Terraform - dependency between modules

Question

I'm trying tell terraform the resource is dependent on other one. The problem is the resources are in separate modules. The dependent resource looks like that:

variable dependency {
  type = "list"
  default = []
}

resource "docker_container" "web" {
  depends_on = "${var.dependency}"
...

Then I 'call' the module:

module "wordpress" {
  source = "../modules/wordpress"
  dependency = [ "${module.provision.res}" ]
}

And I got error:

  on ../modules/wordpress/main.tf line 11, in resource "docker_container" "web":
  11:   depends_on = "${var.dependency}"

A static list expression is required.

It looks like I cannot use variable in 'depends_on'. How to create dependency between modules?

PS: The resource I depend on is a null_resource which provides some some provisioning. I need to rebuild some stuff every time it changes.

Answer 1

To solve the error: "A static list expression is required."

You need to wrap the var.dependency with []:

resource "docker_container" "web" {
  depends_on = ["${var.dependency}"]
...

Update: The syntax above is for terraform<0.12. For terraform >=0.12, as @Maciej Wawrzyńczuk points out, [var.dependency] will just work in this case. ["${var.dependency}"] will also work in 0.12 as backward compatibility but if you run tf 0.12 you probably want to do it the new way.