feat(elasticloadbalancing): classic load balancer supports ec2 instances (#24353)

Introducing the `InstanceTarget` class: an EC2 instance that serves as the target for load balancing. This class allows to register an instance to a load balancer. `InstanceTarget ` takes an instance to register as the target for the  load balancer.

For example,
```ts
const target = new elb.InstanceTarget(instance);
elb.addTarget(target);
```
creates an InstanceTarget object with the specified instance and then adds it to the load balancer.

> [CONTRIBUTING GUIDE]: https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md
> [DESIGN GUIDELINES]: https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md

Closes #23500.

----

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

Author: pattasai

packages/@aws-cdk/aws-elasticloadbalancing/README.md

@@ -48,3 +48,20 @@ lb.addListener({
   allowConnectionsFrom: [mySecurityGroup],
 });
 ```
+
+### Adding Ec2 Instance as a target for the load balancer
+
+You can add an EC2 instance to the load balancer by calling using `new InstanceTarget` as the argument to `addTarget()`:
+
+```ts
+const lb = new elb.LoadBalancer(this, 'LB', {
+  vpc,
+});
+// instance to add as the target for load balancer.
+const instance = new Instance(stack, 'targetInstance', {
+  vpc: vpc,
+  instanceType: InstanceType.of(InstanceClass.BURSTABLE2, InstanceSize.MICRO),
+  machineImage: new AmazonLinuxImage(),
+});
+lb.addTarget(elb.InstanceTarget(instance));
+```

packages/@aws-cdk/aws-elasticloadbalancing/lib/load-balancer.ts

@@ -1,5 +1,5 @@
 import {
-  Connections, IConnectable, ISecurityGroup, IVpc, Peer, Port,
+  Connections, IConnectable, Instance, ISecurityGroup, IVpc, Peer, Port,
   SecurityGroup, SelectedSubnets, SubnetSelection, SubnetType,
 } from '@aws-cdk/aws-ec2';
 import { Duration, Lazy, Resource } from '@aws-cdk/core';
@@ -251,20 +251,21 @@ export class LoadBalancer extends Resource implements IConnectable {
 
   private readonly instancePorts: number[] = [];
   private readonly targets: ILoadBalancerTarget[] = [];
+  private readonly instanceIds: string[] = [];
 
   constructor(scope: Construct, id: string, props: LoadBalancerProps) {
     super(scope, id);
 
     this.securityGroup = new SecurityGroup(this, 'SecurityGroup', { vpc: props.vpc, allowAllOutbound: false });
     this.connections = new Connections({ securityGroups: [this.securityGroup] });
-
     // Depending on whether the ELB has public or internal IPs, pick the right backend subnets
     const selectedSubnets: SelectedSubnets = loadBalancerSubnets(props);
 
     this.elb = new CfnLoadBalancer(this, 'Resource', {
       securityGroups: [this.securityGroup.securityGroupId],
       subnets: selectedSubnets.subnetIds,
       listeners: Lazy.any({ produce: () => this.listeners }),
+      instances: Lazy.list({ produce: () => this.instanceIds }, { omitEmpty: true }),
       scheme: props.internetFacing ? 'internet-facing' : 'internal',
       healthCheck: props.healthCheck && healthCheckToJSON(props.healthCheck),
       crossZone: props.crossZone ?? true,
@@ -398,6 +399,33 @@ export class LoadBalancer extends Resource implements IConnectable {
       Port.tcp(instancePort),
       `Port ${instancePort} LB to fleet`);
   }
+
+  /**
+   * Add instance to the load balancer.
+   * @internal
+   */
+  public _addInstanceId(instanceId: string) {
+    this.instanceIds.push(instanceId);
+  }
+}
+
+/**
+ * An EC2 instance that is the target for load balancing
