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Cross-Service Subscription

Services can subscribe to Events from other Services by configuring Rules to forward events from one Event Bus to another.

tip

This is also known as "Bus to Bus Event Routing" (read more).

Subscribe to a Service's Events​

In your infra code, you can use subscribe to forward events from one service to another. It will create an CloudWatch Event Rule to match events by their name and forward them to the other Service's Event Bus.

import { aws_events, aws_events_targets } from "aws-cdk-lib";

const A = new Service(..);
const B = new Service(..);

A.subscribe(B, {
  events: ["MyEvent"]
});

In the example above, all events with the name "MyEvent" will be sent from the source Event Bus of service A to the target Event Bus of service B. This allows you to easily route events between different services in your application, using the power and flexibility of AWS Event Bridge.

Doing it Manually​

You can also do this manually using the aws_events.Rule Construct if you want to customize the Event Pattern.

import { aws_events, aws_events_targets } from "aws-cdk-lib";

const A = new Service(..);
const B = new Service(..);

new aws_events.Rule(stack, "Rule", {
  // send from service A
  eventBus: A.events.bus,
  eventPattern: {
    // select all events with the name "MyEvent"
    detailType: ["MyEvent"]
  },
  targets: [
    // send to service B
    new aws_events_targets.EventBus(B.events.bus)
  ]
})

The detail-type of Events emitted in an Eventual Service will be the name of the event type.

export const myEvent = event("myEvent");
//                               ^ detail-type

The detail will be the event payload serialized as JSON.