feat(ecs-service-extensions): Target tracking policies for Service Ex…

…tensions (aws#17101)

----
This PR adds `desiredCount`, `targetCpuUtilization` and `targetMemoryUtilization` to the service construct. It also adds `requestsPerTarget` to the `HttpLoadBalancerExtension` props to allow adding target tracking policy based on the ALB request count.

It will be followed by another PR to configure queue auto scaling for the SQS Queues in the `QueueExtension`.

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

packages/@aws-cdk-containers/ecs-service-extensions/README.md

@@ -154,43 +154,68 @@ const nameService = new Service(stack, 'name', {
 });
 ```
 
+## Task Auto-Scaling
+
+You can configure the task count of a service to match demand. The recommended way of achieving this is to configure target tracking policies for your service which scales in and out in order to keep metrics around target values.
+
+You need to configure an auto scaling target for the service by setting the `minTaskCount` (defaults to 1) and `maxTaskCount` in the `Service` construct. Then you can specify target values for "CPU Utilization" or "Memory Utilization" across all tasks in your service. Note that the `desiredCount` value will be set to `undefined` if the auto scaling target is configured.
+
+If you want to configure auto-scaling policies based on resources like Application Load Balancer or SQS Queues, you can set the corresponding resource-specific fields in the extension. For example, you can enable target tracking scaling based on Application Load Balancer request count as follows:
+
+```ts
+const stack = new cdk.Stack();
+const environment = new Environment(stack, 'production');
+const serviceDescription = new ServiceDescription();
+
+serviceDescription.add(new Container({
+  cpu: 256,
+  memoryMiB: 512,
+  trafficPort: 80,
+  image: ecs.ContainerImage.fromRegistry('my-alb'),
+}));
+
+// Add the extension with target `requestsPerTarget` value set
+serviceDescription.add(new HttpLoadBalancerExtension({ requestsPerTarget: 10 }));
+
+// Configure the auto scaling target
+new Service(stack, 'my-service', {
+  environment,
+  serviceDescription,
+  desiredCount: 5,
+  // Task auto-scaling constuct for the service
+  autoScaleTaskCount: {
+    maxTaskCount: 10,
+    targetCpuUtilization: 70,
+    targetMemoryUtilization: 50,
+  },
+});
+```
+
+You can also define your own service extensions for [other auto-scaling policies](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service-auto-scaling.html) for your service by making use of the `scalableTaskCount` attribute of the `Service` class.
+
 ## Creating your own custom `ServiceExtension`
 
 In addition to using the default service extensions that come with this module, you
 can choose to implement your own custom service extensions. The `ServiceExtension`
 class is an abstract class you can implement yourself. The following example
 implements a custom service extension that could be added to a service in order to
-autoscale it based on CPU:
+autoscale it based on scaling intervals of SQS Queue size:
 
 ```ts
 export class MyCustomAutoscaling extends ServiceExtension {
   constructor() {
     super('my-custom-autoscaling');
-  }
-
-  // This function modifies properties of the service prior
-  // to construct creation.
-  public modifyServiceProps(props: ServiceBuild) {
-    return {
-      ...props,
-
-      // Initially launch 10 copies of the service
-      desiredCount: 10
-    } as ServiceBuild;
+    // Scaling intervals for the step scaling policy
+    this.scalingSteps = [{ upper: 0, change: -1 }, { lower: 100, change: +1 }, { lower: 500, change: +5 }];
+    this.sqsQueue = new sqs.Queue(this.scope, 'my-queue');
   }
 
   // This hook utilizes the resulting service construct
   // once it is created
   public useService(service: ecs.Ec2Service | ecs.FargateService) {
-    const scalingTarget = service.autoScaleTaskCount({
-      minCapacity: 5, // Min 5 tasks
-      maxCapacity: 20 // Max 20 tasks
-    });
-
-    scalingTarget.scaleOnCpuUtilization('TargetCpuUtilization50', {
-      targetUtilizationPercent: 50,
-      scaleInCooldown: cdk.Duration.seconds(60),
-      scaleOutCooldown: cdk.Duration.seconds(60),
+    this.parentService.scalableTaskCount.scaleOnMetric('QueueMessagesVisibleScaling', {
+      metric: this.sqsQueue.metricApproximateNumberOfMessagesVisible(),
+      scalingSteps: this.scalingSteps,
     });
   }
 }

packages/@aws-cdk-containers/ecs-service-extensions/lib/extensions/http-load-balancer.ts

@@ -8,16 +8,24 @@ import { ServiceExtension, ServiceBuild } from './extension-interfaces';
 // eslint-disable-next-line no-duplicate-imports, import/order
 import { Construct } from '@aws-cdk/core';
 
