Automatically scale a virtual machine scale set with Azure PowerShell

When you create a scale set, you define the number of VM instances that you wish to run. As your application demand changes, you can automatically increase or decrease the number of VM instances. The ability to autoscale lets you keep up with customer demand or respond to application performance changes throughout the lifecycle of your app.

This article shows you how to create autoscale rules with Azure PowerShell that monitor the performance of the VM instances in your scale set. These autoscale rules increase or decrease the number of VM instances in response to these performance metrics. You can also complete these steps with the Azure CLI 2.0 or in the Azure portal.

Prerequisites

To create autoscale rules, you need an existing virtual machine scale set. You can create a scale set with the Azure portal, Azure PowerShell, or Azure CLI 2.0.

To make it easier to create the autoscale rules, define some variables for your scale set. The following example defines variables for the scale set named myScaleSet in the resource group named myResourceGroup and in the East US region. Your subscription ID is obtained with Get-AzureRmSubscription. If you have multiple subscriptions associated with your account, only the first subscription is returned. Adjust the names and subscription ID as follows:

$mySubscriptionId = (Get-AzureRmSubscription).Id
$myResourceGroup = "myResourceGroupScaleSet"
$myScaleSet = "myScaleSet"
$myLocation = "East US"

Create a rule to automatically scale out

If your application demand increases, the load on the VM instances in your scale set increases. If this increased load is consistent, rather than just a brief demand, you can configure autoscale rules to increase the number of VM instances in the scale set. When these VM instances are created and your applications are deployed, the scale set starts to distribute traffic to them through the load balancer. You control what metrics to monitor, such as CPU or disk, how long the application load must meet a given threshold, and how many VM instances to add to the scale set.

Let's create a rule with New-AzureRmAutoscaleRule that increases the number of VM instances in a scale set when the average CPU load is greater than 70% over a 5-minute period. When the rule triggers, the number of VM instances is increased by 20%. In scale sets with a small number of VM instances, you could leave out -ScaleActionScaleType and only specify -ScaleActionValue to increase by 1 or 2 instances. In scale sets with a large number of VM instances, an increase of 10% or 20% VM instances may be more appropriate.

The following parameters are used for this rule:

Parameter Explanation Value
-MetricName The performance metric to monitor and apply scale set actions on. Percentage CPU
-TimeGrain How often the metrics are collected for analysis. 1 minute
-MetricStatistic Defines how the collected metrics should be aggregated for analysis. Average
-TimeWindow The amount of time monitored before the metric and threshold values are compared. 10 minutes
-Operator Operator used to compare the metric data against the threshold. Greater Than
-Threshold The value that causes the autoscale rule to trigger an action. 70%
-ScaleActionDirection Defines if the scale set should scale up or down when the rule applies. Increase
–ScaleActionScaleType Indicates that the number of VM instances should be changed by a percentage amount. Percent Change
-ScaleActionValue The percentage of VM instances should be changed when the rule triggers. 20
-ScaleActionCooldown The amount of time to wait before the rule is applied again so that the autoscale actions have time to take effect. 5 minutes

The following example creates an object named myRuleScaleOut that holds this scale up rule. The -MetricResourceId uses the variables previously defined for the subscription ID, resource group name, and scale set name:

$myRuleScaleOut = New-AzureRmAutoscaleRule `
  -MetricName "Percentage CPU" `
  -MetricResourceId /subscriptions/$mySubscriptionId/resourceGroups/$myResourceGroup/providers/Microsoft.Compute/virtualMachineScaleSets/$myScaleSet `
  -TimeGrain 00:01:00 `
  -MetricStatistic Average `
  -TimeWindow 00:10:00 `
  -Operator GreaterThan `
  -Threshold 70 `
  -ScaleActionDirection Increase `
  –ScaleActionScaleType PercentChangeCount `
  -ScaleActionValue 20 `
  -ScaleActionCooldown 00:05:00

Create a rule to automatically scale in

On an evening or weekend, your application demand may decrease. If this decreased load is consistent over a period of time, you can configure autoscale rules to decrease the number of VM instances in the scale set. This scale-in action reduces the cost to run your scale set as you only run the number of instances required to meet the current demand.

