Is it possible to provision backend address pool separately from application gateway

Question

I want to provision my infrastructure with terraform and to have some shared infrastructure. For example, to have 1 shared application gateway and multiple application specific webapps sitting behind it.

Is it possible to provision this with separate terraform projects?

terraform project that would be for a shared infrastructure - to create an empty/default app gateway.
other projects that would create a webapp and some extra configuration for that shared app gw - backend_address_pool, probe, backend_http_settings, etc.

Could not find how can you do it in documentation. There is this - application_gateway example where everything is provided in 1 go and then this - network_interface_application_gateway_backend_address_pool_association where you can make a assotiation between app gw and a network interface, but not a webapp.

EDIT

To expand a bit on what I want to achieve - the application gateway will be one application gateway for the whole non-prod environment (hence "shared") and it is there to save the cost. Behind it I want to configure multiple applications for multiple environments, for example, "Accounts.DEV", "Accounts.UAT", "Calculator.Dev", etc. Hope this makes my intentions a bit clearer.

For now I am trying to create empty Application gateway in the shared project (with a default pool, front-end config and rules). And then, after each application deployment to run some extra az cli logic (documentation).

Nancy Xiong Nancy Xiong · Accepted Answer · 2019-12-05T06:41:01

It is possible to provision the application gateway and web apps separately.

By default, this application_gateway example creates an empty backend pool without any targets with one default HTTP setting, one listener for 80 port, and a basic rule for this backend pool. When you want to associate your backend web apps behind this application gateway, you need to target default_site_hostname of your web app to the backend pool and modify some specific configurations to match your backend web apps.

For example,

In the azurerm_app_service project, you can add the value of default_site_hostname for an app service at the provision time or use the data source to access an existing app service.

output "default_site_hostname" {
  value = "${azurerm_app_service.test.default_site_hostname}"
}

In the azurerm_application_gateway project, you can add the value of default_site_hostname to the fqdns, then associate the backend pool with them.

# since these variables are re-used - a locals block makes this more maintainable
locals {
  backend_address_pool_name      = "${azurerm_virtual_network.test.name}-beap"
  frontend_port_name             = "${azurerm_virtual_network.test.name}-feport"
  frontend_ip_configuration_name = "${azurerm_virtual_network.test.name}-feip"
  http_setting_name              = "${azurerm_virtual_network.test.name}-be-htst"
  listener_name                  = "${azurerm_virtual_network.test.name}-httplstn"
  request_routing_rule_name      = "${azurerm_virtual_network.test.name}-rqrt"
  redirect_configuration_name    = "${azurerm_virtual_network.test.name}-rdrcfg"
  fqdns                          = ["${azurerm_app_service.test.default_site_hostname}","${data.azurerm_app_service.example.default_site_hostname}"]
...

 backend_address_pool {
    name = "${local.backend_address_pool_name}"
    fqdns = "${local.fqdns}"
  }

Is it possible to provision backend address pool separately from application gateway

2 Answers