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-microserviceTo 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.
topic |
Topic name which your server subscription will belong to |
subscription |
Subscription name which your server will listen to |
replyTopic |
Topic name which your client subscription will belong to |
replySubscription |
Subscription name which your client will listen to |
noAck |
If false, manual acknowledgment mode enabled |
init |
If false, topics and subscriptions will not be created, only validated |
checkExistence |
If false, topics and subscriptions will not be checked, only used. This only applies when init is false |
useAttributes |
Only applicable for client. If true, pattern and correlationId will be sent via message attributes. This is useful if message consumer is not NestJs microservice or you have message filtering on subscription |
client |
Additional client options (read more here) |
publisher |
Additional topic publisher options (read more here) |
subscriber |
Additional subscriber options (read more here) |
scopedEnvKey |
Scope topics and subscriptions to avoid losing messages when several people are working on the same code base. Will prefixes topics and subscriptions with this key (read more here) |
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();
}
}
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