[ecs] Step Scaling policy does not create all steps

[kyler-hyuna]

I'm using the scaleOnMetric method of the FargateService construct. My requirements need 2 policies.

construct.scaling.scaleOnMetric('scale-up-policy', {
  scalingSteps: [
    {
      lower: 65,
      change: +2,
    },
    {
      lower: 85,
      change: +3,
    },
  ],
  metric: construct.service.metricCpuUtilization({
    statistic: 'Maximum',
    period: cdk.Duration.minutes(1),
  }),
  cooldown: cdk.Duration.minutes(2),
});

construct.scaling.scaleOnMetric('scale-down-policy', {
  scalingSteps: [
    {
      upper: 50,
      change: -1,
    },
    {
      upper: 40,
      change: -2,
    },
  ],
  metric: construct.service.metricCpuUtilization({
    statistic: 'Average',
  }),
});

However it never creates the 65 step. It only creates >85 step.

Reproduction Steps

• Create a FargateService
• Add autoscaling
• Make step scaling policies as outlined above

What did you expect to happen?

I should have 2 alarms created with the actions:

• low alarm

  • remove 1 task < 50% CPU
  • remove 2 tasks < 40 % CPU

• high alarm

  • add 2 tasks > 65% CPU
  • add 3 tasks > 85% CPU

What actually happened?

Environment

• CLI Version : 1.61.1
• Framework Version: 1.61.1
• Node.js Version: v14.8.0
• OS : mac os
• Language (Version): typescript

Other

---

This is 🐛 Bug Report

[kyler-hyuna]

Even worse I added in the 65 step twice to test, and now it works. This is definitely a bug

construct.scaling.scaleOnMetric('scale-up-policy', {
  scalingSteps: [
    {
      lower: 65,
      change: +2,
    },
    {
      lower: 65, // Duplicate of above
      change: +2,
    },
    {
      lower: 85,
      change: +3,
    },
  ],
  metric: construct.service.metricCpuUtilization({
    statistic: 'Maximum',
    period: cdk.Duration.minutes(1),
  }),
  cooldown: cdk.Duration.minutes(2),
});

[olivier-schmitt]

Same issue in with the Python version; it's a very annoying bug.
Is there a workaround?

[kyler-hyuna]

@olivier-schmitt try this

Even worse I added in the 65 step twice to test, and now it works. This is definitely a bug

construct.scaling.scaleOnMetric('scale-up-policy', {
  scalingSteps: [
    {
      lower: 65,
      change: +2,
    },
    {
      lower: 65, // Duplicate of above
      change: +2,
    },
    {
      lower: 85,
      change: +3,
    },
  ],
  metric: construct.service.metricCpuUtilization({
    statistic: 'Maximum',
    period: cdk.Duration.minutes(1),
  }),
  cooldown: cdk.Duration.minutes(2),
});

[SoManyHs]

Hi @kyler-hyuna ,

What happens when you use the aws cli to describe the service/autoscaling policy?

[kyler-hyuna]

Hey @SoManyHs

The code:

scaleOnMetric('scale-up-policy', {
    scalingSteps: [
      {
        lower: 40,
        change: +3,
      },
      {
        // FIXME: https://github.com/aws/aws-cdk/issues/10141
        lower: 40,
        change: +3,
      },
      {
        lower: 60,
        change: +5,
      },
    ],
    metric: construct.service.metricCpuUtilization({
      statistic: 'Maximum',
      period: cdk.Duration.minutes(1),
    }),
    cooldown: cdk.Duration.minutes(2),
  });

The describe (just kept the scaling part of it):

"StepScalingPolicyConfiguration": {
    "AdjustmentType": "ChangeInCapacity",
    "StepAdjustments": [
        {
            "MetricIntervalLowerBound": 0.0,
            "MetricIntervalUpperBound": 20.0,
            "ScalingAdjustment": 3
        },
        {
            "MetricIntervalLowerBound": 20.0,
            "ScalingAdjustment": 5
        }
    ],
    "Cooldown": 120,
    "MetricAggregationType": "Maximum"
}

[coleman-c]

We are seeing similar unexpected behaviors. I'm using the .netcore CDK version 1.76.0, and nodejs v10.19.0

The configuration

[ { "Lower": 300, "Change": 1 }, { "Lower": 600, "Change": 2 }, { "Lower": 900, "Change": 3 } ]

Results in the following:

[
  {
    "MetricIntervalLowerBound": 0,
    "MetricIntervalUpperBound": 300,
    "ScalingAdjustment": 2
  },
  {
    "MetricIntervalLowerBound": 300,
    "ScalingAdjustment": 3
  }
]

Wheras I think is should be:

[
  {
    "MetricIntervalLowerBound": 300,
    "MetricIntervalUpperBound": 600,
    "ScalingAdjustment": 1
  },
  {
    "MetricIntervalLowerBound": 600,
    "MetricIntervalUpperBound": 900,
    "ScalingAdjustment": 2
  },
  {
    "MetricIntervalLowerBound": 900,
    "ScalingAdjustment": 3
  }
]

[YuryShchanouskiTR]

Any updates?

[matheushent]

Any update on it? I'm facing this.

[ruancomelli]

Hi @SoManyHs
What should be the expected behaviour when the user does not provide any intervals starting from lower=0? Should we prepend a dummy scaling interval that takes no actions (that is, change=undefined)? If so, I can readily work on a PR for that.

[github-actions]

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[kyler-hyuna]

Thanks for fixing this @ruancomelli 🎉