Attaching VM Scale Set to Application Gateway's backend pool

Question

Can anyone provide an example of how to add an Azure VM Scale Set (VMSS) to an application gateway's backend pool using Terraform.

I can only find one example that I cannot get to work for me. Although terraform plan and terraform apply do not error, the VMSS is not added to the backend pool. I wonder if my problems are caused by the fact the backend_address_pool output from the app gateway resource is a block as opposed to a single attribute.

I am using Terraform 0.12.5, with AzureRM provider 1.31.0.

App Gateway resource output:

output "name" {
  value = azurerm_application_gateway.pool[*].name
}

output "id" {
  value = azurerm_application_gateway.pool[*].id
}

output "backend_address_pool" {
  value = azurerm_application_gateway.pool[*].backend_address_pool
}

VMSS module declaration:

module "vmss_example" {
...
application_gateway_backend_address_pool_ids = ["${module.app_gateway_example[0].id}/backendAddressPools/my-backend-address-pool"]
}

---

After the resources have been created, rerunning terraform plan wants to change the vmss from:

...
- ip_configuration {
              - application_gateway_backend_address_pool_ids = [] -> null
              - application_security_group_ids               = [] -> null
              - load_balancer_backend_address_pool_ids       = [] -> null
              - load_balancer_inbound_nat_rules_ids          = [] -> null
              - name                                         = "ipconfig" -> null
              - primary                                      = true -> null
              - subnet_id                                    = "/subscriptions/#########-####-####-####-############/resourceGroups/demo-modules-rg/providers/Microsoft.Network/virtualNetworks/vnetdemodemo01/subnets/subnetdemovm01" -> null
            }
...

to:

...
+ ip_configuration {
              + application_gateway_backend_address_pool_ids = (known after apply)
              + application_security_group_ids               = []
              + load_balancer_backend_address_pool_ids       = []
              + load_balancer_inbound_nat_rules_ids          = (known after apply)
              + name                                         = "ipconfig"
              + primary                                      = true
              + subnet_id                                    = "/subscriptions/#########-####-####-####-############/resourceGroups/demo-modules-rg/providers/Microsoft.Network/virtualNetworks/vnetdemodemo01/subnets/subnetdemovm01"
            }
...

Which I believe means that it is trying to update application_gateway_backend_address_pool_ids.

Any help, pointers or suggestions will be gratefully received. TIA

Answer 1

Yes, you just need to use application_gateway_backend_address_pool_ids to specify an array of references to backend address pools of application gateways in the ip_configuration block.

For example, this works on my side with Terraform v0.12.5 + provider.azurerm v1.32.0 :

ip_configuration {
  name                                   = "TestIPConfiguration"
  primary                                = true
  subnet_id                              = "${azurerm_subnet.backend.id}"
  application_gateway_backend_address_pool_ids = "${azurerm_application_gateway.network.backend_address_pool[*].id}"
}

Or, this also works application_gateway_backend_address_pool_ids = ["${azurerm_application_gateway.network.backend_address_pool[0].id}"]

Check the output

output "backend_address_pool" {
    value = "${azurerm_application_gateway.network.backend_address_pool[*].id}"
}

Answer 2

Note that in version 3.0.0 of the azurerm provider the backend_address_pool of the application_gateway is now a Set instead of a List.

If you only have one backend_address_pool that you need to reference, you can use the one() built-in function like this:

ip_configuration {
  name                                   = "TestIPConfiguration"
  primary                                = true
  subnet_id                              = "${azurerm_subnet.backend.id}"
  application_gateway_backend_address_pool_ids = one(azurerm_application_gateway.network.backend_address_pool[*].id)
}