fix(elb): allow inverting load balancer task count metric statistics (#387)

# Summary

To monitor network load balancer metrics, track the maximum `HealthyHostCount` and the minimum `UnHealthyHostCount` as per the guidelines at
<https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-cloudwatch-metrics.html#metric-statistics>.

Fixes #392

# Tests
```
yarn build
```
The `package:js` build step has succeeded.

---

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

Author: jrchen-amzn

API.md

@@ -102,6 +102,19 @@ interface BaseLoadBalancedEc2ServiceMonitoringProps
     string,
     MinProcessedBytesThreshold
   >;
+
+  /**
+   * Invert the statistics of `HealthyHostCount` and `UnHealthyHostCount`.
+   *
+   * When `invertLoadBalancerTaskCountMetricsStatistics` is set to false, the minimum of `HealthyHostCount` and the maximum of `UnHealthyHostCount` are monitored.
+   * When `invertLoadBalancerTaskCountMetricsStatistics` is set to true, the maximum of `HealthyHostCount` and the minimum of `UnHealthyHostCount` are monitored.
+   *
+   * `invertLoadBalancerTaskCountMetricsStatistics` is recommended to set to true as per the guidelines at
+https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-cloudwatch-metrics.html#metric-statistics
+   *
+   * @default false
+   */
+  readonly invertLoadBalancerTaskCountMetricsStatistics?: boolean;
 }
 
 /**
@@ -187,7 +200,8 @@ export class Ec2ServiceMonitoring extends Monitoring {
       this.loadBalancerMetricFactory = createLoadBalancerMetricFactory(
         this.metricFactory,
         props.loadBalancer!,
-        props.targetGroup!
+        props.targetGroup!,
+        props.invertLoadBalancerTaskCountMetricsStatistics
       );
       this.healthyTaskCountMetric =
         this.loadBalancerMetricFactory.metricHealthyTaskCount();

lib/monitoring/aws-ecs-patterns/Ec2ServiceMonitoring.ts

@@ -102,6 +102,19 @@ interface BaseLoadBalancedFargateServiceMonitoringProps
     string,
     MinProcessedBytesThreshold
   >;
+
+  /**
+   * Invert the statistics of `HealthyHostCount` and `UnHealthyHostCount`.
+   *
+   * When `invertLoadBalancerTaskCountMetricsStatistics` is set to false, the minimum of `HealthyHostCount` and the maximum of `UnHealthyHostCount` are monitored.
+   * When `invertLoadBalancerTaskCountMetricsStatistics` is set to true, the maximum of `HealthyHostCount` and the minimum of `UnHealthyHostCount` are monitored.
+   *
+   * `invertLoadBalancerTaskCountMetricsStatistics` is recommended to set to true as per the guidelines at
+https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-cloudwatch-metrics.html#metric-statistics
+   *
+   * @default false
+   */
+  readonly invertLoadBalancerTaskCountMetricsStatistics?: boolean;
 }
 
 /**
@@ -190,7 +203,8 @@ export class FargateServiceMonitoring extends Monitoring {
       this.loadBalancerMetricFactory = createLoadBalancerMetricFactory(
         this.metricFactory,
         props.loadBalancer!,
-        props.targetGroup!
+        props.targetGroup!,
+        props.invertLoadBalancerTaskCountMetricsStatistics
       );
       this.healthyTaskCountMetric =
         this.loadBalancerMetricFactory.metricHealthyTaskCount();

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

@@ -3,7 +3,10 @@ import {
   IApplicationTargetGroup,
 } from "aws-cdk-lib/aws-elasticloadbalancingv2";
 
-import { ILoadBalancerMetricFactory } from "./LoadBalancerMetricFactory";
+import {
+  ILoadBalancerMetricFactory,
+  BaseLoadBalancerMetricFactoryProps,
+} from "./LoadBalancerMetricFactory";
 import {
   HealthyMetricColor,
   MetricFactory,
@@ -14,7 +17,8 @@ import {
 /**
  * Props to create ApplicationLoadBalancerMetricFactory.
  */
-export interface ApplicationLoadBalancerMetricFactoryProps {
+export interface ApplicationLoadBalancerMetricFactoryProps
+  extends BaseLoadBalancerMetricFactoryProps {
   readonly applicationLoadBalancer: IApplicationLoadBalancer;
   readonly applicationTargetGroup: IApplicationTargetGroup;
 }
@@ -28,6 +32,7 @@ export class ApplicationLoadBalancerMetricFactory
   protected readonly metricFactory: MetricFactory;
   protected readonly applicationLoadBalancer: IApplicationLoadBalancer;
   protected readonly applicationTargetGroup: IApplicationTargetGroup;
+  protected readonly invertStatisticsOfTaskCountEnabled: boolean;
 
   constructor(
     metricFactory: MetricFactory,
@@ -36,14 +41,18 @@ export class ApplicationLoadBalancerMetricFactory
     this.metricFactory = metricFactory;
     this.applicationLoadBalancer = props.applicationLoadBalancer;
     this.applicationTargetGroup = props.applicationTargetGroup;
+    this.invertStatisticsOfTaskCountEnabled =
+      props.invertStatisticsOfTaskCountEnabled ?? false;
   }
 
   metricHealthyTaskCount() {
     return this.metricFactory.adaptMetric(
       this.applicationTargetGroup.metrics.healthyHostCount({
         label: "Healthy Tasks",
         color: HealthyMetricColor,
-        statistic: MetricStatistic.MIN,
+        statistic: this.invertStatisticsOfTaskCountEnabled
+          ? MetricStatistic.MAX
+          : MetricStatistic.MIN,
       })
     );
   }
@@ -53,7 +62,9 @@ export class ApplicationLoadBalancerMetricFactory
       this.applicationTargetGroup.metrics.unhealthyHostCount({
         label: "Unhealthy Tasks",
         color: UnhealthyMetricColor,
-        statistic: MetricStatistic.MAX,
+        statistic: this.invertStatisticsOfTaskCountEnabled
+          ? MetricStatistic.MIN
+          : MetricStatistic.MAX,
       })
     );
   }

