How does NestJS support microservices?

Question

Accepted Answer

NestJS has **built-in microservices support** — it can run as a microservice that communicates over various **transports** (TCP, Redis, NATS, RabbitMQ, Kafka, gRPC) instead of HTTP, using a consistent, message-based programming model. The same NestJS structure (modules, services, DI) applies; only the transport and message patterns change.

## Creating a microservice

```ts
// a microservice listens on a transport instead of HTTP
const app = await NestFactory.createMicroservice(AppModule, {
  transport: Transport.TCP,             // or REDIS, NATS, RMQ, KAFKA, GRPC
  options: { host: "0.0.0.0", port: 3001 },
});
await app.listen();
```

## Message patterns vs event patterns

```ts
@Controller()
export class MathController {
  // REQUEST-RESPONSE: respond to a message and RETURN a result (RPC-style)
  @MessagePattern({ cmd: "sum" })
  sum(data: number[]): number {
    return data.reduce((a, b) => a + b, 0);   // the caller awaits this result
  }

// EVENT-BASED: react to an event, NO response expected (fire-and-forget)
  @EventPattern("order_created")
  handleOrderCreated(data: OrderDto) {
    this.processOrder(data);                   // just react; nothing returned
  }
}
```

Two communication styles: **`@MessagePattern`** for request-response (the caller waits for a result, like RPC) and **`@EventPattern`** for events (fire-and-forget, decoupled — the publisher doesn't wait).

## Calling another microservice (the client)

```ts
@Injectable()
export class AppService {
  constructor(@Inject("MATH_SERVICE") private client: ClientProxy) {}

async getSum() {
    // send() → request-response (returns an Observable of the result)
    return this.client.send({ cmd: "sum" }, [1, 2, 3]);
    // emit() → fire an event (no response)
    this.client.emit("order_created", orderData);
  }
}
```

A `ClientProxy` (configured via `ClientsModule`) sends messages/events to other services — `send()` for request-response, `emit()` for events.

## Hybrid applications

```ts
// an app can serve HTTP AND connect microservice transports simultaneously
const app = await NestFactory.create(AppModule);
app.connectMicroservice({ transport: Transport.RMQ, options: {...} });
await app.startAllMicroservices();
await app.listen(3000);   // HTTP too
```

## Transport choice matters

```text
TCP   → simple direct service-to-service
Redis/NATS → lightweight pub/sub messaging
RabbitMQ/Kafka → robust message queues/streams (durability, retries, decoupling)
gRPC  → high-performance, strongly-typed RPC (protobuf)
→ Async message brokers (RMQ/Kafka) give decoupling & resilience; gRPC gives speed+typing.
```

## Why it matters

NestJS's built-in microservices support is significant because it lets you build distributed systems using the *same* familiar NestJS structure (modules, providers, DI, decorators) while swapping HTTP for message-based transports — a major productivity and consistency advantage.

Understanding it is important for architecting scalable systems: the **message-pattern (request-response) vs event-pattern (fire-and-forget) distinction** maps directly to the fundamental microservices communication choice — synchronous RPC-style calls versus asynchronous, decoupled events (the latter, via message brokers, providing the resilience and loose coupling that make microservices robust).

Knowing how to create microservices, define message/event handlers, call other services via `ClientProxy`, build hybrid HTTP+microservice apps, and choose transports (lightweight pub/sub vs durable queues like Kafka/RabbitMQ vs high-performance gRPC) is valuable senior-level knowledge for designing distributed back-ends.

While microservices add real complexity (and aren't always the right choice over a monolith), NestJS makes the implementation clean and consistent, making this an important architectural topic for building large-scale, distributed NestJS systems.