How to use Terraform dynamic block - terraform

Question

I am trying to create a azure app gateway using terraform and my code looks like below:
main.tf
# Create Application Gateway
resource "azurerm_application_gateway" "app_gateway" {
name = var.name
resource_group_name = var.appg_name
location = var.appg_location
.
.
.
# backend_address_pool {
# name = var.backend_address_pool_name
# }
dynamic "backend_address_pool" {
for_each = var.backend_pools
content {
name = backend_pools.value["name"]
fqdns = backend_pools.value["fqdns"]
ip_addresses = backend_pools.value["ip_addresses"]
}
}
backend_http_settings {
name = var.http_setting_name
cookie_based_affinity = var.cookie_based_affinity
path = var.backend_http_settings_path
port = var.http_setting_port
protocol = var.http_setting_protocol
request_timeout = var.http_setting_request_time_out_value
}
.
.
.
}
And here's my variables.tf - only backend_pools is mentioned here of all the variables
variable "backend_pools" {
type = list(map(string))
}
And this is my terraform.tfvars
backend_pools = [
{
name = "pool1"
fqdns = "fqdns1"
ip_addresses = "10.0.0.0"
},
{
name = "pool2"
fqdns = "fqdns2"
ip_addresses = "10.0.0.0"
},
{
name = "pool3"
fqdns = "fqdns3"
ip_addresses = "10.0.0.0"
},
]
What I am worried about is, according to the terraform docs it is mentioned that the data types of both fqdns and ip_addresses are list, then how can I change my variables.tf and terraform.tfvars and also main.tf which contain the dynamic block accordingly.
What I want to do is, create multiple backend pools and pass the values using a variable file or is it possible to create a json file with multiple backend pool and pass it to the backend_pools parameter?
Can someone please help me on this?

Answer 1

First, there is a mistake in your Terraform code:
dynamic "backend_address_pool" {
for_each = var.backend_pools
content {
name = backend_pools.value["name"]
fqdns = backend_pools.value["fqdns"]
ip_addresses = backend_pools.value["ip_addresses"]
}
}
When you use the for_each loop, it should be:
dynamic "backend_address_pool" {
for_each = var.backend_pools
content {
name = each.value["name"]
fqdns = each.value["fqdns"]
ip_addresses = each.value["ip_addresses"]
}
}
When you use the list in the for_each loop, you can try to change the type like this:
for_each = toset(var.backend_pools)
But the better way is to use the map, so you can change your variables like this:
backend_pools = {
pool1 = {
fqdns = "fqdns1"
ip_addresses = "10.0.0.0"
},
pool2 = {
fqdns = "fqdns2"
ip_addresses = "10.0.0.0"
},
pool3 = {
fqdns = "fqdns3"
ip_addresses = "10.0.0.0"
}
}
Then your dynamic block will look like this:
dynamic "backend_address_pool" {
for_each = var.backend_pools
content {
name = each.key
fqdns = each.value["fqdns"]
ip_addresses = each.value["ip_addresses"]
}
}