feat(elasticloadbalancing): add crossZone load balancing (#2787)

Defaults to cross-zone load balancing for a Classic Load Balancer by default, can be switched
off using 'crossZone' property.

Closes #2786.

Author: ScOut3R

packages/@aws-cdk/aws-autoscaling/test/integ.asg-w-classic-loadbalancer.expected.json

@@ -691,6 +691,7 @@
             "Protocol": "http"
           }
         ],
+        "CrossZone": true,
         "HealthCheck": {
           "HealthyThreshold": "2",
           "Interval": "30",

packages/@aws-cdk/aws-codedeploy/test/server/integ.deployment-group.expected.json

@@ -725,6 +725,7 @@
             "Protocol": "http"
           }
         ],
+        "CrossZone": true,
         "Scheme": "internal",
         "SecurityGroups": [
           {

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

@@ -49,6 +49,16 @@ export interface LoadBalancerProps {
    * @default - None.
    */
   readonly healthCheck?: HealthCheck;
+
+  /**
+   * Whether cross zone load balancing is enabled
+   *
+   * This controls whether the load balancer evenly distributes requests
+   * across each availability zone
+   *
+   * @default true
+   */
+  readonly crossZone?: boolean;
 }
 
 /**
@@ -226,6 +236,7 @@ export class LoadBalancer extends Resource implements IConnectable {
       listeners: Lazy.anyValue({ produce: () => this.listeners }),
       scheme: props.internetFacing ? 'internet-facing' : 'internal',
       healthCheck: props.healthCheck && healthCheckToJSON(props.healthCheck),
+      crossZone: (props.crossZone === undefined || props.crossZone) ? true : false
     });
     if (props.internetFacing) {
       this.elb.node.addDependency(...subnets.map(s => s.internetConnectivityEstablished));

packages/@aws-cdk/aws-elasticloadbalancing/test/integ.elb.expected.json

@@ -228,6 +228,7 @@
             "Protocol": "http"
           }
         ],
+        "CrossZone": true,
         "HealthCheck": {
           "HealthyThreshold": "2",
           "Interval": "30",
@@ -255,4 +256,4 @@
       ]
     }
   }
-}
+}

packages/@aws-cdk/aws-elasticloadbalancing/test/test.loadbalancer.ts

@@ -93,7 +93,64 @@ export = {
     }));
 
     test.done();
-  }
+  },
+
+  'enable cross zone load balancing'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+    const vpc = new Vpc(stack, 'VCP');
+
+    // WHEN
+    new LoadBalancer(stack, 'LB', {
+      vpc,
+      crossZone: true,
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::ElasticLoadBalancing::LoadBalancer', {
+      CrossZone: true
+    }));
+
+    test.done();
+  },
+
+  'disable cross zone load balancing'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+    const vpc = new Vpc(stack, 'VCP');
+
+    // WHEN
+    new LoadBalancer(stack, 'LB', {
+      vpc,
+      crossZone: false,
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::ElasticLoadBalancing::LoadBalancer', {
+      CrossZone: false
+    }));
+
+    test.done();
+  },
+
+  'cross zone load balancing enabled by default'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+    const vpc = new Vpc(stack, 'VCP');
+
+    // WHEN
+    new LoadBalancer(stack, 'LB', {
+      vpc,
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::ElasticLoadBalancing::LoadBalancer', {
+      CrossZone: true
+    }));
+
+    test.done();
+  },
+
 };
 
 class FakeTarget implements ILoadBalancerTarget {