Pub/Sub is an asynchronous messaging service that decouples services that produce events from services that process events.

You can use Pub/Sub as messaging-oriented middleware or event ingestion and delivery for streaming analytics pipelines.

Pub/Sub offers durable message storage and real-time message delivery with high availability and consistent performance at scale

To start building Pub/Sub-based microservices, first install the required packages:

$ npm i --save @google-cloud/pubsub nestjs-google-pubsub-microservice

To use the Pub/Sub transporter, pass the following options object to the createMicroservice() method:

const app = await NestFactory.createMicroservice<MicroserviceOptions>(ApplicationModule, {
  strategy: new GCPubSubServer({
    topic: 'cats_topic',
    subscription: 'cats_subscription',
    client: {
      projectId: 'microservice',
    },
  }),
});

The options property is specific to the chosen transporter. The GCloud Pub/Sub transporter exposes the properties described below.

const client = 
  new GCPubSubClient({
      client: {
        apiEndpoint: 'localhost:8681',
        projectId: 'microservice',
      },
    });
client
  .send('pattern', 'Hello world!')
  .subscribe((response) => console.log(response));

In more sophisticated scenarios, you may want to access more information about the incoming request. When using the Pub/Sub transporter, you can access the GCPubSubContext object.

@MessagePattern('notifications')
getNotifications(@Payload() data: number[], @Ctx() context: GCPubSubContext) {
  console.log(`Pattern: ${context.getPattern()}`);
}

To access the original Pub/Sub message (with the attributes, data, ack and nack), use the getMessage() method of the GCPubSubContext object, as follows:

@MessagePattern('notifications')
getNotifications(@Payload() data: number[], @Ctx() context: GCPubSubContext) {
  console.log(context.getMessage());
}

To make sure a message is never lost, Pub/Sub supports message acknowledgements. An acknowledgement is sent back by the consumer to tell Pub/Sub that a particular message has been received, processed and that Pub/Sub is free to delete it. If a consumer dies (its subscription is closed, connection is closed, or TCP connection is lost) without sending an ack, Pub/Sub will understand that a message wasn't processed fully and will re-deliver it.

To enable manual acknowledgment mode, set the noAck property to false:

{
  replyTopic: 'cats_topic_reply',
  replySubscription: 'cats_subscription_reply',
  noAck: false,
  client: {
    projectId: 'microservice',
  },
},

When manual consumer acknowledgements are turned on, we must send a proper acknowledgement from the worker to signal that we are done with a task.

@MessagePattern('notifications')
getNotifications(@Payload() data: number[], @Ctx() context: GCPubSubContext) {
  const originalMsg = context.getMessage();

  originalMsg.ack();
}

Pub/Sub requires a graceful shutdown properly configured in order to work correctly, otherwise some messages acknowledges can be lost. Therefore, don't forget to call client close:

export class GCPubSubController implements OnApplicationShutdown {
  client: ClientProxy;

  constructor() {
    this.client = new GCPubSubClient({});
  }

  onApplicationShutdown() {
    return this.client.close();
  }
}