Terraform - Multi Loops or Maps - amazon-web-services

Question

I am creating few public subnets, private subnets, igw, nat, route_tables, and route_table entry in AWS using terraform. Below is the number of resources I am creating.
terraform.tfvars
vpc_cidr = "10.0.0.0/16"
public_subnet_count = 6
public_subnets_cidr = ["10.0.1.0/24","10.0.2.0/24","10.0.3.0/24","10.0.4.0/24","10.0.5.0/24", "10.0.6.0/24"]
availability_zones = ["us-east-2a", "us-east-2b","us-east-2c","us-east-2d"]
resources.tf
resource "aws_vpc" "vpc" {
cidr_block = var.vpc_cidr
enable_dns_hostnames = true
enable_dns_support = true
tags = {
Name = "${var.environment}-vpc"
Environment = var.environment
}
}
resource "aws_subnet" "public_subnet" {
vpc_id = "${aws_vpc.vpc.id}"
count = var.public_subnet_count
cidr_block = "${element(var.public_subnets_cidr, count.index)}"
availability_zone = length(var.availability_zones) > 1 ? var.availability_zones[count.index % length(var.availability_zones)] : var.availability_zones[0]
map_public_ip_on_launch = false
tags = {
Name = "${var.environment}-${element(var.availability_zones, count.index)}-public-subnet"
Environment = "${var.environment}"
}
}
resource "aws_internet_gateway" "ig" {
count = 1
vpc_id = aws_vpc.vpc.id
tags = {
Name = "${var.environment}-igw"
Environment = "${var.environment}"
}
}
resource "aws_route_table" "public" {
count = length(var.availability_zones)
vpc_id = aws_vpc.vpc.id
tags = {
Name = "${var.environment}-public-route-table"
Environment = "${var.environment}"
}
}
resource "aws_route" "public_internet_gateway" {
count = length(aws_route_table.public.*.id)
route_table_id = element(aws_route_table.public.*.id, count.index)
destination_cidr_block = "0.0.0.0/0"
gateway_id = aws_internet_gateway.ig[0].id
}
resource "aws_route_table_association" "public" {
count = length(var.availability_zones)
subnet_id = element(aws_subnet.public_subnet.*.id, count.index)
route_table_id = element(aws_route_table.public.*.id, count.index)
}
Query in aws_route_table_association.public section.
How do I attach private subnet of a particular az to a route table id. For example if i use 6 private subnets in tfvars, it creates atleast 2 private subnets in one az (ex: us-east-2a). How do i loop and attach 2 subnet from us-east-2a to one route table created for us-east-2a. Kind of map between multiple subnets in one az to route table in that az.
This is to attach each az nat gateway to that az subnet for routing.

Answer 1

You don't need to create a route table per AZ, the route tables are linked to subnets. So, you can just create a single route table and then link all your public subnets to it:
resource "aws_route_table" "public" {
vpc_id = aws_vpc.vpc.id
route {
cidr_block = "0.0.0.0/0"
gateway_id = aws_internet_gateway.ig.id
}
}
resource "aws_route_table_association" "public" {
count = var.public_subnet_count
subnet_id = aws_subnet.public_subnet[count.index].id
route_table_id = aws_route_table.public.id
}
I've included the single route into the route_table to reduce the amount of code, so you can delete the aws_route resource if you use this.
[Edit]
Sorry I just noticed your mention of private subnets, but attaching route tables follows the same principle, and you don't need a NAT gateway per AZ either (that's expensive!) so you could route all private subnets through a single NAT gateway with one route table.
If you really want 4 NAT gateways, then you could do something like:
resource "aws_route_table" "private" {
count = length(var.availability_zones)
vpc_id = aws_vpc.vpc.id
route {
cidr_block = "0.0.0.0/0"
nat_gateway_id = aws_nat_gateway.nat[count.index].id
}
}
resource "aws_route_table_association" "private" {
count = var.private_subnet_count
subnet_id = aws_subnet.private[count.index].id
route_table_id = aws_route_table.private[
count.index % length(var.availability_zones)
].id
}
Since you already use the modulus (%) operator to distribute the subnets, I think you'll get the same result doing it this way.