hi @neil-yechenwei

Thanks for this PR.

Unfortunately the AppSettings and Connection Strings on the azurerm_app_service can be configured on Slots rather than the top-level App Service, which will take precedence when that Slot is made Active - which is why unfortunately both of these blocks have to be set to computed - for example the following Terraform Configuration:

resource "azurerm_resource_group" "test" {
  name     = "some-resource-group"
  location = "West Europe"
}

resource "azurerm_app_service_plan" "test" {
  name                = "example-appserviceplan"
  location            = "${azurerm_resource_group.test.location}"
  resource_group_name = "${azurerm_resource_group.test.name}"

  sku {
    tier = "Standard"
    size = "S1"
  }
}

resource "azurerm_app_service" "test" {
  name                = "example-appservice"
  location            = "${azurerm_resource_group.test.location}"
  resource_group_name = "${azurerm_resource_group.test.name}"
  app_service_plan_id = "${azurerm_app_service_plan.test.id}"
}

resource "azurerm_app_service_slot" "first" {
  name                = "first"
  app_service_name    = "${azurerm_app_service.test.name}"
  location            = "${azurerm_resource_group.test.location}"
  resource_group_name = "${azurerm_resource_group.test.name}"
  app_service_plan_id = "${azurerm_app_service_plan.test.id}"

  site_config {
    dotnet_framework_version = "v4.0"
  }

  app_settings = {
    "SOME_KEY" = "some-value"
  }

  connection_string {
    name  = "Database"
    type  = "SQLServer"
    value = "Server=some-server.mydomain.com;Integrated Security=SSPI"
  }
}

resource "azurerm_app_service_slot" "second" {
  name                = "second"
  app_service_name    = "${azurerm_app_service.test.name}"
  location            = "${azurerm_resource_group.test.location}"
  resource_group_name = "${azurerm_resource_group.test.name}"
  app_service_plan_id = "${azurerm_app_service_plan.test.id}"

  site_config {
    dotnet_framework_version = "v4.0"
  }

  app_settings = {
    "SOME_KEY" = "some-other-value"
  }

  connection_string {
    name  = "Database"
    type  = "SQLServer"
    value = "Server=some-other-server.mydomain.com;Integrated Security=SSPI"
  }
}

resource "azurerm_app_service_active_slot" "test" {
  resource_group_name   = "${azurerm_resource_group.test.name}"
  app_service_name      = "${azurerm_app_service.test.name}"
  app_service_slot_name = "${azurerm_app_service_slot.first.name}"
}

Where changing the app_service_slot_name on the azurerm_app_service_active_slot resource would change both the app_settings and connection_strings blocks on the top level azurerm_app_service to the values from the first (or second) azurerm_app_service_slot depending on which one is active.

As such you can instead empty these blocks out by assigning an empty list to them; as shown below:

...

resource "azurerm_app_service" "test" {
  name                = "example-appservice"
  location            = "${azurerm_resource_group.test.location}"
  resource_group_name = "${azurerm_resource_group.test.name}"
  app_service_plan_id = "${azurerm_app_service_plan.test.id}"

  app_settings = []
  connection_strings = []
}

As such whilst I'd like to thank you for this contribution I'm going to close this PR for the moment since this behaviour is intentional to support this use-case in the Azure API.

Thanks!

@tombuildsstuff , thanks for your comment but I still have some concerns on it. Could you help to explain it? Thanks.
Question # 1:
After removed the "Computed=true" and re-run the test case "TestAccAzureRMAppServiceSlot_connectionStrings" which meets your use-case what you mentioned, it also work fine. So why you think "computed=true" is required for this field connection_string? Thanks.

Question # 2:
This PR is trying to fix the bug "#4151" what the user filed. The issue describes that user cannot delete all connection strings in app service once created one connection string. And I tried your above sample code, it still cannot delete the connection string with config "connection_strings = []" and threw below error. If you could, could you provide an entire sample you tested successfully for deleting all connection string after created connection string? Thanks.
Error message:
Error: Unsupported argument
on main.tf line 30, in resource "azurerm_app_service" "test":
30: connection_string = []
An argument named "connection_string" is not expected here. Did you mean to
define a block of type "connection_string"?

My sample code:
resource "azurerm_resource_group" "test" {
name = "neilappservicetestrg02"
location = "eastus2"
}

resource "azurerm_app_service_plan" "test" {
name = "neilsptest01"
location = "${azurerm_resource_group.test.location}"
resource_group_name = "${azurerm_resource_group.test.name}"

sku {
tier = "Standard"
size = "S1"
}
}

resource "azurerm_app_service" "test" {
name = "neilastest01"
location = "${azurerm_resource_group.test.location}"
resource_group_name = "${azurerm_resource_group.test.name}"
app_service_plan_id = "${azurerm_app_service_plan.test.id}"

connection_string = []
}

Question # 3:
I think it means that the field can be specified or not be specified and the diff verify for the field would be ignored while specifying "optional=true" and "computed=true" in the same time. If my understanding is incorrect, could you help to explain what does the field connection_string mean while specifying "optional =true" and "computed=true" in the same time? Thanks.

Question # 4:
You said "In order to support this use-case in Terraform both of these blocks are marked as 'Computed', which means that if a users configured values for them then we'll use those values, however if they have no values configured then we'll use whatever's returned from Azure." but I have some different idea. According by the doc https://www.terraform.io/docs/extend/schemas/schema-behaviors.html, it means the field is read-only when used "computed=true". The scenario you just said should be covered by the attributed "optional=true". Please see the doc https://www.terraform.io/docs/extend/schemas/schema-behaviors.html, right?

I'm having the same issue. Is there some update on this?

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