fix(appscaling): fix StepScaling (#2522)

Add ScalingTargetId and default AdjustmentType.

Author: piradeepk

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

@@ -83,9 +83,13 @@ export class StepScalingAction extends cdk.Construct implements cloudwatch.IAlar
   constructor(scope: cdk.Construct, id: string, props: StepScalingActionProps) {
     super(scope, id);
 
+    // Cloudformation requires either the ResourceId, ScalableDimension, and ServiceNamespace
+    // properties, or the ScalingTargetId property, but not both.
+    // https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-applicationautoscaling-scalingpolicy.html
     const resource = new CfnScalingPolicy(this, 'Resource', {
       policyName: props.policyName || this.node.uniqueId,
       policyType: 'StepScaling',
+      scalingTargetId: props.scalingTarget.scalableTargetId,
       stepScalingPolicyConfiguration: {
         adjustmentType: props.adjustmentType,
         cooldown: props.cooldownSec,

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

@@ -85,7 +85,7 @@ export class StepScalingPolicy extends cdk.Construct {
       const threshold = intervals[alarms.lowerAlarmIntervalIndex].upper;
 
       this.lowerAction = new StepScalingAction(this, 'LowerPolicy', {
-        adjustmentType: props.adjustmentType,
+        adjustmentType,
         cooldownSec: props.cooldownSec,
         metricAggregationType: aggregationTypeFromMetric(props.metric),
         minAdjustmentMagnitude: props.minAdjustmentMagnitude,
@@ -115,7 +115,7 @@ export class StepScalingPolicy extends cdk.Construct {
       const threshold = intervals[alarms.upperAlarmIntervalIndex].lower;
 
       this.upperAction = new StepScalingAction(this, 'UpperPolicy', {
-        adjustmentType: props.adjustmentType,
+        adjustmentType,
         cooldownSec: props.cooldownSec,
         metricAggregationType: aggregationTypeFromMetric(props.metric),
         minAdjustmentMagnitude: props.minAdjustmentMagnitude,

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

@@ -1,3 +1,4 @@
+import { expect, haveResource } from '@aws-cdk/assert';
 import { SynthUtils } from '@aws-cdk/assert';
 import cloudwatch = require('@aws-cdk/aws-cloudwatch');
 import cdk = require('@aws-cdk/cdk');
@@ -116,6 +117,43 @@ export = {
 
     test.done();
   },
+
+  'test step scaling on metric'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const target = createScalableTarget(stack);
+
+    // WHEN
+    target.scaleOnMetric('Tracking', {
+      metric: new cloudwatch.Metric({ namespace: 'Test', metricName: 'Metric' }),
+      scalingSteps: [
+        { upper: 0, change: -1 },
+        { lower: 100, change: +1 },
+        { lower: 500, change: +5 }
+      ]
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::ApplicationAutoScaling::ScalingPolicy', {
+      PolicyType: "StepScaling",
+      ScalingTargetId: {
+        Ref: "Target3191CF44"
+      },
+      StepScalingPolicyConfiguration: {
+        AdjustmentType: "ChangeInCapacity",
+        MetricAggregationType: "Average",
+        StepAdjustments: [
+          {
+            MetricIntervalUpperBound: 0,
+            ScalingAdjustment: -1
+          }
+        ]
+      }
+
+    }));
+
+    test.done();
+  }
 };
 
 /**