# NestJS Microservices

> Connect services with ClientsModule, @MessagePattern, @EventPattern, and TCP/Redis transport

## When to Use

- You are decomposing a monolith into independent services that communicate over a message broker
- You need request/response messaging between services (command-reply pattern)
- You need one-way event broadcasting to multiple downstream services
- You need to choose a transport (TCP for low-latency internal, Redis/RabbitMQ for durability)

## Instructions

1. **Bootstrap as a microservice:**

```typescript
// main.ts
const app = await NestFactory.createMicroservice<MicroserviceOptions>(AppModule, {
  transport: Transport.REDIS,
  options: { host: 'localhost', port: 6379 },
});
await app.listen();
```

2. **Handle incoming messages:**

```typescript
@Controller()
export class OrdersController {
  @MessagePattern({ cmd: 'get_order' }) // request/response
  getOrder(@Payload() data: { id: string }): Promise<Order> {
    return this.ordersService.findOne(data.id);
  }

  @EventPattern('order_created') // fire-and-forget
  async handleOrderCreated(@Payload() data: OrderCreatedEvent): Promise<void> {
    await this.inventoryService.reserve(data.items);
  }
}
```

3. **Call a microservice from another service:**

```typescript
// register the client in a module
@Module({
  imports: [
    ClientsModule.register([
      {
        name: 'ORDERS_SERVICE',
        transport: Transport.REDIS,
        options: { host: 'localhost', port: 6379 },
      },
    ]),
  ],
})
// inject and use
@Injectable()
export class ApiGatewayService {
  constructor(@InjectClient('ORDERS_SERVICE') private client: ClientProxy) {}

  getOrder(id: string): Observable<Order> {
    return this.client.send<Order>({ cmd: 'get_order' }, { id });
  }

  notifyOrderCreated(event: OrderCreatedEvent): Observable<void> {
    return this.client.emit('order_created', event);
  }
}
```

4. **Hybrid application** (HTTP + microservice on the same app):

```typescript
const app = await NestFactory.create(AppModule);
app.connectMicroservice<MicroserviceOptions>({
  transport: Transport.REDIS,
  options: { host: 'localhost', port: 6379 },
});
await app.startAllMicroservices();
await app.listen(3000);
```

5. **Exception handling:** Throw `RpcException` inside message handlers — it serializes cleanly to the caller.

## Details

**Transport comparison:**

- **TCP** — built-in, no external dependency, lowest latency, point-to-point only
- **Redis** — pub/sub with persistence off by default, good for event fan-out, requires Redis
- **RabbitMQ** — durable queues, dead-letter exchange, complex routing, good for reliable delivery
- **Kafka** — partitioned log, replay, high throughput, best for event sourcing and stream processing
- **NATS** — lightweight, fast, at-most-once delivery by default (NATS JetStream for persistence)

**`send()` vs `emit()`:** `send()` is request/response — it returns an Observable that emits the reply. `emit()` is fire-and-forget — it returns an Observable that completes when the message is sent (no reply). Always `subscribe()` to both, or convert to Promise with `firstValueFrom()`.

**`@Payload()` and `@Ctx()`:** Use `@Payload()` to extract the message payload. Use `@Ctx()` to get the transport context (e.g., `RmqContext` for RabbitMQ to manually acknowledge messages).

**Manual acknowledgment (RabbitMQ):**

```typescript
@MessagePattern('order_created')
async handle(@Payload() data: OrderCreatedEvent, @Ctx() context: RmqContext) {
  const channel = context.getChannelRef();
  const message = context.getMessage();
  await this.processOrder(data);
  channel.ack(message); // only ack after successful processing
}
```

**Serialization:** By default, NestJS serializes/deserializes with `JSON.stringify/parse`. For binary protocols (protobuf, msgpack), configure a custom serializer/deserializer pair.

## Source

https://docs.nestjs.com/microservices/basics

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.
