Creating subnets and assinging rout tables to them - terraform

Question

I am new to terraform and I'm trying to create a VPC with multiple subnets and adding route tables to all those subnets in a for loop manner.
VPC: 10.207.0.0/16
There's number_of_subnets which will create subnets like this: 10.207.x.0/24
This code works fine:
variable "region" {
default = "us-east-1"
}
variable "availability_zone" {
default = "us-east-1a"
}
variable "cidr_block" {
default = "207"
}
variable "number_of_subnets" {
default = 5
}
provider "aws" {
region = var.region
}
resource "aws_vpc" "test_vpc" {
cidr_block = "10.${var.cidr_block}.0.0/16"
instance_tenancy = "default"
enable_dns_support = true
enable_dns_hostnames = true
tags = {
Name = "test_vpc_${var.cidr_block}"
}
}
resource "aws_subnet" "test_subnets" {
count = var.number_of_subnets
vpc_id = aws_vpc.test_vpc.id
cidr_block = "10.${var.cidr_block}.${count.index+1}.0/24" # start from x.x.1.0/24
availability_zone = var.availability_zone
map_public_ip_on_launch = false
tags = {
Name = "test_subnet_${var.cidr_block}_${count.index+1}"
}
}
Now if I try to add this code to the bottom of the same file (everything is in one file called main.tf) to get the subnets and add route table to each:
# get all subnet IDs
data "aws_subnets" "q_subnets" {
filter {
name = "vpc-id"
values = [aws_vpc.test_vpc.id]
}
}
# add route table to all subnets
resource "aws_route_table_association" "rt_assoc_subnet" {
depends_on = [aws_subnet.test_subnets]
for_each = toset(data.aws_subnets.q_subnets.ids)
subnet_id = each.value
route_table_id = aws_route_table.test_rt.id
}
and run terraform apply it will give this error:
invalid for_each argument...
The "for_each" value depends on resource attribute that cannot be deteremined until apply,...
which doesn't make scense. First create vpc, then subnet, then get all subnets...
I also tried depends_on and didn't help.
How would I write this to make it work?
Any help is appreciated.
UPDATE1:
I tried to use aws_subnet.test_subnets.*.id instead of data and it still gives depencendy error:
variable "region" {
default = "us-east-1"
}
variable "availability_zone" {
default = "us-east-1a"
}
variable "cidr_block" {
default = "207"
}
variable "number_of_subnets" {
default = 5
}
provider "aws" {
region = var.region
}
resource "aws_vpc" "test_vpc" {
cidr_block = "10.${var.cidr_block}.0.0/16"
instance_tenancy = "default"
enable_dns_support = true
enable_dns_hostnames = true
tags = {
Name = "test_vpc_${var.cidr_block}"
}
}
resource "aws_route_table" "test_rt" {
vpc_id = aws_vpc.test_vpc.id
route = []
tags = {
Name = "test_rt_${var.cidr_block}"
}
}
resource "aws_subnet" "test_subnets" {
count = var.number_of_subnets
vpc_id = aws_vpc.test_vpc.id
cidr_block = "10.${var.cidr_block}.${count.index+1}.0/24" # start from x.x.1.0/24
availability_zone = var.availability_zone
map_public_ip_on_launch = false
tags = {
Name = "test_subnet_${var.cidr_block}_${count.index+1}"
}
}
output "subnets" {
value = aws_subnet.test_subnets.*.id
}
# add route table to all subnets
resource "aws_route_table_association" "rt_assoc_subnet" {
depends_on = [aws_subnet.test_subnets]
for_each = toset(aws_subnet.test_subnets.*.id)
subnet_id = each.value
route_table_id = aws_route_table.test_rt.id
}
is there another way to pass the subnets to aws_route_table_association without getting dependency error?

Answer 1

Since you are using count, it is very hard to make count work with for_each. It would be better to continue using count for route table association as well. If you decide to go down that route, the only change you need is:
resource "aws_route_table_association" "rt_assoc_subnet" {
count = var.number_of_subnets
subnet_id = aws_subnet.test_subnets.*.id[count.index]
route_table_id = aws_route_table.test_rt.id
}
This will work as intended. However, if you must use for_each I would suggest defining a variable that could be used with it in all the resources you are now using count. If you really want to use for_each with the current code, then you can use the -target option [1]:
terraform apply -target=aws_vpc.test_vpc -target=aws_route_table.test_rt -target=aws_subnet.test_subnets
When running this command, this will be shown in the command output:
│ Warning: Resource targeting is in effect
│
│ You are creating a plan with the -target option, which means that the result of this plan may not represent all of the changes requested by the current
│ configuration.
│
│ The -target option is not for routine use, and is provided only for exceptional situations such as recovering from errors or mistakes, or when Terraform
│ specifically suggests to use it as part of an error message.
After the targeted resources are created, you could re-run terraform apply and it should create the route table associations.
[1] https://www.terraform.io/cli/commands/plan#target-address