How to Add a Rule to an Existing Application Load Balancer Listener using AWS CDK

When I inherited a project, I discovered an application load balancer with a HTTPS Listener that was set up before I began using CDK.

This listener currently has 13 rules in place that route requests based on hostname to different fargate instances, with the last rule acting as a fallback option.

Now, I need to add new rules using CDK before the fallback rules. Since new rules will be added for new fargate services in the future, this process needs to be dynamic.

How can I insert new rules into an existing listener and determine the priority of a specific rule (in this case, the fallback rule) to use as a starting point for priority calculation?

Alternatively, is it feasible to simply add one rule after another with each having a priority of 1, causing the priorities of the existing rules to increment by 1 each time?

I managed to retrieve the existing listener using the following code snippet, but I am unsure how to access its rules:

const httpsListener = elbv2.ApplicationListener.fromLookup(
  this,
  "ALBListenerHTTPS", {
    listenerArn: "my:aws:alb:listener:arn",
  }
);

Answer №1

In the world of software development, every ApplicationListener corresponds to a

 AWS::ElasticLoadBalancingV2::ListenerRule
CloudFormation resource.

When you have an ApplicationListener, you can create new ApplicationListenerRules like this:

Create a new ApplicationListenerRule instance, naming it 'Rule1', with parameters such as the listener and priority.
    ...

However, if you want `aws-cdk` to be responsible for managing load balancer rules, they must all be generated by it. There are two potential solutions to achieve this.

Solution A

The first solution involves importing all existing listener rules into your CloudFormation stack. This can be done by utilizing the option to import load balancer rule resources into CloudFormation stack as described here.

Solution B

An alternative approach is to adjust the priorities of existing listener rules using the command line interface. The goal is to ensure that the priorities of existing rules start at N + 1, where N represents the current count of rules. By following this method, you create room for any new rules defined in `aws-cdk`. Once you have sufficient space available, you can define duplicates of existing rules using `aws-cdk` and subsequently deploy them. After successful deployment, manually created rules can be safely removed.

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