+export interface HttpLoadBalancerProps {
+  /**
+   * The number of ALB requests per target.
+   */
+  readonly requestsPerTarget?: number;
+}
 /**
  * This extension add a public facing load balancer for sending traffic
  * to one or more replicas of the application container.
  */
 export class HttpLoadBalancerExtension extends ServiceExtension {
   private loadBalancer!: alb.IApplicationLoadBalancer;
   private listener!: alb.IApplicationListener;
+  private requestsPerTarget?: number;
 
-  constructor() {
+  constructor(props: HttpLoadBalancerProps = {}) {
     super('load-balancer');
+    this.requestsPerTarget = props.requestsPerTarget;
   }
 
   // Before the service is created, go ahead and create the load balancer itself.
@@ -55,10 +63,21 @@ export class HttpLoadBalancerExtension extends ServiceExtension {
 
   // After the service is created add the service to the load balancer's listener
   public useService(service: ecs.Ec2Service | ecs.FargateService) {
-    this.listener.addTargets(this.parentService.id, {
+    const targetGroup = this.listener.addTargets(this.parentService.id, {
       deregistrationDelay: cdk.Duration.seconds(10),
       port: 80,
       targets: [service],
     });
+
+    if (this.requestsPerTarget) {
+      if (!this.parentService.scalableTaskCount) {
+        throw Error(`Auto scaling target for the service '${this.parentService.id}' hasn't been configured. Please use Service construct to configure 'minTaskCount' and 'maxTaskCount'.`);
+      }
+      this.parentService.scalableTaskCount.scaleOnRequestCount(`${this.parentService.id}-target-request-count-${this.requestsPerTarget}`, {
+        requestsPerTarget: this.requestsPerTarget,
+        targetGroup,
+      });
+      this.parentService.enableAutoScalingPolicy();
+    }
   }
 }

packages/@aws-cdk-containers/ecs-service-extensions/lib/service.ts

@@ -30,6 +30,43 @@ export interface ServiceProps {
    * @default - A task role is automatically created for you.
    */
   readonly taskRole?: iam.IRole;
+
+  /**
+   * The desired number of instantiations of the task definition to keep running on the service.
+   *
+   * @default - When creating the service, default is 1; when updating the service, default uses
+   * the current task number.
+   */
+  readonly desiredCount?: number;
+
+  /**
+   * The options for configuring the auto scaling target.
+   */
+  readonly autoScaleTaskCount?: AutoScalingOptions;
+}
+
+export interface AutoScalingOptions {
+  /**
+   * The minimum number of tasks when scaling in.
+   *
+   * @default - 1
+   */
+  readonly minTaskCount?: number;
+
+  /**
+    * The maximum number of tasks when scaling out.
+    */
+  readonly maxTaskCount: number;
+
+  /**
+   * The target value for CPU utilization across all tasks in the service.
+   */
+  readonly targetCpuUtilization?: number;
+
+  /**
+   * The target value for memory utilization across all tasks in the service.
+   */
+  readonly targetMemoryUtilization?: number;
 }
 
 /**
@@ -75,6 +112,17 @@ export class Service extends Construct {
    */
   public readonly environment: IEnvironment;
 
+  /**
+   * The scalable attribute representing task count.
+   */
+  public readonly scalableTaskCount?: ecs.ScalableTaskCount;
+
+  /**
+   * The flag to track if auto scaling policies have been configured
+   * for the service.
+   */
+  private autoScalingPoliciesEnabled: boolean = false;
+
   /**
    * The generated task definition for this service. It is only
    * generated after .prepare() has been executed.
@@ -160,14 +208,17 @@ export class Service extends Construct {
       }
     }
 
+    // Set desiredCount to `undefined` if auto scaling is configured for the service
+    const desiredCount = props.autoScaleTaskCount ? undefined : (props.desiredCount || 1);
+
     // Give each extension a chance to mutate the service props before
     // service creation
     let serviceProps = {
       cluster: this.cluster,
       taskDefinition: this.taskDefinition,
       minHealthyPercent: 100,
       maxHealthyPercent: 200,
-      desiredCount: 1,
+      desiredCount,
     } as ServiceBuild;
 
     for (const extensions in this.serviceDescription.extensions) {
@@ -219,12 +270,41 @@ export class Service extends Construct {
       throw new Error(`Unknown capacity type for service ${this.id}`);
     }
 