lib/monitoring/aws-loadbalancing/ApplicationLoadBalancerMetricFactory.ts

@@ -46,7 +46,8 @@ function isNetworkTargetGroup(
 export function createLoadBalancerMetricFactory(
   metricFactory: MetricFactory,
   loadBalancer: INetworkLoadBalancer | IApplicationLoadBalancer,
-  targetGroup: INetworkTargetGroup | IApplicationTargetGroup
+  targetGroup: INetworkTargetGroup | IApplicationTargetGroup,
+  invertStatisticsOfTaskCountEnabled?: boolean
 ): ILoadBalancerMetricFactory {
   if (
     isNetworkLoadBalancer(loadBalancer) &&
@@ -55,6 +56,7 @@ export function createLoadBalancerMetricFactory(
     return new NetworkLoadBalancerMetricFactory(metricFactory, {
       networkLoadBalancer: loadBalancer,
       networkTargetGroup: targetGroup,
+      invertStatisticsOfTaskCountEnabled: invertStatisticsOfTaskCountEnabled,
     });
   } else if (
     isApplicationLoadBalancer(loadBalancer) &&
@@ -63,6 +65,7 @@ export function createLoadBalancerMetricFactory(
     return new ApplicationLoadBalancerMetricFactory(metricFactory, {
       applicationLoadBalancer: loadBalancer,
       applicationTargetGroup: targetGroup,
+      invertStatisticsOfTaskCountEnabled: invertStatisticsOfTaskCountEnabled,
     });
   } else {
     throw new Error(
@@ -71,6 +74,24 @@ export function createLoadBalancerMetricFactory(
   }
 }
 
+/**
+ * Base of Monitoring props for load-balancer metric factories.
+ */
+export interface BaseLoadBalancerMetricFactoryProps {
+  /**
+   * Invert the statistics of `HealthyHostCount` and `UnHealthyHostCount`.
+   *
+   * When `invertStatisticsOfTaskCountEnabled` is set to false, the minimum of `HealthyHostCount` and the maximum of `UnHealthyHostCount` are monitored.
+   * When `invertStatisticsOfTaskCountEnabled` is set to true, the maximum of `HealthyHostCount` and the minimum of `UnHealthyHostCount` are monitored.
+   *
+   * `invertStatisticsOfTaskCountEnabled` is recommended to set to true as per the guidelines at
+https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-cloudwatch-metrics.html#metric-statistics
+   *
+   * @default false
+   */
+  readonly invertStatisticsOfTaskCountEnabled?: boolean;
+}
+
 /**
  * Common interface for load-balancer based service metric factories.
  */

lib/monitoring/aws-loadbalancing/LoadBalancerMetricFactory.ts

@@ -3,7 +3,10 @@ import {
   INetworkTargetGroup,
 } from "aws-cdk-lib/aws-elasticloadbalancingv2";
 
-import { ILoadBalancerMetricFactory } from "./LoadBalancerMetricFactory";
+import {
+  ILoadBalancerMetricFactory,
+  BaseLoadBalancerMetricFactoryProps,
+} from "./LoadBalancerMetricFactory";
 import {
   HealthyMetricColor,
   MetricFactory,
@@ -14,7 +17,8 @@ import {
 /**
  * Props to create NetworkLoadBalancerMetricFactory.
  */
-export interface NetworkLoadBalancerMetricFactoryProps {
+export interface NetworkLoadBalancerMetricFactoryProps
+  extends BaseLoadBalancerMetricFactoryProps {
   readonly networkLoadBalancer: INetworkLoadBalancer;
   readonly networkTargetGroup: INetworkTargetGroup;
 }
@@ -28,6 +32,7 @@ export class NetworkLoadBalancerMetricFactory
   protected readonly metricFactory: MetricFactory;
   protected readonly networkLoadBalancer: INetworkLoadBalancer;
   protected readonly networkTargetGroup: INetworkTargetGroup;
+  protected readonly invertStatisticsOfTaskCountEnabled: boolean;
 
   constructor(
     metricFactory: MetricFactory,
@@ -36,14 +41,18 @@ export class NetworkLoadBalancerMetricFactory
     this.metricFactory = metricFactory;
     this.networkLoadBalancer = props.networkLoadBalancer;
     this.networkTargetGroup = props.networkTargetGroup;
+    this.invertStatisticsOfTaskCountEnabled =
+      props.invertStatisticsOfTaskCountEnabled ?? false;
   }
 
   metricHealthyTaskCount() {
     return this.metricFactory.adaptMetric(
       this.networkTargetGroup.metrics.healthyHostCount({
         label: "Healthy Tasks",
         color: HealthyMetricColor,
-        statistic: MetricStatistic.MIN,
+        statistic: this.invertStatisticsOfTaskCountEnabled
+          ? MetricStatistic.MAX
+          : MetricStatistic.MIN,
       })
     );
   }
@@ -53,7 +62,9 @@ export class NetworkLoadBalancerMetricFactory
       this.networkTargetGroup.metrics.unHealthyHostCount({
         label: "Unhealthy Tasks",
         color: UnhealthyMetricColor,
-        statistic: MetricStatistic.MAX,
+        statistic: this.invertStatisticsOfTaskCountEnabled
+          ? MetricStatistic.MIN
+          : MetricStatistic.MAX,
       })
     );
   }