dynamic block on terraform azurerm_virtual_machine_extension settings

Question

I'm creating Azure VM extension resources azurerm_virtual_machine_extension using terraform. However, I'm stuck with below use-case whether I can dynamically create settings block?

variable "test_string" {
  description = "boolean var to attach VM nics to lb"
  type        = string
  default     = ""
}

variable "test_script" {
  description = "script locally"
  type        = string
  default     = ""
}

variable "file_uri" {
  description = "script to download"
  type        = string
  default     = "https://example.com/azure-tests/custom_script.sh"
}


resource "azurerm_virtual_machine_extension" "ama_vm_extension" {

  name                 = "tst-vm-extension"
  virtual_machine_id   = "xxxxx-xxxx-xxxxx"
  publisher            = "Microsoft.Azure.Extensions"
  type                 = "CustomScript"
  type_handler_version = "2.0"

  dynamic settings = <<SETTINGS
  {
    count = var.file_uri != "" ? 1: 0
    "commandToExecute": "sh ${basename(var.file_uri)}",
    "fileUris": ["${var.file_uri}"]
  }
  SETTINGS

  dynamic settings = <<SETTINGS
  {
     count = var.test_string != "" ? 1: 0
     content {
       "commandToExecute": "${var.test_string}"
     }
  }
  SETTINGS

  dynamic settings = <<SETTINGS
  {
     count = var.test_script != "" ? 1: 0
     content {
       "script": "${base64encode(var.test_string)}"
     }
  }
  SETTINGS

}

In the above code, I would like to control the resource behavior with test_string, test_script, file_uri vars. However, terraform allows only one settings block in the resource and I don't know how I can use dynamic block feature here. As we have SETTINGS string in the end.

Your help is much appreciated here.

Thanks, Harsha

If the three settings are mutually exclusive, then have three azurerm_virtual_machine_extension eliminate the dynamic block and move the count to the resource itself. If they are not mutually exclusive, then use an object variable and convert it to json. I don't have time to test this, but that is where I would start. — Christian Pearce

Nick Keenan Nick Keenan · Accepted Answer · 2020-08-24T21:54:20

Terraform's dynamic blocks are meant to be used with nested blocks in resources. Since setting is just a string, you can just use regular conditionals to set the value of setting.

I would probably do something like this:

locals {
  file_uri_settings = jsonencode({
    "commandToExecute" = "sh ${basename(var.file_uri)}",
    "fileUris"         = [var.file_uri]
  })
  test_string_settings = jsonencode({
    "commandToExecute" = var.test_string
  })
  test_script_settings = jsonencode({
    "script" = base64encode(var.test_script)
  })
}

resource "azurerm_virtual_machine_extension" "example" {
  // ...
  settings = var.test_string != "" ? local.test_string_settings : (var.test_script != "" ? local.test_script_settings : local.file_uri_settings)
  // ...
}

Just to clean it up, I pull the different options into a local block. Then you can chain ternary operators to select the correct string to use.

dynamic block on terraform azurerm_virtual_machine_extension settings

1 Answers