allocate_alb_rule_priority.py

# Source: https://stackoverflow.com/questions/50003378/automatically-set-listenerrule-priority-in-cloudformation-template

import random
import uuid
import traceback
import boto3


# Member must have value less than or equal to 50000
ALB_RULE_PRIORITY_RANGE = 1, 50000


def handler(event, context):
    physical_resource_id = event.get("PhysicalResourceId", str(uuid.uuid4()))
    response_data = {}
    listener_arn = event["ResourceProperties"]["ListenerArn"]
    if event["RequestType"] == "Create":
        elbv2_client = boto3.client("elbv2")
        result = elbv2_client.describe_rules(ListenerArn=listener_arn)
        in_use = list(
            filter(lambda s: s.isdecimal(), [r["Priority"] for r in result["Rules"]])
        )
        priority = None
        while not priority or priority in in_use:
            priority = str(random.randint(*ALB_RULE_PRIORITY_RANGE))
        response_data = {"Priority": priority}
    return {"PhysicalResourceId": physical_resource_id, "Data": response_data}

external-stack-usage.ts

    const loadBalancerName = `${titleCase(stage)}PublicLoadBalancer`
    const loadBalancer = elbv2.ApplicationLoadBalancer.fromLookup(
      this,
      `LoadBalancer`,
      {
        loadBalancerTags: { Name: loadBalancerName },
      }
    )
    const listener = elbv2.ApplicationListener.fromLookup(this, `Listener`, {
      loadBalancerTags: { Name: loadBalancerName },
      listenerProtocol: elbv2.ApplicationProtocol.HTTPS,
    })

    const hostedZone = route53.HostedZone.fromLookup(this, `HostedZone`, {
      domainName: "blahblahblah.io",
    })

    let subdomain = `${client}-snowplow-collector`
    if (stage === STAGE.DEVELOPMENT) subdomain = `${stage}-${subdomain}`

    const recordName = `${subdomain}.blahblahblah.io`
    new route53.ARecord(this, `AliasRecord`, {
      zone: hostedZone,
      recordName,
      target: route53.RecordTarget.fromAlias(
        new alias.LoadBalancerTarget(loadBalancer)
      ),
    })

    const targetGroup = new elbv2.ApplicationTargetGroup(scope, `TargetGroup`, {
      port: 80,
      healthCheck: {
        port: "traffic-port",
        path: "/health",
      },
      targets: [service],
      vpc: vpc,
    })

    const nextListenerRulePriorityLambda = new CustomResource(
      this,
      "nextListenerRulePriority",
      {
        resourceType: "Custom::AllocateAlbRulePriority",
        serviceToken: Fn.importValue("AllocateAlbRulePriority-serviceToken"),
        properties: {
          ListenerArn: listener.listenerArn,
        },
      }
    )

    const nextListenerRulePriority = nextListenerRulePriorityLambda.getAttString(
      "Priority"
    )

    const listenerRule = new elbv2.ApplicationListenerRule(
      this,
      "ListenerRule",
      {
        action: elbv2.ListenerAction.forward([targetGroup]),
        conditions: [elbv2.ListenerCondition.hostHeaders([recordName])],
        listener,
        priority: 9999,
      }
    )

    // Source: https://github.com/aws/aws-cdk/issues/9296#issuecomment-723294027
    const albRuleEscapeHatch = listenerRule.node
      .defaultChild as elbv2.CfnListenerRule
    albRuleEscapeHatch.priority = Token.asNumber(nextListenerRulePriority)
  }

lambdas.ts

import * as cdk from "@aws-cdk/core"
import * as logs from "@aws-cdk/aws-logs"
import * as path from "path"
import * as iam from "@aws-cdk/aws-iam"
import * as lambda from "@aws-cdk/aws-lambda"
import * as cr from "@aws-cdk/custom-resources"
import { CfnOutput } from "@aws-cdk/core"

/**
 * Central lambdas used throughout the team.
 */
export default function createLambdas(scope: cdk.Stack) {
  const fn = new lambda.Function(scope, "AllocateAlbRulePriority", {
    runtime: lambda.Runtime.PYTHON_3_6,
    handler: "allocate_alb_rule_priority.handler",
    timeout: cdk.Duration.seconds(30),
    code: lambda.Code.fromAsset(
      path.join(__dirname, "..", "..", "lambdas", "allocate-alb-rule-priority")
    ),
    initialPolicy: [
      new iam.PolicyStatement({
        sid: "DescribeRulesPolicy",
        actions: ["elasticloadbalancing:DescribeRules"],
        resources: ["*"],
      }),
    ],
  })

  const provider = new cr.Provider(scope, "AllocateAlbRulePriorityProvider", {
    onEventHandler: fn,
    logRetention: logs.RetentionDays.ONE_DAY,
  })

  new cdk.CfnOutput(scope, "AllocateAlbRulePriorityServiceToken", {
    value: provider.serviceToken,
    exportName: "AllocateAlbRulePriority-serviceToken",
  })
}

You saved me hours of my life.
Thank you!

glad I could help @Lasim!

Could you please share some instructions on how exactly to use this code ? Thanks.