+    // Create the auto scaling target and configure target tracking policies after the service is created
+    if (props.autoScaleTaskCount) {
+      this.scalableTaskCount = this.ecsService.autoScaleTaskCount({
+        maxCapacity: props.autoScaleTaskCount.maxTaskCount,
+        minCapacity: props.autoScaleTaskCount.minTaskCount,
+      });
+
+      if (props.autoScaleTaskCount.targetCpuUtilization) {
+        const targetUtilizationPercent = props.autoScaleTaskCount.targetCpuUtilization;
+        this.scalableTaskCount.scaleOnCpuUtilization(`${this.id}-target-cpu-utilization-${targetUtilizationPercent}`, {
+          targetUtilizationPercent,
+        });
+        this.enableAutoScalingPolicy();
+      }
+
+      if (props.autoScaleTaskCount.targetMemoryUtilization) {
+        const targetUtilizationPercent = props.autoScaleTaskCount.targetMemoryUtilization;
+        this.scalableTaskCount.scaleOnMemoryUtilization(`${this.id}-target-memory-utilization-${targetUtilizationPercent}`, {
+          targetUtilizationPercent,
+        });
+        this.enableAutoScalingPolicy();
+      }
+    }
+
     // Now give all extensions a chance to use the service
     for (const extensions in this.serviceDescription.extensions) {
       if (this.serviceDescription.extensions[extensions]) {
         this.serviceDescription.extensions[extensions].useService(this.ecsService);
       }
     }
+
+    // Error out if the auto scaling target is created but no scaling policies have been configured
+    if (this.scalableTaskCount && !this.autoScalingPoliciesEnabled) {
+      throw Error(`The auto scaling target for the service '${this.id}' has been created but no auto scaling policies have been configured.`);
+    }
   }
 
   /**
@@ -266,4 +346,12 @@ export class Service extends Construct {
 
     return this.urls[urlName];
   }
+
+  /**
+   * This helper method is used to set the `autoScalingPoliciesEnabled` attribute
+   * whenever an auto scaling policy is configured for the service.
+   */
+  public enableAutoScalingPolicy() {
+    this.autoScalingPoliciesEnabled = true;
+  }
 }

packages/@aws-cdk-containers/ecs-service-extensions/test/http-load-balancer.test.ts

@@ -72,4 +72,72 @@ describe('http load balancer', () => {
 
   });
 
+  test('allows scaling on request count for the HTTP load balancer', () => {
+    // GIVEN
+    const stack = new cdk.Stack();
+
+    // WHEN
+    const environment = new Environment(stack, 'production');
+    const serviceDescription = new ServiceDescription();
+
+    serviceDescription.add(new Container({
+      cpu: 256,
+      memoryMiB: 512,
+      trafficPort: 80,
+      image: ecs.ContainerImage.fromRegistry('nathanpeck/name'),
+    }));
+
+    serviceDescription.add(new HttpLoadBalancerExtension({ requestsPerTarget: 100 }));
+
+    new Service(stack, 'my-service', {
+      environment,
+      serviceDescription,
+      autoScaleTaskCount: {
+        maxTaskCount: 5,
+      },
+    });
+
+    // THEN
+    expect(stack).toHaveResourceLike('AWS::ApplicationAutoScaling::ScalableTarget', {
+      MaxCapacity: 5,
+      MinCapacity: 1,
+    });
+
+    expect(stack).toHaveResourceLike('AWS::ApplicationAutoScaling::ScalingPolicy', {
+      PolicyType: 'TargetTrackingScaling',
+      TargetTrackingScalingPolicyConfiguration: {
+        PredefinedMetricSpecification: {
+          PredefinedMetricType: 'ALBRequestCountPerTarget',
+        },
+        TargetValue: 100,
+      },
+    });
+  });
+
+  test('should error when adding scaling policy if scaling target has not been configured', () => {
+    // GIVEN
+    const stack = new cdk.Stack();
+
+    // WHEN
+    const environment = new Environment(stack, 'production');
+    const serviceDescription = new ServiceDescription();
+
+    serviceDescription.add(new Container({
+      cpu: 256,
+      memoryMiB: 512,
+      trafficPort: 80,
+      image: ecs.ContainerImage.fromRegistry('nathanpeck/name'),
+    }));
+
+    serviceDescription.add(new HttpLoadBalancerExtension({ requestsPerTarget: 100 }));
+
+    // THEN
+    expect(() => {
+      new Service(stack, 'my-service', {
+        environment,
+        serviceDescription,
+      });
+    }).toThrow(/Auto scaling target for the service 'my-service' hasn't been configured. Please use Service construct to configure 'minTaskCount' and 'maxTaskCount'./);
+  });
+
 });