Using subnets launched VPC in RDS

Terraform CDK for Terraform
1

Hello all, this is my first question here. Apologize if something goes wrong, I’m also a newbie in Terraform,

Anyway, I have two modules in my AWS Terraform script, and it should be as simple as possible, just to help me to understand how it works.

In the vpc module, I create the VPC itself, subnets (let’s say 6 of them, 3 public, 3 private), internet gateway, route tables, and NAT gateway.

########################################
# starting private subnet - API
resource "aws_subnet" "api-subnets" {
  count = var.api_subnets_size
  vpc_id = aws_vpc.main-pub-vpc.id
  cidr_block = "10.0.${count.index+10}.0/24"
  availability_zone = data.aws_availability_zones.available.names[count.index]
  map_public_ip_on_launch = false
  tags = {
    "Name" = "${var.prefix}-subnet-api-${count.index+1}",
    "cliente" = var.client,
    "ambiente" = "dev"
  }
}

exposing them to other modules (I think they are exported as a list)

output "api_subnet_ids" {
  value = aws_subnet.api-subnets[*].id
}

In the rds module, I create a Postgres instance in the private subnets, so I have to get them from the vpc module to create the subnet group. Easy, right?

First I have to declare rds module in my main.tf and get that output from the vpc module.

module "vpc" {
  source = "./modules/vpc"
  prefix = var.prefix
  client = var.client
  api_subnets_size = var.api_subnets_size
  api_subnet_ids = module.vpc.api_subnet_ids <== HERE
}

Then the variable is used to set up the subnet ids list in the rds module:

module "rds" {
  source = "./modules/rds"
  prefix = var.prefix
  client = var.client
  api_subnet_ids = module.vpc.api_subnet_ids <== AND HERE
}

Finally, I use this variable to set the subnets in the rds subnets group:

resource "aws_db_subnet_group" "api-subnet-grp" {
  name = "${var.prefix}-api-subnet-grp"
  subnet_ids = api_subnet_ids  # -> why doesn't work here? It is a list in both source and destination parameters
  description = "subnet group to allow Multi AZ on this instance"
  tags = {
    "Name" = "${var.prefix}-api-subnet-grp",
    "cliente" = var.client,
    "ambiente" = "dev"
  }
}

Apparently, the subnet_ids property type of resource aws_db_subnet_group is different from subnets property type of resource aws_subnet list of subnets…

I am completely lost here. I really don’t know what I did wrong or how to proceed to move forward. It was supposed to be easy.

Thanks,
Renato

2

Assuming you are just talking about hcl (you posted in the CDK for Terraform section)…
Did you perhaps miss defining a api_subnet_ids variable within your rds module? At the very least, it’s passed as a variable but the reference is missing var. in your example.

1 Like
3

Hi @jsteinich , thanks for replying.

Yes, I did create the output in the vpc module for db-subnets:

output "db_subnet_ids" {
  value = aws_subnet.db-subnets[*].id
}

Also, I created the variable db-subnets in the rds module:

variable "db_subnet_ids" {}

Finally, in main.tf the information gets linked to each other:

module "rds" {
  source = "./modules/rds"
  db_subnet_ids = module.vpc.db_subnet_ids

  # other stuff
}

Did I miss something?

Regards

4

Do you have api_subnet_ids or var.api_subnet_ids when creating your aws_db_subnet_group resource?

What error do you get?

5

Hello @jsteinich

I deleted all tfstate files and ran terraform init again and voilá, it worked. Really don’t know why…