How to add listener rules in a loop; with priority derived from loop index?

In continuation to my previous quest, now I am stuck at the following juncture. I read a file clients.json and then create stack resources based on the input from the file. This file contains an array. And for each item of an array I create similar resources. In the following code, I am adding listener rules in a loop for containers which are also created in the same loop.

//add listener for application inside load balancer
const myListener = alb.addListener("myListener", {});
// rule with priority 1 added here.

for (let i = 0; i < clients.length; i++) {
  const tenantId = clients[i].tenantId;
  const domainName = clients[i].dnsName;

  const taskDefinition = new ecs.FargateTaskDefinition(
    this,
    `serviceTask${tenantId}`,
    {}
  );

  // now create container for clients[i]
  // ...

  // now add listeners to existing LB
  const myTarget = new elbv2.ApplicationTargetGroup(
    this,
    `myService${tenantId}targetgroup`,
    {}
  );

  const listenerRule = new elbv2.ApplicationListenerRule(
    this,
    "listenerRule" + tenantId,
    {
      listener: myListener,
      /*priority: i + 2; */ /*rules with priority 2 and above*/
      priority: clients.length - i + 1,
      action: ListenerAction.forward([myTarget]),
      conditions: [elbv2.ListenerCondition.hostHeaders([domainName])],
    }
  );
}

Look at the priority field in the last block. One line is commented and the current approach is uncommented. Both the approaches are similar. All of this is in a for-loop. In the commented part, I am starting from 2 and going till the length of clients array. In the other approach, I am starting from the end.

Suppose there are 4 elements in the array. So the listener rule priorities will be - 5, 4, 3, 2.

In a new deployment, suppose the first element of the clients array is removed. So the updated priorities will be - 4, 3, 2.

This is not much of a hassle. cdk deploy runs perfectly. This is because rule with priority 5 is simply deleted and no new rule has to be created. The remaining containers map to the same priority as before.

Now suppose the last element if the clients array is removed. So the updated cloudformation template will map the containers to the priorities like this,

Notice that containers A, B, C were earlier mapped to 5, 4, 3 respectively, but after removal of the last element, they are mapped to 4, 3, 2 respectively. This gives "ListenerRule already exist" error during deployment. Because when A is getting mapped to priority 4, it notices that 4 priority is already mapped to B. And so on...

So, how should I handle such a deployment update where a high priority rule (one with low priority value) is removed and other rules shift by one.

The CloudFormation AWS::ElasticLoadBalancingV2::ListenerRule docs explain why you're getting the error when re-assigning the rule priorities:

> A listener can't have multiple rules with the same priority. If you try to reorder rules by updating their priorities, do not specify a new priority if an existing rule already uses this priority, as this can cause an error. If you need to reuse a priority with a different rule, you must remove it as a priority first, and then specify it in a subsequent update.

In other words, you're looking to set unique, semi-permanent priority values, while accomodating subsequent insertions and deletions.

One approach would be to assign an explicit priority value to each entry in clients.json. These priorities would be set with a gap of, say, 100 to accomodate later reordering (rules apparently accept priority values up to 50000).

• containerA: 100
• containerB: 200
• containerC: 300

If a new customer comes along, you can give her top priority with containerD: 50, for instance, without touching the existing priorities.