Azure VMSS Custom Data refresh the exisiting VMs

Azure VMSS Custom Data refresh the existing VMs using terraform.

For this kind of requirement, you can use create_before_destroy along with lifecycle rules and a null resource using commands which can trigger whenever there is a change in custom data.

This setup works as same as AWS Auto Scaling and meets the requikrement.

Configuration:

resource "null_resource" "vmss_reimage_trigger" {
  triggers = {
    
    custom_data_hash = filemd5("custom_data.sh")
  }

  provisioner "local-exec" {
    command = "echo 'Changes detected, will re-image VMSS'"
  }
}


resource "azurerm_windows_virtual_machine_scale_set" "vmss" {
  name                = "vmss-demo"
  location            = azurerm_resource_group.rg.location
  resource_group_name = azurerm_resource_group.rg.name
  admin_username      = "adminuser"
  admin_password      = "P@55w0rd1234!"  
  sku                 = "Standard_F2"
  instances           = 1

  custom_data = base64encode(file("custom_data.sh"))

  os_disk {
    storage_account_type = "Standard_LRS"
    caching              = "ReadWrite"
  }

  source_image_reference {
    publisher = "MicrosoftWindowsServer"
    offer     = "WindowsServer"
    sku       = "2019-Datacenter"
    version   = "latest" 
  }

  upgrade_mode = "Automatic"

  health_probe_id = azurerm_lb_probe.lb_probe.id

  automatic_instance_repair {
    enabled      = true
    grace_period = "PT10M"
  }

  network_interface {
    name    = "example"
    primary = true

    ip_configuration {
      name      = "internal"
      primary   = true
      subnet_id = azurerm_subnet.internal.id
      load_balancer_backend_address_pool_ids = [azurerm_lb_backend_address_pool.bap.id]
    }
  }

  lifecycle {
    create_before_destroy = true
  }
  depends_on = [null_resource.vmss_reimage_trigger]  
}

Deployment:

refer:

azurerm_windows_virtual_machine_scale_set | Resources | hashicorp/azurerm | Terraform | Terraform Registry

The lifecycle Meta-Argument - Configuration Language | Terraform | HashiCorp Developer