClient.produce({
      topic: this.topic,
      key: product.id,
      value: JSON.stringify(event)
    });
  }
}

Dependency Injection with TypeScript

We use the inversify library for dependency injection:

// src/infrastructure/ioc/container.ts
import { Container } from 'inversify';
import { MongoClient } from 'mongodb';
import { KafkaClient } from '../kafka';
import { ProductRepository } from '../../repositories/product-repository';
import { MongoDBProductRepository } from '../../repositories/mongodb-product-repository';
import { ProductService } from '../../domain/services/product-service';
import { ProductEventProducer } from '../../events/producers/product-event-producer';
import { ProductController } from '../../api/controllers/product-controller';
import { AppConfig } from '../../config/app-config';
import TYPES from './types';

const container = new Container();

// Config
container.bind<AppConfig>(TYPES.AppConfig).to(AppConfig).inSingletonScope();

// Infrastructure
container.bind<MongoClient>(TYPES.MongoClient).toDynamicValue((context) => {
  const config = context.container.get<AppConfig>(TYPES.AppConfig);
  return new MongoClient(config.mongoDbUri);
}).inSingletonScope();

container.bind<KafkaClient>(TYPES.KafkaClient).toDynamicValue((context) => {
  const config = context.container.get<AppConfig>(TYPES.AppConfig);
  return new KafkaClient(config.kafkaBrokers);
}).inSingletonScope();

// Repositories
container.bind<ProductRepository>(TYPES.ProductRepository).toDynamicValue((context) => {
  const mongoClient = context.container.get<MongoClient>(TYPES.MongoClient);
  return new MongoDBProductRepository(mongoClient);
}).inSingletonScope();

// Event Producers
container.bind<ProductEventProducer>(TYPES.ProductEventProducer).toDynamicValue((context) => {
  const kafkaClient = context.container.get<KafkaClient>(TYPES.KafkaClient);
  return new ProductEventProducer(kafkaClient);
}).inSingletonScope();

// Services
container.bind<ProductService>(TYPES.ProductService).toDynamicValue((context) => {
  const repository = context.container.get<ProductRepository>(TYPES.ProductRepository);
  const eventProducer = context.container.get<ProductEventProducer>(TYPES.ProductEventProducer);
  return new ProductService(repository, eventProducer);
}).inSingletonScope();

// Controllers
container.bind<ProductController>(TYPES.ProductController).toDynamicValue((context) => {
  const service = context.container.get<ProductService>(TYPES.ProductService);
  return new ProductController(service);
}).inSingletonScope();

export default container;

// src/infrastructure/ioc/types.ts
const TYPES = {
  AppConfig: Symbol.for('AppConfig'),
  MongoClient: Symbol.for('MongoClient'),
  KafkaClient: Symbol.for('KafkaClient'),
  ProductRepository: Symbol.for('ProductRepository'),
  ProductEventProducer: Symbol.for('ProductEventProducer'),
  ProductService: Symbol.for('ProductService'),
  ProductController: Symbol.for('ProductController')
};

export default TYPES;

Error Handling with TypeScript

Use custom error classes:

// src/utils/errors.ts
export class AppError extends Error {
  constructor(
    public readonly message: string,
    public readonly statusCode: number = 500,
    public readonly code: string = 'INTERNAL_ERROR',
    public readonly details?: any
  ) {
    super(message);
    this.name = this.constructor.name;
    Error.captureStackTrace(this, this.constructor);
  }
}

export class NotFoundError extends AppError {
  constructor(resource: string, id: string) {
    super(`${resource} with id ${id} not found`, 404, 'NOT_FOUND');
    this.name = this.constructor.name;
  }
}

export class ValidationError extends AppError {
  constructor(message: string, details?: any) {
    super(message, 400, 'VALIDATION_ERROR', details);
    this.name = this.constructor.name;
  }
}

export class ConflictError extends AppError {
  constructor(message: string) {
    super(message, 409, 'CONFLICT_ERROR');
    this.name = this.constructor.name;
  }
}

export class AuthorizationError extends AppError {
  constructor(message: string = 'Unauthorized') {
    super(message, 401, 'UNAUTHORIZED');
    this.name = this.constructor.name;
  }
}

// src/api/middleware/error-handler.ts
import { Request, Response, NextFunction } from 'express';
import { AppError } from '../../utils/errors';
import { logger } from '../../infrastructure/observability';

export function errorHandler(
  error: Error, 
  req: Request, 
  res: Response, 
  next: NextFunction
): void {
  logger.error('Request error', {
    error: error.message,
    stack: error.stack,
    path: req.path,
    method: req.method
  });

  if (error instanceof AppError) {
    res.status(error.statusCode).json({
      error: {
        code: error.code,
        message: error.message,
        details: error.details
      }
    });
    return;
  }

  res.status(500).json({
    error: {
      code: 'INTERNAL_SERVER_ERROR',
      message: 'An unexpected error occurred'
    }
  });
}

Testing TypeScript Services

We use Jest for testing TypeScript services:

