Overview of Azure Virtual Machine Scale Sets

Azure Virtual Machine Scale Sets (VMSS) provide a way to deploy and manage a set of identical, auto-scaling virtual machines. They are ideal for building highly available and scalable applications that can respond to demand or scheduled events. VMSS allows you to automatically increase or decrease the number of virtual machines based on metrics like CPU utilization or a custom metric.

Key Features

• Automatic Scaling: Define rules to automatically scale out (add VMs) or scale in (remove VMs) based on performance metrics or a schedule.
• High Availability: Distribute VMs across multiple fault domains and availability zones for resilience.
• Simplified Management: Deploy and manage a fleet of VMs with a single configuration.
• Application Health: Integrate with Azure Load Balancer and Application Gateway for seamless traffic distribution and health probes.
• Customization: Use custom images or marketplace images, and configure extensions for software installation and configuration.

Creating a Virtual Machine Scale Set

You can create a VMSS using the Azure portal, Azure CLI, PowerShell, or ARM templates. The process typically involves defining:

• Operating System: Choose from a variety of Linux or Windows images.
• Instance Count: The initial number of VMs.
• VM Size: The size of each virtual machine.
• Storage: Configure OS and data disks.
• Networking: Virtual network and subnet configuration.
• Load Balancer: Integration with Azure Load Balancer or Application Gateway.
• Scaling Rules: Define metrics and thresholds for automatic scaling.

Using Azure CLI:

az vmss create \
  --resource-group MyResourceGroup \
  --name MyVmss \
  --image UbuntuLTS \
  --vm-sku Standard_D2s_v3 \
  --instance-count 2 \
  --upgrade-policy-mode Automatic \
  --admin-username azureuser \
  --generate-ssh-keys
            

Managing a Virtual Machine Scale Set

Once created, you can manage your VMSS through various operations:

• Scale: Manually change the instance count.
• Upgrade: Update VM instances with new configurations or images.
• Health Monitoring: Check the health of individual VM instances.
• Restart/Deallocate: Perform maintenance operations on VMs.
• Delete: Remove the scale set and all its resources.

Scaling Operations

VMSS can scale automatically based on predefined rules. You can configure rules to trigger scaling actions based on:

• Metrics: CPU utilization, network in/out, memory usage (requires Azure Monitor agent).
• Schedules: Scale out or in at specific times of the day or week.

Networking Integration

VMSS integrates seamlessly with Azure Virtual Networks (VNet). Each VM instance in the scale set gets its own network interface connected to the VNet. You can configure:

• Load Balancer: Distribute incoming traffic across VM instances.
• Application Gateway: For advanced traffic management, SSL termination, and Web Application Firewall (WAF).
• Network Security Groups (NSGs): Control network traffic to and from VM instances.
• Public IP Addresses: Assign public IPs to instances if required, though often managed by the load balancer.

Load Balancing

Azure Load Balancer is commonly used with VMSS to distribute incoming network traffic to the VM instances. It provides:

• High Availability: Redirects traffic away from unhealthy instances.
• Scalability: Handles varying traffic loads.
• Health Probes: Monitors the health of individual VM instances.

Application Deployment

Deploying applications to VMSS can be done in several ways:

• Custom Images: Pre-configure your VMs with your application installed using custom images.
• VM Extensions: Use extensions like Custom Script Extension or Desired State Configuration (DSC) to install and configure applications post-deployment.
• Configuration Management Tools: Integrate with tools like Chef, Puppet, or Ansible.

Monitoring and Diagnostics

Monitor the health and performance of your VMSS and its instances using:

• Azure Monitor: Collects metrics and logs for analysis and alerting.
• VM Scale Set Health: Built-in health checks for individual instances.
• Boot Diagnostics: Capture console output and screenshots for troubleshooting boot issues.

Best Practices

• Use Managed Disks: For better performance and availability.
• Choose the Right Upgrade Policy: 'Automatic' for seamless updates, 'Rolling' for controlled rollouts, or 'Manual' for full control.
• Implement Health Probes: Crucial for load balancers to route traffic correctly.
• Optimize Scaling Rules: Set appropriate thresholds to avoid over- or under-provisioning.
• Use Custom Script Extensions: For idempotent and repeatable application deployments.
• Consider Availability Zones: For maximum resilience against data center failures.

Troubleshooting

Common issues include:

• Scaling failures: Review scale set logs and Azure Activity Logs.
• Application errors: Use boot diagnostics, custom script logs, and application logs.
• Network connectivity: Verify NSG rules, VNet configurations, and load balancer settings.