AutoScalingGroup.scaleOnMetric with MathExpression as metric created invalid template

[leocurrie]

When attempting to create an Autoscaling Group that uses a Math Expression as the scaling metric, CDK will generate a template that is rejected by CloudFormation.

Reproduction Steps

Starting from the Application Load Balancer Typescript example (https://github.com/aws-samples/aws-cdk-examples/blob/master/typescript/application-load-balancer/index.ts), add the following:

import { MathExpression } from '@aws-cdk/aws-cloudwatch';

asg.scaleOnMetric('MyScalingMetric', {
      metric: new MathExpression({
        expression: 'activeConnections / processedBytes',
        usingMetrics: {
          activeConnections: lb.metricActiveConnectionCount({
            statistic: 'avg',
            period: Duration.minutes(5)
          }),
          processedBytes: lb.metricProcessedBytes({
            statistic: 'avg',
            period: Duration.minutes(5)
          })
        }
      }),
      adjustmentType: AdjustmentType.CHANGE_IN_CAPACITY,
      scalingSteps: [
        { change: -1, lower: 0, upper: 49 },
        { change: 0, lower: 50, upper: 99 },
        { change: 1, lower: 100 }
      ]
    });

(I'm not suggesting that this is a useful metric! It's just an example)
run cdk stack synth to generate the stack.
Note in the generated output, the generated ASGMyScalingMetricUpperAlarm ASGMyScalingMetricLowerAlarm both include the Period property, which according to the docs, should not be supplied for Math expression metrics: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-cw-alarm.html#cfn-cloudwatch-alarms-period

When deploying the stack, the upper and lower alarms will fail to be created with an error.

Error Log

Period should not be set if list of Metrics is set. (Service: AmazonCloudWatch; Status Code: 400; Error Code: ValidationError; Request ID: 869ad658-bae4-4006-b93f-5bd085eeaa17)

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Environment

• **CLI Version : 1.20.0
• **Framework Version: 1.20.0
• **OS : MacOSX
• **Language : Typescript

Other

• Looks like the period is hard-coded in https://github.com/aws/aws-cdk/blob/master/packages/@aws-cdk/aws-autoscaling/lib/step-scaling-policy.ts#L107
• Editing the generated template to remove the Period property from the alarms resolves the problem.

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This is 🐛 Bug Report

[SomayaB]

HI @leocurrie, thanks for reporting this issue. We are actively working on this.

[mordr]

Just to clarify, is this now usable in CFN for target tracking, or only for step scaling? From the doc, seems to suggest no.