Define Module Dependencies in Terraform

Question

How can we ensure that the first module “frontend_1” is executed before the rest?

module "frontend_1" {
  source = "/modules/frontend-app"
}
module "frontend_2" {
  source = "/modules/frontend-app"
}
module "frontend_3" {
  source = "/modules/frontend-app"
}

Also - there're a couple of recorded Gitissues re this problem but a good workaround has not been presented in any of them. As a start - you can check this one so that you can get familiar with the essence of my question - https://github.com/hashicorp/terraform/issues/10462

And how to make an external for a module resource be built before the module gets executed - so that the "count" can be computed if it depends on a value which is calculated from such an external resource? For example - if you need to use the ID of a newly created VPC in the "count" in a module which creates multiple AWS Security Groups within that VPC?

Answer 1

As mentioned in the question, I was able to make a workaround for you using the depends on variable.

Refer to this for Terraform dependencies.

Assume we have 2 modules, one which defines the vpc and subnets, the second to define the various range of security groups to be used in the Infrastructure.

Since we have the dependency such that all security groups are to be made only after the successful creation of vpc in the VPC module, it can be met by the following strategy.

variable "vpc_arn" {
   description = "The ARN of the VPC which is created in the VPC module"
}

resource "null_resource" "vpc_found" {
  triggers = {
    vpc_name = "${var.vpc_arn}"
  }
}

resource "aws_security_group" "allow_all" {

  depends_on = ["null_resource.vpc_found"]

  name        = "allow_all"
  description = "Allow all inbound traffic"
  vpc_id      = "${var.vpc_arn}"
  ......
}

Refer to this for null resources.