script.ps1

param(
	[Parameter(mandatory=$true)]
	[string]$scalingPlan,
	[Parameter(mandatory=$true)]
	[string]$scalingPlanRG,
	[Parameter(mandatory=$true)]
	[string]$hostPoolRG,
	[Parameter()]
	[String]$subscription = "your-subscription-name-here",
	[Parameter(mandatory=$true)]
	[String[]]$hostPools,
	[Parameter(mandatory=$true)]
	[bool]$scalingPlanEnabled,
	[Parameter(mandatory=$true)]
	[bool]$startVMs,
	[Parameter(mandatory=$true)]
	[string]$patchWindow,
	[Parameter(mandatory=$true)]
	[bool]$patchingEnabled
)

<#

	Before patching set $scalingPlanEnabled to $false and $startVMs to $true to let VMs running before patch window

	After patching is over set $scalingPlanEnabled back to $true and set $startVMs to $false

#>

if ($patchingEnabled) {
	# Login to Azure
	Connect-AzAccount -Identity
	# Select subscription scope
	Select-AzSubscription $subscription
	# Exist if no hostPools provided
	if ($hostPools.Count -eq 0) {
		throw "No hostPool(s) provided."
	}
	# Get HostPoolReferences for the selected scalingplan
	$hpRef = Get-AzWvdScalingPlan -Name $scalingPlan -ResourceGroupName $scalingPlanRG | select *
	# Iterate through all hostpoolreferences
	foreach ($ref in $hpRef.HostPoolReference) {
		# If hostpool is in provided hostPool array, change scaling plan enabled to the selected setting
		if ($ref.HostPoolArmPath.split("/")[-1] -in $hostPools) {
			$ref.ScalingPlanEnabled = $scalingPlanEnabled
		}
	}
	# Update scaling plan settings. This will enable or disable autoscale for the selected hostpools using the provided scale plan. 
	Update-AzWvdScalingPlan -Name $scalingPlan -ResourceGroupName $scalingPlanRG -HostPoolReference $hpRef.HostPoolReference
	# Iterate through each hostpool and start all hosts.
	# Additionally enable drainmode using the same variable used for enabling/disabling the scalingPlan.
	foreach ($hp in $hostPools) {
		Get-AzWvdSessionHost -HostPoolName $hp -ResourceGroupName $hostPoolRG | select * | % {
			if ($startVMs) {
				Start-AzVM -Id $_.ResourceId
			}
			$vm = Get-AZVM -ResourceId $_.ResourceId
			if ($vm.Tags['vm-update'] -eq $patchWindow) {
				Update-AzWvdSessionHost -ResourceGroupName $hostPoolRG -HostPoolName $hp -Name $_.Name.split("/")[-1] -AllowNewSession:$scalingPlanEnabled
			}
		}
	}
}