Tutorial: Automatically scale a Virtual Machine Scale Set with the Azure CLI

Article
08/19/2024

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. In this tutorial you learn how to:

Use autoscale with a scale set
Create and use autoscale rules
Stress-test VM instances and trigger autoscale rules
Autoscale back in as demand is reduced

If you don't have an Azure subscription, create a trial account before you begin.

Prerequisites

If you prefer to run CLI reference commands locally, install the Azure CLI. If you're running on Windows or macOS, consider running Azure CLI in a Docker container. For more information, see How to run the Azure CLI in a Docker container.
- If you're using a local installation, sign in to the Azure CLI by using the az login command. To finish the authentication process, follow the steps displayed in your terminal. For other sign-in options, see Sign in with the Azure CLI.
- When you're prompted, install the Azure CLI extension on first use. For more information about extensions, see Use extensions with the Azure CLI.
- Run az version to find the version and dependent libraries that are installed. To upgrade to the latest version, run az upgrade.

This tutorial requires version 2.0.32 or later of the Azure CLI.

Create a scale set

Create a resource group with az group create.

az group create --name myResourceGroup --location chinanorth2

Now create a Virtual Machine Scale Set with az vmss create. The following example creates a scale set with an instance count of 2, and generates SSH keys if they don't exist.

az vmss create \
  --resource-group myResourceGroup \
  --name myScaleSet \
  --image <SKU image> \
  --orchestration-mode Flexible \
  --instance-count 2 \
  --admin-username azureuser \
  --generate-ssh-keys

Define an autoscale profile

To enable autoscale on a scale set, you first define an autoscale profile. This profile defines the default, minimum, and maximum scale set capacity. These limits let you control cost by not continually creating VM instances, and balance acceptable performance with a minimum number of instances that remain in a scale-in event. Create an autoscale profile with az monitor autoscale create. The following example sets the default, and minimum, capacity of 2 VM instances, and a maximum of 10:

az monitor autoscale create \
  --resource-group myResourceGroup \
  --resource myScaleSet \
  --resource-type Microsoft.Compute/virtualMachineScaleSets \
  --name autoscale \
  --min-count 2 \
  --max-count 10 \
  --count 2

Create a rule to autoscale 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, how long the application load must meet a given threshold, and how many VM instances to add to the scale set.

Create a rule with az monitor autoscale rule create 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 three.

az monitor autoscale rule create \
  --resource-group myResourceGroup \
  --autoscale-name autoscale \
  --condition "Percentage CPU > 70 avg 5m" \
  --scale out 3

Create a rule to autoscale 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 another rule with az monitor autoscale rule create that decreases the number of VM instances in a scale set when the average CPU load then drops below 30% over a 5-minute period. The following example defines the rule to scale in the number of VM instances by one.

az monitor autoscale rule create \
  --resource-group myResourceGroup \
  --autoscale-name autoscale \
  --condition "Percentage CPU < 30 avg 5m" \
  --scale in 1

Generate CPU load on scale set

To test the autoscale rules, generate some CPU load on the VM instances in the scale set. This simulated CPU load causes the autoscales to scale out and increase the number of VM instances. As the simulated CPU load is then decreased, the autoscale rules scale in and reduce the number of VM instances.

To connect to an individual instance, see Tutorial: Connect to Virtual Machine Scale Set instances

Once logged in, install the stress or stress-ng utility. Start 10 stress workers that generate CPU load. These workers run for 420 seconds, which is enough to cause the autoscale rules to implement the desired action.

sudo apt-get update
sudo apt-get -y install stress
sudo stress --cpu 10 --timeout 420 &

sudo dnf install stress-ng
sudo  stress-ng --cpu 10 --timeout 420s --metrics-brief &

sudo zypper install stress-ng
sudo stress-ng --cpu 10 --timeout 420s --metrics-brief &

When stress shows output similar to stress: info: [2688] dispatching hogs: 10 cpu, 0 io, 0 vm, 0 hdd, press the Enter key to return to the prompt.

To confirm that stress generates CPU load, examine the active system load with the top utility:

top

Exit top, then close your connection to the VM instance. stress continues to run on the VM instance.

Ctrl-c
exit

Connect to second VM instance with the port number listed from the previous az vmss list-instance-connection-info:

ssh azureuser@13.92.224.66 -p 50003

Install and run stress or stress-ng, then start ten workers on this second VM instance.

sudo apt-get -y install stress
sudo stress --cpu 10 --timeout 420 &

sudo dnf install stress-ng
sudo  stress-ng --cpu 10 --timeout 420s --metrics-brief &

sudo zypper install stress-ng
sudo stress-ng --cpu 10 --timeout 420s --metrics-brief &

Again, when stress shows output similar to stress: info: [2713] dispatching hogs: 10 cpu, 0 io, 0 vm, 0 hdd, press the Enter key to return to the prompt.

Close your connection to the second VM instance. stress continues to run on the VM instance.

exit

Monitor the active autoscale rules

To monitor the number of VM instances in your scale set, use watch. It takes 5 minutes for the autoscale rules to begin the scale-out process in response to the CPU load generated by stress on each of the VM instances:

watch az vmss list-instances \
  --resource-group myResourceGroup \
  --name myScaleSet \
  --output table

Once the CPU threshold has been met, the autoscale rules increase the number of VM instances in the scale set. The following output shows three VMs created as the scale set autoscales out:

Every 2.0s: az vmss list-instances --resource-group myResourceGroup --name myScaleSet --output table

  InstanceId  LatestModelApplied    Location    Name          ProvisioningState    ResourceGroup    VmId
------------  --------------------  ----------  ------------  -------------------  ---------------  ------------------------------------
           1  True                  chinanorth2      myScaleSet_1  Succeeded            myResourceGroup  4f92f350-2b68-464f-8a01-e5e590557955
           2  True                  chinanorth2      myScaleSet_2  Succeeded            myResourceGroup  d734cd3d-fb38-4302-817c-cfe35655d48e
           4  True                  chinanorth2      myScaleSet_4  Creating             myResourceGroup  061b4c90-0d73-49fc-a066-19eab0b3d95c
           5  True                  chinanorth2      myScaleSet_5  Creating             myResourceGroup  4beff8b9-4e65-40cb-9652-43899309da27
           6  True                  chinanorth2      myScaleSet_6  Creating             myResourceGroup  9e4133dd-2c57-490e-ae45-90513ce3b336

Once stress stops on the initial VM instances, the average CPU load returns to normal. After another 5 minutes, the autoscale rules then scale in the number of VM instances. Scale in actions remove VM instances with the highest IDs first. When a scale set uses Availability Sets or Availability Zones, scale in actions are evenly distributed across those VM instances. The following example output shows one VM instance deleted as the scale set autoscales in:

6  True                  chinanorth2      myScaleSet_6  Deleting             myResourceGroup  9e4133dd-2c57-490e-ae45-90513ce3b336

Exit watch with Ctrl-c. The scale set continues to scale in every 5 minutes and remove one VM instance until the minimum instance count of two is reached.

Clean up resources

To remove your scale set and additional resources, delete the resource group and all its resources with az group delete. The --no-wait parameter returns control to the prompt without waiting for the operation to complete. The --yes parameter confirms that you wish to delete the resources without an additional prompt to do so.

az group delete --name myResourceGroup --yes --no-wait

Next steps

In this tutorial, you learned how to automatically scale in or out a scale set with the Azure CLI:

Use autoscale with a scale set
Create and use autoscale rules
Stress-test VM instances and trigger autoscale rules
Autoscale back in as demand is reduced