// test/unit/domain/services/product-service.test.ts
import { ProductService } from '../../../../src/domain/services/product-service';
import { Product, ProductCategory } from '../../../../src/domain/models/product';
import { NotFoundError } from '../../../../src/utils/errors';

describe('ProductService', () => {
  const mockProduct = new Product(
    '1',
    'Test Product',
    'Test Description',
    9.99,
    ProductCategory.ELECTRONICS,
    'TEST-123',
    100
  );

  const mockRepository = {
    findById: jest.fn(),
    findAll: jest.fn(),
    findByCategory: jest.fn(),
    save: jest.fn(),
    update: jest.fn(),
    delete: jest.fn()
  };

  const mockEventProducer = {
    productCreated: jest.fn(),
    productUpdated: jest.fn()
  };

  const productService = new ProductService(mockRepository, mockEventProducer);

  beforeEach(() => {
    jest.clearAllMocks();
  });

  describe('getProductById', () => {
    it('should return a product when found', async () => {
      mockRepository.findById.mockResolvedValue(mockProduct);
      
      const result = await productService.getProductById('1');
      
      expect(result).toEqual(mockProduct);
      expect(mockRepository.findById).toHaveBeenCalledWith('1');
    });

    it('should return null when product not found', async () => {
      mockRepository.findById.mockResolvedValue(null);
      
      const result = await productService.getProductById('999');
      
      expect(result).toBeNull();
      expect(mockRepository.findById).toHaveBeenCalledWith('999');
    });
  });

  describe('createProduct', () => {
    it('should create and return a new product', async () => {
      const productData = {
        name: 'New Product',
        summary: 'New Description',
        price: 19.99,
        category: ProductCategory.CLOTHING,
        sku: 'NEW-123',
        inventoryCount: 50
      };
      
      mockRepository.save.mockImplementation(product => Promise.resolve(product));
      
      const result = await productService.createProduct(productData);
      
      expect(result).toMatchObject(productData);
      expect(mockRepository.save).toHaveBeenCalledTimes(1);
      expect(mockEventProducer.productCreated).toHaveBeenCalledWith(expect.objectContaining(productData));
    });
  });

  describe('updateProduct', () => {
    it('should update and return the product', async () => {
      const changes = { price: 29.99, inventoryCount: 75 };
      const updatedProduct = { ...mockProduct, ...changes };
      
      mockRepository.findById.mockResolvedValue(mockProduct);
      mockRepository.update.mockResolvedValue(updatedProduct);
      
      const result = await productService.updateProduct('1', changes);
      
      expect(result).toEqual(updatedProduct);
      expect(mockRepository.findById).toHaveBeenCalledWith('1');
      expect(mockRepository.update).toHaveBeenCalledWith(expect.objectContaining(changes));
      expect(mockEventProducer.productUpdated).toHaveBeenCalledWith(
        expect.anything(),
        expect.objectContaining(changes)
      );
    });

    it('should throw NotFoundError when product not found', async () => {
      mockRepository.findById.mockResolvedValue(null);
      
      await expect(productService.updateProduct('999', { price: 29.99 }))
        .rejects
        .toThrow(NotFoundError);
      
      expect(mockRepository.update).not.toHaveBeenCalled();
      expect(mockEventProducer.productUpdated).not.toHaveBeenCalled();
    });
  });
});

Building and Packaging TypeScript Services

Our template includes optimized build scripts:

// package.json (excerpt)
{
  "scripts": {
    "build": "tsc",
    "start": "node dist/app.js",
    "dev": "ts-node-dev --respawn --transpile-only src/app.ts",
    "lint": "eslint src --ext .ts",
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage"
  }
}

Dockerizing TypeScript Services

Our Docker setup uses multi-stage builds for optimal container size:

# Dockerfile
FROM node:18-alpine AS builder

WORKDIR /app

COPY package*.json ./
RUN npm ci

COPY tsconfig.json ./
COPY src/ ./src/

RUN npm run build

FROM node:18-alpine

WORKDIR /app

COPY package*.json ./
RUN npm ci --production

COPY --from=builder /app/dist ./dist

ENV NODE_ENV=production
EXPOSE 3000

HEALTHCHECK --interval=30s --timeout=5s --start-period=5s --retries=3 \
  CMD wget -qO- http://localhost:3000/health || exit 1

CMD ["node", "dist/app.js"]

CI/CD for TypeScript Services

Our CI/CD workflow ensures proper TypeScript builds:

# .github/workflows/main.yml (excerpt)
jobs:
  test:
    name: Test
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Setup Node.js
        uses: actions/setup-node@v3
        with:
          node-version: '18'
          cache: 'npm'
      - run: npm ci
      - run: npm run lint
      - run: npm test
      
  build:
    name: Build and Push
    needs: test
    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Setup Node.js
        uses: actions/setup-node@v3
        with:
          node-version: '18'
          cache: 'npm'
      - run: npm ci
      - run: npm run build
      # Then Docker build and push steps...

TypeScript Best Practices at FlowMart

  1. Use Interfaces for Public APIs: Define interfaces for repositories, services, and controllers.

  2. Embrace Type Inference: Let TypeScript infer types where it makes sense to reduce verbosity.

  3. Use Discriminated Unions: For handling different event types or command patterns.

  4. Leverage Utility Types: Use built-in utility types like Partial<T>, Readonly<T>, Pick<T>, etc.

  5. Strict Null Checks: Always enable strictNullChecks to prevent null/undefined errors.

  6. Immutability: Use readonly properties and const variables to enforce immutability.

  7. Error Handling: Use custom error classes with type information.

  8. Asynchronous Code: Use async/await consistently with proper error handling.

  9. Use Enums for Constants: Define related constants as TypeScript enums.

  10. Module Structure: Organize code into cohesive modules with clear interfaces.

  • Express.js: Web framework
  • inversify: Dependency injection
  • zod: Runtime validation
  • winston: Logging
  • prom-client: Prometheus metrics
  • jaeger-client: Distributed tracing
  • mongodb: Database driver
  • kafkajs: Kafka client
  • jest: Testing framework
  • supertest: API testing
  • ts-node-dev: Development server

Next Steps

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