Acquiring subnet availability zone ids in bulk, in a module - amazon-web-services

Question

The module I'm working on represents one app which is deployed to a VPC. The VPC is declared elsewhere.
The relevant data path includes these resources:
variable "vpc_id" { }
data "aws_subnets" "private" {
filter {
name = "vpc-id"
values = [data.aws_vpc.vpc.id]
}
filter {
name = "tag:Visibility"
values = ["private"]
}
}
data "aws_subnet" "private" {
for_each = toset(data.aws_subnets.private.ids)
vpc_id = data.aws_vpc.vpc.id
id = each.value
}
resource "aws_rds_cluster" "database" {
availability_zones = data.aws_subnet.private.*.availability_zones
}
That feels like the correct syntax, though it is a verbose chain of data retrieval. However, when I terraform plan it:
│ Error: Unsupported attribute
│
│ on ../../../../../appmodule/rds_postgres.tf line 23, in resource "aws_rds_cluster" "webapp":
│ 23: availability_zones = data.aws_subnet.private.*.availability_zone_id
│
│ This object does not have an attribute named "availability_zone_id".
I'm using aws-provider 4.18.0 and Terraform v1.1.2. The documentation for the subnet data source shows that availability_zone_id
Am I missing something obvious here?

Answer 1

As mentioned in the comments, you can get the list of AZs by using the values built-in function [1]. This is necessary as the data source you are relying on to provide the AZs is in a key value format due to for_each meta-argument use:
data "aws_subnet" "private" {
for_each = toset(data.aws_subnets.private.ids)
vpc_id = data.aws_vpc.vpc.id
id = each.value
}
The change you need to make is:
resource "aws_rds_cluster" "database" {
availability_zones = values(data.aws_subnet.private)[*].availability_zone
}
A test with an output and a default VPC shows the following result:
+ subnet_azs = [
+ "us-east-1b",
+ "us-east-1c",
+ "us-east-1d",
+ "us-east-1a",
+ "us-east-1f",
+ "us-east-1e",
]
As you can see, it is already a list so you can use it as is.
Note that there is an explanation why you should use the availability_zone attribute:
availability_zone_id - (Optional) ID of the Availability Zone for the subnet. This argument is not supported in all regions or partitions. If necessary, use availability_zone instead
[1] https://www.terraform.io/language/functions/values