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// Microservices Architecture with Event Sourcing & CQRS // Domain: E-commerce Order Management System // 1. Domain Events interface DomainEvent { id: string; aggregateId: string; version: number; timestamp: Date; type: string; } class OrderCreatedEvent implements DomainEvent { constructor( public readonly id: string, public readonly aggregateId: string, public readonly version: number, public readonly timestamp: Date, public readonly type: string = 'OrderCreated', public readonly customerId: string, public readonly items: OrderItem[], public readonly totalAmount: number ) {} } class OrderStatusChangedEvent implements DomainEvent { constructor( public readonly id: string, public readonly aggregateId: string, public readonly version: number, public readonly timestamp: Date, public readonly type: string = 'OrderStatusChanged', public readonly previousStatus: OrderStatus, public readonly newStatus: OrderStatus ) {} } // 2. Aggregate Root with Event Sourcing class OrderAggregate { private events: DomainEvent[] = []; private _version = 0; constructor( private _id: string, private _customerId: string = '', private _items: OrderItem[] = [], private _status: OrderStatus = OrderStatus.Pending, private _totalAmount = 0 ) {} // Command Handlers public static create( id: string, customerId: string, items: OrderItem[] ): OrderAggregate { const aggregate = new OrderAggregate(id); const totalAmount = items.reduce((sum, item) => sum + item.price * item.quantity, 0); const event = new OrderCreatedEvent( uuidv4(), id, 1, new Date(), 'OrderCreated', customerId, items, totalAmount ); aggregate.apply(event); return aggregate; } public changeStatus(newStatus: OrderStatus): void { if (this._status === newStatus) return; if (!this.isValidStatusTransition(this._status, newStatus)) { throw new Error(`Invalid status transition from ${this._status} to ${newStatus}`); } const event = new OrderStatusChangedEvent( uuidv4(), this._id, this._version + 1, new Date(), 'OrderStatusChanged', this._status, newStatus ); this.apply(event); } // Event Application private apply(event: DomainEvent): void { switch (event.type) { case 'OrderCreated': this.applyOrderCreated(event as OrderCreatedEvent); break; case 'OrderStatusChanged': this.applyOrderStatusChanged(event as OrderStatusChangedEvent); break; } this.events.push(event); this._version = event.version; } private applyOrderCreated(event: OrderCreatedEvent): void { this._customerId = event.customerId; this._items = event.items; this._totalAmount = event.totalAmount; this._status = OrderStatus.Pending; } private applyOrderStatusChanged(event: OrderStatusChangedEvent): void { this._status = event.newStatus; } // Business Logic private isValidStatusTransition(from: OrderStatus, to: OrderStatus): boolean { const validTransitions: Record<OrderStatus, OrderStatus[]> = { [OrderStatus.Pending]: [OrderStatus.Confirmed, OrderStatus.Cancelled], [OrderStatus.Confirmed]: [OrderStatus.Processing, OrderStatus.Cancelled], [OrderStatus.Processing]: [OrderStatus.Shipped, OrderStatus.Cancelled], [OrderStatus.Shipped]: [OrderStatus.Delivered], [OrderStatus.Delivered]: [], [OrderStatus.Cancelled]: [] }; return validTransitions[from].includes(to); } public getUncommittedEvents(): DomainEvent[] { return [...this.events]; } public markEventsAsCommitted(): void { this.events = []; } // Getters get id(): string { return this._id; } get customerId(): string { return this._customerId; } get status(): OrderStatus { return this._status; } get version(): number { return this._version; } } // 3. Event Store Implementation interface EventStore { saveEvents(aggregateId: string, events: DomainEvent[], expectedVersion: number): Promise<void>; getEventsForAggregate(aggregateId: string): Promise<DomainEvent[]>; } class PostgreSQLEventStore implements EventStore { constructor(private db: any) {} async saveEvents(aggregateId: string, events: DomainEvent[], expectedVersion: number): Promise<void> { const client = await this.db.connect(); try { await client.query('BEGIN'); // Optimistic concurrency check const { rows } = await client.query( 'SELECT MAX(version) as current_version FROM events WHERE aggregate_id = $1', [aggregateId] ); const currentVersion = rows[0]?.current_version || 0; if (currentVersion !== expectedVersion) { throw new Error(`Concurrency conflict. Expected version ${expectedVersion}, but current version is ${currentVersion}`); } // Insert events for (const event of events) { await client.query( `INSERT INTO events (id, aggregate_id, version, timestamp, type, data) VALUES ($1, $2, $3, $4, $5, $6)`, [event.id, event.aggregateId, event.version, event.timestamp, event.type, JSON.stringify(event)] ); } await client.query('COMMIT'); } catch (error) { await client.query('ROLLBACK'); throw error; } finally { client.release(); } } async getEventsForAggregate(aggregateId: string): Promise<DomainEvent[]> { const { rows } = await this.db.query( 'SELECT data FROM events WHERE aggregate_id = $1 ORDER BY version', [aggregateId] ); return rows.map(row => JSON.parse(row.data)); } } // 4. Repository with Event Sourcing class OrderRepository { constructor(private eventStore: EventStore) {} async save(order: OrderAggregate): Promise<void> { const events = order.getUncommittedEvents(); if (events.length === 0) return; await this.eventStore.saveEvents(order.id, events, order.version - events.length); order.markEventsAsCommitted(); } async getById(id: string): Promise<OrderAggregate | null> { const events = await this.eventStore.getEventsForAggregate(id); if (events.length === 0) return null; const order = new OrderAggregate(id); events.forEach(event => order['apply'](event)); return order; } } // 5. Command