feat(aws-applicationautoscaling): Allow autoscaling with "M out of N" datapoints (#17441)

This PR closes #17433. It adds a `datapointsToAlarm` property to the `StepScalingPolicy` construct which allows auto-scaling activities to trigger when only a portion of the data points in the evaluation periods are breaching.

Motivation: Some metrics may have a certain amount of noise/randomness and in these cases it may make more sense to not require that all data points must be breaching for auto-scaling activity to trigger.
----

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

Author: stephenwiebe

packages/@aws-cdk/aws-applicationautoscaling/README.md

@@ -106,6 +106,37 @@ capacity.scaleOnMetric('ScaleToCPU', {
 The AutoScaling construct library will create the required CloudWatch alarms and
 AutoScaling policies for you.
 
+### Scaling based on multiple datapoints
+
+The Step Scaling configuration above will initiate a scaling event when a single
+datapoint of the scaling metric is breaching a scaling step breakpoint. In cases
+where you might want to initiate scaling actions on a larger number of datapoints
+(ie in order to smooth out randomness in the metric data), you can use the
+optional `evaluationPeriods` and `datapointsToAlarm` properties:
+
+```ts
+declare const capacity: ScalableAttribute;
+declare const cpuUtilization: cloudwatch.Metric;
+
+capacity.scaleOnMetric('ScaleToCPUWithMultipleDatapoints', {
+  metric: cpuUtilization,
+  scalingSteps: [
+    { upper: 10, change: -1 },
+    { lower: 50, change: +1 },
+    { lower: 70, change: +3 },
+  ],
+
+  // if the cpuUtilization metric has a period of 1 minute, then data points
+  // in the last 10 minutes will be evaluated
+  evaluationPeriods: 10,
+
+  // Only trigger a scaling action when 6 datapoints out of the last 10 are
+  // breaching. If this is left unspecified, then ALL datapoints in the 
+  // evaluation period must be breaching to trigger a scaling action
+  datapointsToAlarm: 6
+});
+```
+
 ## Target Tracking Scaling
 
 This type of scaling scales in and out in order to keep a metric (typically

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

@@ -58,10 +58,26 @@ export interface BasicStepScalingPolicyProps {
    * Raising this value can be used to smooth out the metric, at the expense
    * of slower response times.
    *
+   * If `datapointsToAlarm` is not set, then all data points in the evaluation period
+   * must meet the criteria to trigger a scaling action.
+   *
    * @default 1
    */
   readonly evaluationPeriods?: number;
 
+  /**
+   * The number of data points out of the evaluation periods that must be breaching to
+   * trigger a scaling action
+   *
+   * Creates an "M out of N" alarm, where this property is the M and the value set for
+   * `evaluationPeriods` is the N value.
+   *
+   * Only has meaning if `evaluationPeriods != 1`.
+   *
+   * @default `evaluationPeriods`
+   */
+  readonly datapointsToAlarm?: number;
+
   /**
    * Aggregation to apply to all data points over the evaluation periods
    *
@@ -99,6 +115,10 @@ export class StepScalingPolicy extends CoreConstruct {
       throw new Error('You must supply at least 2 intervals for autoscaling');
     }
 
+    if (props.datapointsToAlarm !== undefined && props.datapointsToAlarm < 1) {
+      throw new RangeError(`datapointsToAlarm cannot be less than 1, got: ${props.datapointsToAlarm}`);
+    }
+
     const adjustmentType = props.adjustmentType || AdjustmentType.CHANGE_IN_CAPACITY;
     const changesAreAbsolute = adjustmentType === AdjustmentType.EXACT_CAPACITY;
 
@@ -130,6 +150,7 @@ export class StepScalingPolicy extends CoreConstruct {
         alarmDescription: 'Lower threshold scaling alarm',
         comparisonOperator: cloudwatch.ComparisonOperator.LESS_THAN_OR_EQUAL_TO_THRESHOLD,
         evaluationPeriods: props.evaluationPeriods ?? 1,
+        datapointsToAlarm: props.datapointsToAlarm,
         threshold,
       });
       this.lowerAlarm.addAlarmAction(new StepScalingAlarmAction(this.lowerAction));
@@ -160,6 +181,7 @@ export class StepScalingPolicy extends CoreConstruct {
         alarmDescription: 'Upper threshold scaling alarm',
         comparisonOperator: cloudwatch.ComparisonOperator.GREATER_THAN_OR_EQUAL_TO_THRESHOLD,
         evaluationPeriods: props.evaluationPeriods ?? 1,
+        datapointsToAlarm: props.datapointsToAlarm,
         threshold,
       });
       this.upperAlarm.addAlarmAction(new StepScalingAlarmAction(this.upperAction));

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

@@ -227,6 +227,62 @@ describe('step scaling policy', () => {
 
 
   });
+
+  test('step scaling with evaluation period & data points to alarm configured', () => {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const target = createScalableTarget(stack);
+
+    // WHEN
+    target.scaleOnMetric('Tracking', {
+      metric: new cloudwatch.Metric({ namespace: 'Test', metricName: 'Metric', statistic: 'p99' }),
+      scalingSteps: [
+        { upper: 0, change: -1 },
+        { lower: 100, change: +1 },
+        { lower: 500, change: +5 },
+      ],
+      evaluationPeriods: 10,
+      datapointsToAlarm: 6,
+      metricAggregationType: appscaling.MetricAggregationType.MAXIMUM,
+    });
+
+    // THEN
+    expect(stack).toHaveResourceLike('AWS::ApplicationAutoScaling::ScalingPolicy', {
+      PolicyType: 'StepScaling',
+      StepScalingPolicyConfiguration: {
+        AdjustmentType: 'ChangeInCapacity',
+        MetricAggregationType: 'Maximum',
+      },
+    });
+    expect(stack).toHaveResource('AWS::CloudWatch::Alarm', {
+      ComparisonOperator: 'GreaterThanOrEqualToThreshold',
+      EvaluationPeriods: 10,
+      DatapointsToAlarm: 6,
+      ExtendedStatistic: 'p99',
+      MetricName: 'Metric',
+      Namespace: 'Test',
+      Threshold: 100,
+    });
+  });
+
+  test('step scaling with invalid datapointsToAlarm throws error', () => {
+    const stack = new cdk.Stack();
+    const target = createScalableTarget(stack);
+
+    expect(() => {
+      target.scaleOnMetric('Tracking', {
+        metric: new cloudwatch.Metric({ namespace: 'Test', metricName: 'Metric', statistic: 'p99' }),
+        scalingSteps: [
+          { upper: 0, change: -1 },
+          { lower: 100, change: +1 },
+          { lower: 500, change: +5 },
+        ],
+        evaluationPeriods: 10,
+        datapointsToAlarm: 0,
+        metricAggregationType: appscaling.MetricAggregationType.MAXIMUM,
+      });
+    }).toThrow('datapointsToAlarm cannot be less than 1, got: 0');
+  });
 });
 
 /**