add dowtime prompt

AzureBasicLoadBalancerUpgrade/module/AzureBasicLoadBalancerUpgrade/modules/Start-AzBasicLoadBalancerUpgrade/Start-AzBasicLoadBalancerUpgrade.psm1

@@ -173,6 +173,7 @@ function Start-AzBasicLoadBalancerUpgrade {
         [Parameter(Mandatory = $false)][switch] $outputMigrationValiationObj,
         [Parameter(Mandatory = $false)][int32] $defaultJobWaitTimeout = (New-Timespan -Minutes 10).TotalSeconds,
         [Parameter(Mandatory = $false)][switch] $force,
+        [Parameter(Mandatory = $false)][boolean] $confirm = $true,
         [Parameter(Mandatory = $false)][switch] $Pre
     )
 
@@ -194,6 +195,20 @@ function Start-AzBasicLoadBalancerUpgrade {
         Write-Error "The path '$recoveryBackupPath' specified with parameter recoveryBackupPath must exist and be a valid directory." -terminateOnError
     }
 
+    # warn user about application downtime
+    If ((($confirm) -and !$force) -and !(Test-Path -Path (Join-Path -Path $RecoveryBackupPath -ChildPath 'Start-AzBasicLoadBalancerUpgrade.log'))) {
+        $message = "This operation will cause downtime for the application(s) using the Basic Load Balancer--usually a few minutes--see https://aka.ms/BasicLBMigrateDowntime. Are you sure you want to continue? [y/N]"
+        $result = Read-Host -Prompt $message
+        If ($result -ne 'y' -and $result -ne 'yes') {
+            Write-Host "Operation cancelled by user"
+            return
+        }
+        Else {
+            Write-Host -ForegroundColor Green "`nSpecify parameter ``-confirm `$false`` to skip this prompt in the future`n"
+            Start-Sleep -Seconds 2
+        }
+    }
+
     log -Message "############################## Initializing Start-AzBasicLoadBalancerUpgrade ##############################"
 
     log -Message "[Start-AzBasicLoadBalancerUpgrade] PowerShell Version: $($PSVersionTable.PSVersion.ToString())"