AWS Events: Create Event Bridge target from an rule defined in a different stack

[lucashfreitas]

I have a CDK stack that defines all event bridges rules and exports their ARNs as outputs. I would like to create a target for that rules in other stacks where my consumers are.

I can access the role from an exported ARN without issues:

    //that works
    const eventBusRule = cdk.aws_events.Rule.fromEventRuleArn(
      this,
      "EventBridgeRuleArn",
      'exportedRuleArn
    );

The Object returned from FromEventRuleArn doesn't have properties to AddTarget. aws_events also doesn't have any way to create a new target passing a rule as a reference/option.

[pahud]

fromEventRuleArn actually returns IRule

aws-cdk/packages/@aws-cdk/aws-events/lib/rule.ts

Line 100 in cea1039

public static fromEventRuleArn(scope: Construct, id: string, eventRuleArn: string): IRule {

If you need the addTarget() method you will need to create a new Rule object

[lucashfreitas]

It is possible to create a new rule object from an existent resource Without creating a new one?

[pahud]

When you import an existing rule, which is not created and managed by CDK, you actually get an IRule which is sort of a reference you can only refer to its attribute like Arn or ID. In this case you actually didn't instantiate the Rule L2 construct class. In CDK when you "import" any existing resource, you will not get a full L2 construct class of that resource as it's not "created and managed" by CDK.

[lucashfreitas]

Yeah, I am aware of this, trying to find out if there is any workaround that can be used to create a Target Object with a given rule ARN. Will have a look on Cloudformation docs :) Thank you very much @pahud.

[pahud]

@lucashfreitas If we look at AWS::Events::Rule there's a Targets property where you can define multiple events targets for the rule to be created by cloudformation:

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-events-rule.html#cfn-events-rule-targets

Given the rule is already created in your case and all we know is just its ARN, I don't think cloudformation is able to "link" the target(s) to the existing rule. I believe that's the key limitation.

[pahud]

Looking at putTargets method in SDK, I believe it should be possible to create a custom construct with custom-resources and pass the Rule and EventBusName as the props to this custom construct and it should technically possible to add "target(s)" to the existing rule with CDK. Under the hood, it just calls the putTargets() to add target(s) to the existing rule.