Terraform EB environment not finding subnet with explicit dependency - terraform

Question

I have a Terraform configuration with various AWS resources in one file, including a VPC, three private, three public subnets, and an EB environment.
My Terraform version is 0.12.0, and the AWS provider version is ~> 2.12
The VPC looks like this:
resource "aws_vpc" "terraform-vpc" {
cidr_block = "${var.cidr_block}"
assign_generated_ipv6_cidr_block = true
}
My six subnets look like this:
resource "aws_subnet" "private-a" {
vpc_id = "${aws_vpc.terraform-vpc.id}"
availability_zone = "eu-west-2a"
cidr_block = "10.0.1.0/24"
}
resource "aws_subnet" "public-a" {
vpc_id = "${aws_vpc.terraform-vpc.id}"
availability_zone = "eu-west-2a"
cidr_block = "10.0.2.0/24"
}
resource "aws_subnet" "private-b" {
vpc_id = "${aws_vpc.terraform-vpc.id}"
availability_zone = "eu-west-2b"
cidr_block = "10.0.3.0/24"
}
resource "aws_subnet" "public-b" {
vpc_id = "${aws_vpc.terraform-vpc.id}"
availability_zone = "eu-west-2b"
cidr_block = "10.0.4.0/24"
}
resource "aws_subnet" "private-c" {
vpc_id = "${aws_vpc.terraform-vpc.id}"
availability_zone = "eu-west-2c"
cidr_block = "10.0.5.0/24"
}
resource "aws_subnet" "public-c" {
vpc_id = "${aws_vpc.terraform-vpc.id}"
availability_zone = "eu-west-2c"
cidr_block = "10.0.6.0/24"
}
Then, in my aws_elastic_beanstalk_environment resource I have:
...
setting {
namespace = "aws:ec2:vpc"
name = "Subnets"
value = "#{aws_subnet.private-a.id},#{aws_subnet.private-b.id},#{aws_subnet.private-c.id}"
}
setting {
namespace = "aws:ec2:vpc"
name = "ELBSubnets"
value = "#{aws_subnet.public-a.id},#{aws_subnet.public-b.id},#{aws_subnet.public-c.id}"
}
...
depends_on = [
aws_security_group.default, # created earlier in the same file
aws_subnet.public-a,
aws_subnet.public-b,
aws_subnet.public-c,
aws_subnet.private-a,
aws_subnet.private-b,
aws_subnet.private-c
]
The problem is I always get an error that one of the subnets does not exist, for example:
Error: ConfigurationValidationException: Configuration validation exception:
Invalid option value: '["#{aws_subnet.private-c.id}","#{aws_subnet.private-b.id}","#{aws_subnet.private-a.id}"]'
(Namespace: 'aws:ec2:vpc', OptionName: 'Subnets'): The subnet '#{aws_subnet.private-c.id}' does not exist.
It is always related to a different subnet, sometimes one of the private ones used for the Subnets option, sometimes one of the public ones used in the ELBSubnets option.
What baffles is me is that I have explicitly defined them as dependencies, although I think it should work even without the explicit dependency. And in the terraform apply logs all of the subnets are always created before the eb environment:
aws_subnet.private-c: Creation complete after 1s [id=subnet-some-id]
aws_subnet.public-a: Creation complete after 1s [id=subnet-some-id]
aws_subnet.public-c: Creation complete after 1s [id=subnet-some-id]
aws_subnet.public-b: Creation complete after 1s [id=subnet-some-id]
aws_subnet.private-a: Creation complete after 6s [id=subnet-some-id]
aws_subnet.private-b: Creation complete after 6s [id=subnet-some-id]
...
aws_elastic_beanstalk_environment.default: Creating...
Upon checking in AWS, everything the subnets are created as expected and as reported by apply. What could be the reason for this error?

Answer 1

As ydaetskcor mentioned in the interpolation of the subnets and used # instead of $, so #{aws_subnet.public-a.id},#{aws_subnet.public-b.id} should be ${aws_subnet.public-a.id},${aws_subnet.public-b.id}.