Create a rule with New-AzureRmAutoscaleRule that decreases the number of VM instances in a scale set when the average CPU load then drops below 30% over a 10-minute period. When the rule triggers, the number of VM instances is decreased by 20%.The following example creates an object named myRuleScaleDown that holds this scale up rule. The -MetricResourceId uses the variables previously defined for the subscription ID, resource group name, and scale set name:

$myRuleScaleIn = New-AzureRmAutoscaleRule `
  -MetricName "Percentage CPU" `
  -MetricResourceId /subscriptions/$mySubscriptionId/resourceGroups/$myResourceGroup/providers/Microsoft.Compute/virtualMachineScaleSets/$myScaleSet `
  -Operator LessThan `
  -MetricStatistic Average `
  -Threshold 30 `
  -TimeGrain 00:01:00 `
  -TimeWindow 00:10:00 `
  -ScaleActionCooldown 00:05:00 `
  -ScaleActionDirection Decrease `
  –ScaleActionScaleType PercentChangeCount `
  -ScaleActionValue 20

Define an autoscale profile

To associate your autoscale rules with a scale set, you create a profile. The autoscale profile defines the default, minimum, and maximum scale set capacity, and associates your autoscale rules. Create an autoscale profile with New-AzureRmAutoscaleProfile. The following example sets the default, and minimum, capacity of 2 VM instances, and a maximum of 10. The scale out and scale in rules created in the preceding steps are then attached:

$myScaleProfile = New-AzureRmAutoscaleProfile `
  -DefaultCapacity 2  `
  -MaximumCapacity 10 `
  -MinimumCapacity 2 `
  -Rules $myRuleScaleOut,$myRuleScaleIn `
  -Name "autoprofile"

Apply autoscale rules to a scale set

The final step is to apply the autoscale profile to your scale set. Your scale is then able to automatically scale in or out based on the application demand. Apply the autoscale profile with Add-AzureRmAutoscaleSetting as follows:

Add-AzureRmAutoscaleSetting `
  -Location $myLocation `
  -Name "autosetting" `
  -ResourceGroup $myResourceGroup `
  -TargetResourceId /subscriptions/$mySubscriptionId/resourceGroups/$myResourceGroup/providers/Microsoft.Compute/virtualMachineScaleSets/$myScaleSet `
  -AutoscaleProfiles $myScaleProfile

Monitor number of instances in a scale set

To see the number and status of VM instances, view a list of instances in your scale set with Get-AzureRmVmssVM. The status indicates if the VM instance is provisioning as the scale set automatically scales out, or is deprovisioning as the scale automatically scales in. The following example views the VM instance status for the scale set named myScaleSet in the resource group named myResourceGroup:

Get-AzureRmVmssVM -ResourceGroupName "myResourceGroup" -VMScaleSetName "myScaleSet"

Autoscale based on a schedule

The previous examples automatically scaled a scale set in or out with basic host metrics such as CPU usage. You can also create autoscale rules based on schedules. These schedule-based rules allow you to automatically scale out the number of VM instances ahead of an anticipated increase in application demand, such as core work hours, and then automatically scale in the number of instances at a time that you anticipate less demand, such as the weekend.

To create autoscale rules based on a schedule rather than host metrics, use the Azure portal. Schedule-based rules cannot currently be created with Azure PowerShell.

Next steps

In this article, you learned how to use autoscale rules to scale horizontally and increase or decrease the number of VM instances in your scale set. You can also scale vertically to increase or decrease the VM instance size. For more information, see Vertical autoscale with Virtual Machine Scale sets.

For information on how to manage your VM instances, see Manage virtual machine scale sets with Azure PowerShell.

To learn how to generate alerts when your autoscale rules trigger, see Use autoscale actions to send email and webhook alert notifications in Azure Monitor. You can also Use audit logs to send email and webhook alert notifications in Azure Monitor.