Handlers (Write Side) interface Command { aggregateId: string; } class CreateOrderCommand implements Command { constructor( public readonly aggregateId: string, public readonly customerId: string, public readonly items: OrderItem[] ) {} } class ChangeOrderStatusCommand implements Command { constructor( public readonly aggregateId: string, public readonly newStatus: OrderStatus ) {} } class OrderCommandHandler { constructor( private repository: OrderRepository, private eventBus: EventBus ) {} async handle(command: CreateOrderCommand): Promise<void> { const order = OrderAggregate.create( command.aggregateId, command.customerId, command.items ); await this.repository.save(order); // Publish events to event bus const events = order.getUncommittedEvents(); for (const event of events) { await this.eventBus.publish(event); } } async handle(command: ChangeOrderStatusCommand): Promise<void> { const order = await this.repository.getById(command.aggregateId); if (!order) throw new Error(`Order ${command.aggregateId} not found`); order.changeStatus(command.newStatus); await this.repository.save(order); const events = order.getUncommittedEvents(); for (const event of events) { await this.eventBus.publish(event); } } } // 6. Read Models (Query Side) interface OrderReadModel { id: string; customerId: string; status: string; totalAmount: number; items: any[]; createdAt: Date; updatedAt: Date; } class OrderProjection { constructor(private db: any) {} async handleOrderCreated(event: OrderCreatedEvent): Promise<void> { await this.db.query( `INSERT INTO order_read_model (id, customer_id, status, total_amount, items, created_at, updated_at) VALUES ($1, $2, $3, $4, $5, $6, $7)`, [ event.aggregateId, event.customerId, 'Pending', event.totalAmount, JSON.stringify(event.items), event.timestamp, event.timestamp ] ); } async handleOrderStatusChanged(event: OrderStatusChangedEvent): Promise<void> { await this.db.query( `UPDATE order_read_model SET status = $1, updated_at = $2 WHERE id = $3`, [event.newStatus, event.timestamp, event.aggregateId] ); } } // 7. Query Handlers (Read Side) class OrderQueryHandler { constructor(private db: any) {} async getOrderById(id: string): Promise<OrderReadModel | null> { const { rows } = await this.db.query( 'SELECT * FROM order_read_model WHERE id = $1', [id] ); return rows[0] || null; } async getOrdersByCustomerId(customerId: string): Promise<OrderReadModel[]> { const { rows } = await this.db.query( 'SELECT * FROM order_read_model WHERE customer_id = $1 ORDER BY created_at DESC', [customerId] ); return rows; } async getOrdersByStatus(status: string): Promise<OrderReadModel[]> { const { rows } = await this.db.query( 'SELECT * FROM order_read_model WHERE status = $1 ORDER BY created_at DESC', [status] ); return rows; } } // 8. Event Bus for Cross-Service Communication interface EventBus { publish(event: DomainEvent): Promise<void>; subscribe(eventType: string, handler: (event: DomainEvent) => Promise<void>): void; } class RabbitMQEventBus implements EventBus { constructor(private connection: any) {} async publish(event: DomainEvent): Promise<void> { const channel = await this.connection.createChannel(); const exchange = 'domain_events'; await channel.assertExchange(exchange, 'topic', { durable: true }); const routingKey = `order.${event.type.toLowerCase()}`; const message = Buffer.from(JSON.stringify(event)); channel.publish(exchange, routingKey, message, { persistent: true }); await channel.close(); } subscribe(eventType: string, handler: (event: DomainEvent) => Promise<void>): void { // Implementation for subscribing to events // This would set up RabbitMQ consumers } } // 9. API Gateway & Service Discovery class OrderMicroservice { constructor( private commandHandler: OrderCommandHandler, private queryHandler: OrderQueryHandler ) {} // REST API Endpoints async createOrder(req: any, res: any): Promise<void> { try { const command = new CreateOrderCommand( uuidv4(), req.body.customerId, req.body.items ); await this.commandHandler.handle(command); res.status(201).json({ orderId: command.aggregateId }); } catch (error) { res.status(400).json({ error: error.message }); } } async getOrder(req: any, res: any): Promise<void> { try { const order = await this.queryHandler.getOrderById(req.params.id); if (!order) { return res.status(404).json({ error: 'Order not found' }); } res.json(order); } catch (error) { res.status(500).json({ error: error.message }); } } async updateOrderStatus(req: any, res: any): Promise<void> { try { const command = new ChangeOrderStatusCommand( req.params.id, req.body.status ); await this.commandHandler.handle(command); res.status(200).json({ success: true }); } catch (error) { res.status(400).json({ error: error.message }); } } } // Supporting Types interface OrderItem { productId: string; quantity: number; price: number; } enum OrderStatus { Pending = 'Pending', Confirmed = 'Confirmed', Processing = 'Processing', Shipped = 'Shipped', Delivered = 'Delivered', Cancelled = 'Cancelled' } // 10. Docker Compose for Local Development /* version: '3.8' services: postgres: image: postgres:15 environment: POSTGRES_DB: eventstore POSTGRES_USER: user POSTGRES_PASSWORD: password volumes: - postgres_data:/var/lib/postgresql/data ports: - "5432:5432" rabbitmq: image: rabbitmq:3-management environment: RABBITMQ_DEFAULT_USER: user RABBITMQ_DEFAULT_PASS: password ports: - "5672:5672" - "15672:15672" order-service: build: ./order-service environment: DATABASE_URL: postgresql://user:password@postgres:5432/eventstore RABBITMQ_URL: amqp://user:password@rabbitmq:5672 ports: - "3001:3000" depends_on: - postgres - rabbitmq volumes: postgres_data: */ export { OrderAggregate, OrderCommandHandler, OrderQueryHandler, OrderMicroservice };