[elasticloadbalancingv2] Support References or CfnParameters in priority

It would be great to be able to set the value of the priority dynamically using a Reference :

ApplicationListenerRule.Builder.create(stack, "ApplicationListenerRule")
                                .priority(token) // token.getValueAsNumber() does't work 
                                                 // since it generates a random number ( out of [0,50000] ) at synthesize stage
                                .build();

• Actually I want to deploy the same ressoures defined in a stack in new stacks by setting the generated template in the service catalog ( there is a CFN parameter that will be used for the path but it’s not important ). Theses stacks uses the same ALB ( of another Stack ), and creates a new ALB Rule for the new ressources.

  I’m trying to recode this with CDK/java ( assuming I have a Lambda that generates the priority ) but it’s seems to be impossible

AllocateAlbRulePriorityCustomResource:
    Type: Custom::AllocateAlbRulePriority
    [...]

  ListenerRule:
    Type: AWS::ElasticLoadBalancingV2::ListenerRule
    Properties:
      Priority: !GetAtt AllocateAlbRulePriorityCustomResource.Priority
      [...]

• Another use case is to set that priority as a CfnParameter, but when we call getValueAsNumber() it will generate a random number probably out of the allowed ranges.

Proposed solution

• Add a new priority setter accepting Reference as argument.
• or/and Remove the check on priorityValue.

I vote for option 1.

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This is a 🚀 Feature Request

[AlJohri]

Thank you for sharing @garrettux! That worked for me as well. Here’s my solution with the lambda code as well: https://gist.github.com/AlJohri/fa18bf58b338888934a140c8b6214fdd

Would love any comments if there’s a cleaner way to go about things.

[garrettux]

FWIW I was having the same issue and was able to work around it with an escape hatch:

        const nextListenerRulePriorityLambda = new CustomResource(this, 'nextListenerRulePriority', {
            resourceType: 'Custom::AllocateAlbRulePriority',
            serviceToken: `arn:aws:lambda:${region}:${account}:function:AllocateAlbRulePriority`,
            properties:{
                ListenerArn: listenerArn
            }
        });

        const nextListenerRulePriority = nextListenerRulePriorityLambda.getAttString('Priority');
		
        const albPredeployListenerRule = new ApplicationListenerRule(this, 'albPredeployListenerRule', {
            priority: 9999,
            listener: this.listener
        });
		
        // https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-elasticloadbalancingv2/lib/alb/application-listener-rule.ts#L231-L236
        const albRuleEscapeHatch = albPredeployListenerRule.node.defaultChild as CfnListenerRule;
        albRuleEscapeHatch.priority = Token.asNumber(nextListenerRulePriority);