Lately, I’ve been thinking about how modern applications handle complexity and scale. It’s a challenge I face regularly in my work, and I wanted to share a practical approach I’ve found effective. This article details how to construct a resilient event-driven microservices system using NestJS, Redis Streams, and MongoDB. I hope it provides you with a solid foundation for your own projects.

Event-driven architecture fundamentally changes how services communicate. Instead of services calling each other directly, they emit events. Other services listen for these events and react accordingly. This creates a system that is more resilient to failure and easier to scale. Have you considered what happens when one service in a chain is temporarily unavailable?

Let’s start with the event bus, the central nervous system of our architecture. We’ll use Redis Streams for its reliable message delivery and persistence. Here is a basic service to publish an event.

// event-bus.service.ts
async publish(event: BaseEvent): Promise<string> {
  const eventId = await this.redis.xadd(
    'events-stream',
    '*',
    'type', event.type,
    'data', JSON.stringify(event),
    'timestamp', event.timestamp.toISOString()
  );
  return eventId;
}

Each service that needs to process events will run a consumer. This code sets up a consumer group to reliably read events, ensuring that even if a service restarts, it doesn’t lose its place in the stream.

// inventory.service.ts
async startOrderConsumer() {
  await this.redis.xgroup('CREATE', 'events-stream', 'inventory-group', '$', 'MKSTREAM');
  
  while (true) {
    const streams = await this.redis.xreadgroup(
      'GROUP', 'inventory-group', 'inventory-service',
      'COUNT', 10,
      'BLOCK', 1000,
      'STREAMS', 'events-stream', '>'
    );
    // ... process messages and acknowledge them
  }
}

This pattern allows the inventory service to react to an ORDER_CREATED event by reserving stock, without the order service needing to know anything about the inventory system’s internal logic. What other business processes could be triggered by a new order?

For data persistence, MongoDB works well with its flexible document model. We can store a complete history of all events, which is invaluable for auditing and debugging. This also enables event sourcing, where you can rebuild an application’s state by replaying past events.

// event-store.service.ts
async saveEvent(event: BaseEvent) {
  await this.eventModel.create({
    _id: event.id,
    type: event.type,
    data: event.data,
    timestamp: event.timestamp,
    correlationId: event.correlationId
  });
}

The Command Query Responsibility Segregation (CQRS) pattern fits naturally here. Commands, like “Create Order,” change the system state and emit events. Queries, like “Get Order History,” read from optimized data views. This separation allows you to scale read and write operations independently. How might your database load change if reads and writes are separated?

Handling failures gracefully is critical. If processing an event fails, we need retry mechanisms. Redis Streams allows us to claim pending messages that have not been acknowledged, making it possible to re-process them after a delay or move them to a dead-letter queue for manual inspection.

Monitoring is the final piece. Since events flow asynchronously, traditional debugging can be difficult. By logging correlation IDs and using distributed tracing, you can follow a single business transaction as it moves through multiple services. This visibility is essential for maintaining a healthy system in production.

Building this kind of system requires careful planning, but the payoff is significant. You gain a platform that can handle high loads, is tolerant of individual service failures, and can evolve over time as you add new features that react to existing events.

I hope this exploration of event-driven microservices gives you some ideas for your next project. If you found this useful, please like and share it with your network. I’d love to hear about your experiences or answer any questions in the comments below.