+ */
+export class InstanceTarget implements ILoadBalancerTarget {
+  readonly connections: Connections;
+  /**
+   * Create a new Instance target.
+   *
+   * @param instance Instance to register to.
+   */
+  constructor(public readonly instance: Instance) {
+    this.connections = instance.connections;
+  }
+
+  public attachToClassicLB(loadBalancer: LoadBalancer): void {
+    loadBalancer._addInstanceId(this.instance.instanceId);
+  }
 }
 
 /**

packages/@aws-cdk/aws-elasticloadbalancing/package.json

@@ -83,6 +83,7 @@
     "@aws-cdk/assertions": "0.0.0",
     "@aws-cdk/cdk-build-tools": "0.0.0",
     "@aws-cdk/integ-runner": "0.0.0",
+    "@aws-cdk/integ-tests": "0.0.0",
     "@aws-cdk/cfn2ts": "0.0.0",
     "@aws-cdk/pkglint": "0.0.0",
     "@types/jest": "^27.5.2"

packages/@aws-cdk/aws-elasticloadbalancing/test/integ.instanceTarget.elb.js.snapshot/InstanceTargetTestDefaultTestDeployAssertAF607556.assets.json

@@ -0,0 +1,19 @@
+{
+  "version": "29.0.0",
+  "files": {
+    "21fbb51d7b23f6a6c262b46a9caee79d744a3ac019fd45422d988b96d44b2a22": {
+      "source": {
+        "path": "InstanceTargetTestDefaultTestDeployAssertAF607556.template.json",
+        "packaging": "file"
+      },
+      "destinations": {
+        "current_account-current_region": {
+          "bucketName": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}",
+          "objectKey": "21fbb51d7b23f6a6c262b46a9caee79d744a3ac019fd45422d988b96d44b2a22.json",
+          "assumeRoleArn": "arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-file-publishing-role-${AWS::AccountId}-${AWS::Region}"
+        }
+      }
+    }
+  },
+  "dockerImages": {}
+}

packages/@aws-cdk/aws-elasticloadbalancing/test/integ.instanceTarget.elb.js.snapshot/InstanceTargetTestDefaultTestDeployAssertAF607556.template.json

@@ -0,0 +1,36 @@
+{
+ "Parameters": {
+  "BootstrapVersion": {
+   "Type": "AWS::SSM::Parameter::Value<String>",
+   "Default": "/cdk-bootstrap/hnb659fds/version",
+   "Description": "Version of the CDK Bootstrap resources in this environment, automatically retrieved from SSM Parameter Store. [cdk:skip]"
+  }
+ },
+ "Rules": {
+  "CheckBootstrapVersion": {
+   "Assertions": [
+    {
+     "Assert": {
+      "Fn::Not": [
+       {
+        "Fn::Contains": [
+         [
+          "1",
+          "2",
+          "3",
+          "4",
+          "5"
+         ],
+         {
+          "Ref": "BootstrapVersion"
+         }
+        ]
+       }
+      ]
+     },
+     "AssertDescription": "CDK bootstrap stack version 6 required. Please run 'cdk bootstrap' with a recent version of the CDK CLI."
+    }
+   ]
+  }
+ }
+}

packages/@aws-cdk/aws-elasticloadbalancing/test/integ.instanceTarget.elb.js.snapshot/aws-cdk-elb-instance-target-integ.assets.json

@@ -0,0 +1,19 @@
+{
+  "version": "29.0.0",
+  "files": {
+    "11ca0111a871a53be970c5db0c5a24d4146213fd59f6d172b6fc1bc3de206cf9": {
+      "source": {
+        "path": "aws-cdk-elb-instance-target-integ.template.json",
+        "packaging": "file"
+      },
+      "destinations": {
+        "current_account-current_region": {
+          "bucketName": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}",
+          "objectKey": "11ca0111a871a53be970c5db0c5a24d4146213fd59f6d172b6fc1bc3de206cf9.json",
+          "assumeRoleArn": "arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-file-publishing-role-${AWS::AccountId}-${AWS::Region}"
+        }
+      }
+    }
+  },
+  "dockerImages": {}
+}