feat(elbv2): support ALB target for NLB (#16687)

This PR allows NLB to have a single ALB as the target.

Fixes: #16679 

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-elasticloadbalancingv2-targets/lib/alb-target.ts

@@ -0,0 +1,45 @@
+import * as elbv2 from '@aws-cdk/aws-elasticloadbalancingv2';
+
+/**
+ * A single Application Load Balancer as the target for load balancing.
+ */
+export class AlbArnTarget implements elbv2.INetworkLoadBalancerTarget {
+  /**
+   * Create a new alb target
+   *
+   * @param albArn The ARN of the application load balancer to load balance to
+   * @param port The port on which the target is listening
+   */
+  constructor(private readonly albArn: string, private readonly port: number) {
+  }
+
+  /**
+   * Register this alb target with a load balancer
+   *
+   * Don't call this, it is called automatically when you add the target to a
+   * load balancer.
+   */
+  public attachToNetworkTargetGroup(targetGroup: elbv2.INetworkTargetGroup): elbv2.LoadBalancerTargetProps {
+    return this.attach(targetGroup);
+  }
+
+  private attach(_targetGroup: elbv2.ITargetGroup): elbv2.LoadBalancerTargetProps {
+    return {
+      targetType: elbv2.TargetType.ALB,
+      targetJson: { id: this.albArn, port: this.port },
+    };
+  }
+}
+
+/**
+ * A single Application Load Balancer as the target for load balancing.
+ */
+export class AlbTarget extends AlbArnTarget {
+  /**
+   * @param alb The application load balancer to load balance to
+   * @param port The port on which the target is listening
+   */
+  constructor(alb: elbv2.ApplicationLoadBalancer, port: number) {
+    super(alb.loadBalancerArn, port);
+  }
+}

packages/@aws-cdk/aws-elasticloadbalancingv2-targets/lib/index.ts

@@ -1,3 +1,4 @@
+export * from './alb-target';
 export * from './ip-target';
 export * from './instance-target';
 export * from './lambda-target';

packages/@aws-cdk/aws-elasticloadbalancingv2-targets/package.json

@@ -70,7 +70,9 @@
     "@aws-cdk/cdk-integ-tools": "0.0.0",
     "@aws-cdk/pkglint": "0.0.0",
     "@types/jest": "^26.0.24",
-    "jest": "^26.6.3"
+    "jest": "^26.6.3",
+    "@aws-cdk/aws-ecs": "0.0.0",
+    "@aws-cdk/aws-ecs-patterns": "0.0.0"
   },
   "dependencies": {
     "@aws-cdk/aws-ec2": "0.0.0",

packages/@aws-cdk/aws-elasticloadbalancingv2-targets/test/alb-target.test.ts

@@ -0,0 +1,68 @@
+import { expect, haveResource } from '@aws-cdk/assert-internal';
+import * as ec2 from '@aws-cdk/aws-ec2';
+import * as elbv2 from '@aws-cdk/aws-elasticloadbalancingv2';
+import { Stack } from '@aws-cdk/core';
+import * as targets from '../lib';
+
+test('Can create target groups with alb target', () => {
+  // GIVEN
+  const stack = new Stack();
+  const vpc = new ec2.Vpc(stack, 'Stack');
+  const alb = new elbv2.ApplicationLoadBalancer(stack, 'ALB', { vpc });
+  const nlb = new elbv2.NetworkLoadBalancer(stack, 'NLB', { vpc });
+  const listener = nlb.addListener('Listener', { port: 80 });
+
+  // WHEN
+  listener.addTargets('Targets', {
+    targets: [new targets.AlbTarget(alb, 80)],
+    port: 80,
+  });
+
+  // THEN
+  expect(stack).to(haveResource('AWS::ElasticLoadBalancingV2::TargetGroup', {
+    Port: 80,
+    Protocol: 'TCP',
+    Targets: [
+      {
+        Id: {
+          Ref: 'ALBAEE750D2',
+        },
+        Port: 80,
+      },
+    ],
+    TargetType: 'alb',
+    VpcId: {
+      Ref: 'Stack8A423254',
+    },
+  }));
+});
+
+test('Can create target groups with alb arn target', () => {
+  // GIVEN
+  const stack = new Stack();
+  const vpc = new ec2.Vpc(stack, 'Stack');
+  const nlb = new elbv2.NetworkLoadBalancer(stack, 'NLB', { vpc });
+  const listener = nlb.addListener('Listener', { port: 80 });
+
+  // WHEN
+  listener.addTargets('Targets', {
+    targets: [new targets.AlbArnTarget('MOCK_ARN', 80)],
+    port: 80,
+  });
+
+  // THEN
+  expect(stack).to(haveResource('AWS::ElasticLoadBalancingV2::TargetGroup', {
+    Port: 80,
+    Protocol: 'TCP',
+    Targets: [
+      {
+        Id: 'MOCK_ARN',
+        Port: 80,
+      },
+    ],
+    TargetType: 'alb',
+    VpcId: {
+      Ref: 'Stack8A423254',
+    },
+  }));
+});