I’ve been thinking a lot lately about how modern applications need to handle massive scale while remaining resilient. This led me down the path of distributed event-driven systems, which combine the power of event sourcing with reliable messaging. Let me share what I’ve learned about building such systems using NestJS, EventStore, and RabbitMQ.

Event-driven architecture fundamentally changes how services communicate. Instead of direct API calls, services emit events that other services react to. This approach creates systems that are more scalable and fault-tolerant. Have you ever wondered how large e-commerce platforms handle thousands of orders without dropping requests?

Let’s start with EventStore implementation. This database specializes in storing events as immutable facts. Here’s how I typically structure base events:

export class OrderCreatedEvent extends BaseEvent {
  constructor(
    public readonly orderId: string,
    public readonly customerId: string,
    public readonly totalAmount: number,
    version: number
  ) {
    super(orderId, version, 'OrderCreatedEvent');
  }
}

Each event represents something that happened in the system. They’re stored sequentially, creating an audit trail that’s invaluable for debugging. What happens when you need to replay events to rebuild state?

Now let’s look at RabbitMQ integration. Message brokers ensure reliable delivery between services. Here’s how I set up a connection:

async connectRabbitMQ(): Promise<Channel> {
  const connection = await amqp.connect('amqp://localhost:5672');
  const channel = await connection.createChannel();
  await channel.assertExchange('order-events', 'topic', { durable: true });
  return channel;
}

The real power comes from combining these technologies. EventStore handles event persistence while RabbitMQ manages cross-service communication. This separation allows each service to process events at its own pace.

Error handling becomes crucial in distributed systems. I implement dead letter queues for failed messages:

await channel.assertQueue('order-processing-dlq', {
  durable: true,
  deadLetterExchange: 'order-events'
});

This ensures that problematic messages don’t block the entire system and can be retried or analyzed separately. How would you handle messages that consistently fail processing?

Monitoring is another critical aspect. I use structured logging to track events across services:

logger.log({
  eventId: event.eventId,
  service: 'order-service',
  timestamp: new Date().toISOString(),
  details: 'Order processing started'
});

This creates a clear trail that helps when debugging complex workflows across multiple services.

Testing distributed systems requires careful planning. I focus on testing event handlers in isolation:

it('should update inventory when order is placed', async () => {
  const event = new OrderCreatedEvent('order-123', 'customer-456', 100, 1);
  await inventoryHandler.handle(event);
  expect(inventoryRepository.update).toHaveBeenCalled();
});

Unit tests verify individual components work correctly, while integration tests ensure they work together.

Performance optimization involves several strategies. I use connection pooling for database access and implement backpressure handling for message queues. Batch processing of events can significantly improve throughput when dealing with high volumes.

One common challenge is ensuring exactly-once processing. I solve this by making event handlers idempotent:

async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
  const processed = await this.checkIfProcessed(event.eventId);
  if (processed) return;
  
  // Process the event
  await this.markAsProcessed(event.eventId);
}

This prevents duplicate processing while maintaining system reliability.

Building distributed systems requires careful consideration of failure scenarios. Network partitions, service outages, and message delays are inevitable. The architecture must be designed to handle these gracefully.

I hope this gives you a solid foundation for building your own event-driven systems. The combination of NestJS, EventStore, and RabbitMQ provides a powerful toolkit for creating scalable, resilient applications. What challenges are you facing with your current architecture?

If you found this helpful, please share it with others who might benefit. I’d love to hear about your experiences in the comments below.