azurerm_virtual_machine_scale_set: allow empty zone specification

[Supermathie]

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Description

We are trying to implement VMSSes that take advantage of zones if available. We would like the azurerm_virtual_machine_scale_set resource (and others for which you can specify zones, if any) to interpret a blank list of zones as "the zones are not specified".

The way the existing code behaves is:

resource "azurerm_virtual_machine_scale_set" "app" {
  …
  zones = []
}

# terraform plan →

Error: Error running plan: 1 error(s) occurred:

* azurerm_virtual_machine_scale_set.app: zones: attribute supports 1 item as a minimum, config has 0 declared

resource "azurerm_virtual_machine_scale_set" "app" {
  …
  zones = [1,2,3]
}

# terraform plan →

-/+ azurerm_virtual_machine_scale_set.app (new resource required)
…
      zones.#:                                                                                              "0" => "3" (forces new resource)
      zones.0:                                                                                              "" => "1" (forces new resource)
      zones.1:                                                                                              "" => "2" (forces new resource)
      zones.2:                                                                                              "" => "3" (forces new resource)

Why would we do such a crazy thing? The reason is that the following terraform could would easily let us take advantage of availability zones if present:

locals {
  _region_az_map = {
    "centralus" = "1,2,3"
  }
}

resource "azurerm_virtual_machine_scale_set" "app" {
  …
  zones = ["${compact(split(",", lookup(local._region_az_map, azurerm_resource_group.app.location, "1")))}"]
  …
}

New or Affected Resource(s)

• azurerm_virtual_machine_scale_set

Potential Terraform Configuration

resource "azurerm_virtual_machine_scale_set" "app" {
  …
  zones = []
}

References

• https://docs.microsoft.com/en-us/azure/availability-zones/az-overview

[ghost]

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