feat(ecs-patterns): improve ALB/NLB type guards (#480)

Fixes #476

This is an alternative to PR #477 without a breaking API change.

---

_By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license_

Author: Eugene Cheung

.vscode/settings.json

@@ -1,6 +1,6 @@
 {
   "editor.codeActionsOnSave": {
-    "source.fixAll.eslint": true
+    "source.fixAll.eslint": "explicit"
   },
   "eslint.validate": ["javascript", "typescript"]
 }

lib/monitoring/aws-loadbalancing/LoadBalancerMetricFactory.ts

@@ -12,29 +12,27 @@ import { MetricFactory, MetricWithAlarmSupport } from "../../common";
 // It's not possible to use instanceOf with typescript interfaces, so we have to use type guards to differentiate
 // between the interfaces. As another problem, the LoadBalancer/TargetGroup for both Network and Application types
 // don't have distinguished fields that could be used to differentiate between both types, so we resort to using
-// the name of the class below.
-//
-// Ideally the 2 interfaces would provide a specific field to distinguish both types.
+// checking for unique methods in their metrics interfaces.
 function isApplicationLoadBalancer(
   loadBalancer: INetworkLoadBalancer | IApplicationLoadBalancer
 ): loadBalancer is IApplicationLoadBalancer {
-  return loadBalancer.constructor.name.indexOf("Application") != -1;
+  return !!(loadBalancer.metrics as any).httpRedirectCount;
 }
 function isNetworkLoadBalancer(
   loadBalancer: INetworkLoadBalancer | IApplicationLoadBalancer
 ): loadBalancer is INetworkLoadBalancer {
-  return loadBalancer.constructor.name.indexOf("Network") != -1;
+  return !isApplicationLoadBalancer(loadBalancer);
 }
 
 function isApplicationTargetGroup(
   targetGroup: INetworkTargetGroup | IApplicationTargetGroup
 ): targetGroup is IApplicationTargetGroup {
-  return targetGroup.constructor.name.indexOf("Application") != -1;
+  return !!(targetGroup.metrics as any).httpCodeTarget;
 }
 function isNetworkTargetGroup(
   targetGroup: INetworkTargetGroup | IApplicationTargetGroup
 ): targetGroup is INetworkTargetGroup {
-  return targetGroup.constructor.name.indexOf("Network") != -1;
+  return !isApplicationTargetGroup(targetGroup);
 }
 
 /**

test/monitoring/aws-ecs-patterns/FargateServiceMonitoring.test.ts

@@ -11,7 +11,10 @@ import {
   ApplicationLoadBalancedFargateService,
   NetworkLoadBalancedFargateService,
 } from "aws-cdk-lib/aws-ecs-patterns";
-import { NetworkLoadBalancer } from "aws-cdk-lib/aws-elasticloadbalancingv2";
+import {
+  NetworkLoadBalancer,
+  NetworkTargetGroup,
+} from "aws-cdk-lib/aws-elasticloadbalancingv2";
 
 import { AlarmWithAnnotation, FargateServiceMonitoring } from "../../../lib";
 import { addMonitoringDashboardsToStack } from "../../utils/SnapshotUtil";
@@ -483,3 +486,46 @@ test("snapshot test: with imported service", () => {
   addMonitoringDashboardsToStack(stack, monitoring);
   expect(Template.fromStack(stack)).toMatchSnapshot();
 });
+
+test("snapshot test: with imported NLB", () => {
+  const stack = new Stack();
+
+  const scope = new TestMonitoringScope(stack, "Scope");
+
+  const cluster = new Cluster(stack, "Cluster");
+  const image = new EcrImage(new Repository(stack, "Repository"), "DummyImage");
+  const taskDefinition = new FargateTaskDefinition(stack, "TaskDef", {});
+
+  taskDefinition
+    .addContainer("Container", { image })
+    .addPortMappings({ containerPort: 8080 });
+
+  const fargateService = new FargateService(stack, "Service", {
+    cluster,
+    taskDefinition,
+  });
+  const networkLoadBalancer =
+    NetworkLoadBalancer.fromNetworkLoadBalancerAttributes(stack, "NLB", {
+      loadBalancerArn:
+        "arn:aws:elasticloadbalancing:us-west-2:123456789012:loadbalancer/net/LoadBalancer/123",
+    });
+  const networkTargetGroup = NetworkTargetGroup.fromTargetGroupAttributes(
+    stack,
+    "NTG",
+    {
+      targetGroupArn:
+        "arn:aws:elasticloadbalancing:us-west-2:123456789012:targetgroup/TargetGroup/123",
+      loadBalancerArns: networkLoadBalancer.loadBalancerArn,
+    }
+  );
+
+  const monitoring = new FargateServiceMonitoring(scope, {
+    fargateService,
+    loadBalancer: networkLoadBalancer,
+    targetGroup: networkTargetGroup,
+    alarmFriendlyName: "DummyFargateService",
+  });
+
+  addMonitoringDashboardsToStack(stack, monitoring);
+  expect(Template.fromStack(stack)).toMatchSnapshot();
+});