NestJS Google Cloud Pub/Sub Microservice Transport

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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

#### Installation

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

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

#### Overview

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

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

#### Options

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

#### Client

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

#### Context

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.

`typescript
@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:

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

#### Message acknowledgement

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:

`typescript
{
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.

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

originalMsg.ack();
}
`

#### Shutdown

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:

`typescript
export class GCPubSubController implements OnApplicationShutdown {
client: ClientProxy;

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

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