fix(cloudwatch): move SNS Alarm Action to aws-cloudwatch-actions (#…

…2688)

In accordance with new guidelines, we're centralizing cross-service
integrations into their own package. In this case, centralizing
CloudWatch Alarm Actions into `@aws-cdk/aws-cloudwatch-actions`.

Not moving AutoScaling classes because doing so would introduce circular
dependencies.

BREAKING CHANGE: using an SNS topic as CloudWatch Alarm Actxion now
requires an integration object from the `@aws-cdk/aws-cloudwatch-actions`
package.

packages/@aws-cdk/aws-applicationautoscaling/lib/step-scaling-action.ts

@@ -1,4 +1,3 @@
-import cloudwatch = require('@aws-cdk/aws-cloudwatch');
 import cdk = require('@aws-cdk/cdk');
 import { CfnScalingPolicy } from './applicationautoscaling.generated';
 import { IScalableTarget } from './scalable-target';
@@ -67,17 +66,12 @@ export interface StepScalingActionProps {
  *
  * This Action must be used as the target of a CloudWatch alarm to take effect.
  */
-export class StepScalingAction extends cdk.Construct implements cloudwatch.IAlarmAction {
+export class StepScalingAction extends cdk.Construct {
   /**
    * ARN of the scaling policy
    */
   public readonly scalingPolicyArn: string;
 
-  /**
-   * ARN when this scaling policy is used as an Alarm action
-   */
-  public readonly alarmActionArn: string;
-
   private readonly adjustments = new Array<CfnScalingPolicy.StepAdjustmentProperty>();
 
   constructor(scope: cdk.Construct, id: string, props: StepScalingActionProps) {
@@ -100,7 +94,6 @@ export class StepScalingAction extends cdk.Construct implements cloudwatch.IAlar
     });
 
     this.scalingPolicyArn = resource.scalingPolicyArn;
-    this.alarmActionArn = this.scalingPolicyArn;
   }
 
   /**

packages/@aws-cdk/aws-applicationautoscaling/lib/step-scaling-policy.ts

@@ -108,7 +108,7 @@ export class StepScalingPolicy extends cdk.Construct {
         evaluationPeriods: 1,
         threshold,
       });
-      this.lowerAlarm.addAlarmAction(this.lowerAction);
+      this.lowerAlarm.addAlarmAction(new StepScalingAlarmAction(this.lowerAction));
     }
 
     if (alarms.upperAlarmIntervalIndex !== undefined) {
@@ -138,7 +138,7 @@ export class StepScalingPolicy extends cdk.Construct {
         evaluationPeriods: 1,
         threshold,
       });
-      this.upperAlarm.addAlarmAction(this.upperAction);
+      this.upperAlarm.addAlarmAction(new StepScalingAlarmAction(this.upperAction));
     }
   }
 }
@@ -191,4 +191,22 @@ function aggregationTypeFromMetric(metric: cloudwatch.Metric): MetricAggregation
     default:
       throw new Error(`Cannot only scale on 'Minimum', 'Maximum', 'Average' metrics, got ${metric.statistic}`);
   }
-}
+}
+
+/**
+ * Use a StepScalingAction as an Alarm Action
+ *
+ * This class is here and not in aws-cloudwatch-actions because this library
+ * needs to use the class, and otherwise we'd have a circular dependency:
+ *
+ * aws-autoscaling -> aws-cloudwatch-actions (for using the Action)
+ * aws-cloudwatch-actions -> aws-autoscaling (for the definition of IStepScalingAction)
+ */
+class StepScalingAlarmAction implements cloudwatch.IAlarmAction {
+  constructor(private readonly stepScalingAction: StepScalingAction) {
+  }
+
+  public bind(_scope: cdk.Construct, _alarm: cloudwatch.IAlarm): cloudwatch.AlarmActionConfig {
+    return { alarmActionArn: this.stepScalingAction.scalingPolicyArn };
+  }
+}