How to allow a user to specify an arbitrary number of resources for an environment?

Terraform
1

I am attempting to provide the ability to specify an arbitrary number of resources in a variables file and based on the configuration specified therein use various Terraform programming constructs (if/then, for each, etc.) to turn it into relevant calls to a module which creates/manages a resource (an AWS instance in this case). For example, we could have one configuration (variables.tf) such as the below for a UAT environment (VPC) with two instance components:

"vpc" = {
    "vpc_id" = "uat-1"
    "vpc_cidr_block" = "1.2.3.4/5"
    "vpc_name" = "org-uat"
}
"instances" = {
    "db" = {
        "subnet_id" = "db-sub-1"
        "subnet_name" = "uat-db-1"
        "subnet__cidr_block" = "11.22.33.44/5" 
    }
    "web" = {
        "subnet_id" = "web-sub-1"
        "subnet_name" = "uat-web-1"
        "subnet__cidr_block" = "10.21.32.43/5" 
    }
}

and another for a development environment (VPC) that has three instance components:

 "vpc" = {
    "vpc_id" = "dev-1"
    "vpc_cidr_block" = "1.2.33.4/56"
    "vpc_name" = "org-dev"
}
"instances" = {
    "app" = {
        "subnet_id" = "app-sub-1"
        "subnet_name" = "dev-app-1"
        "subnet_cidr_block" = "10.21.32.44/56" 
    }    
    "db" = {
        "subnet_id" = "db-sub-1"
        "subnet_name" = "dev-db-1"
        "subnet_cidr_block" = "11.22.33.44/5" 
    }
    "web" = {
        "subnet_id" = "web-sub-1"
        "subnet_name" = "dev-web-1"
        "subnet_cidr_block" = "10.21.32.43/5" 
    }
}

The idea is to loop over the instances and to call a module for instance management. There are for loops and if/then/else constructs that make it possible to do something like this within the terraform.tf in the environment’s directory (the below is not valid HCL, only pseudocode):

for instance in ${var.instances}:
        module "instance" {
            source                      = "../../modules/api/dev"
           vpc_cidr_block         = ${var.vpc.vpc_cidr_block}
           vpc_id                        = ${var.vpc.vpc_id}
           vpc_name                 = ${var.vpc.vpc_name}
           subnet_cidr_block   = ${instance.subnet_cidr_block}
           subnet_name           = ${instance.subnet_name}
           subnet_id                  = ${instance.subnet_id}
}

where the above is looping over items in the instances map/dictionary and calls an “instance” module for each. The instance module just configures an AWS instance based on the VPC and subnet variable values passed as inputs:

data "aws_vpc" "example_vpc" {
  cidr_block = var.vpc_cidr_block

  tags = {
    Name = var.vpc_name
  }
}

data "aws_subnet" "example_subnet" {
  vpc_id             = data.aws_vpc.example_vpc.id
  cidr_block         = var.subnet_cidr_block
  availability_zone  = var.subnet_availability_zone

  tags = {
    Name = var.subnet_name
  }
}

resource "aws_key_pair" "example_aws_key_pair" {
  key_name    = "example_key"
  public_key  = file("~/.ssh/terraform.pub")
}

resource "aws_instance" "example_aws_instance" {
  key_name        = aws_key_pair.example_aws_key_pair.key_name
  ami             = "ami-b374d5a5"
  instance_type   = "t2.micro"
  subnet_id       = data.aws_subnet.example_subnet.id

  connection {
    type        = "ssh"
    user        = "terraform"
    private_key = file("~/.ssh/terraform")
    host        = self.public_ip
  }

  tags = {
    "Name"      : "dev_terraform_${terraform.workspace}_${var.environment}"
    "Terraform" : "true"
  }
}

The inputs for the above module are declared like so:

variable "environment" {}
variable "subnet_availability_zone" {}
variable "subnet_cidr_block" {}
variable "subnet_name" {}
variable "vpc_cidr_block" {}
variable "vpc_id" {}
variable "vpc_name" {}

Does this appear to be a reasonable approach to allow a user to configure/manage an infrastructure containing one to many AWS instances, assuming the user can define these instances in a variables/config file?

I am new with Terraform so there may be some really obvious errors in my approach or (attempted) implementation, please advise if so, any help is very kindly appreciated!