Terraform: adding depends_on to your custom modules

Association between some resources is deferred and takes some time. If you don’t explicitly pass the association resource as a variable to your custom module, terraform might fail to provision. Terraform’s modules do not support depends_on out of the box, but it is not difficult to emulate it.

# File fancy-app-module/variables.tf
variable depends_on { default = [], type = "list"}

# File my-app.tf
module "app" {
    source  = "modules/fancy-app-module"
    # Wait for resources and associations to be created
    depends_on = [
        "${aws_alb_target_group.app.arn}"
    ]
}
resource "aws_alb_target_group" "app" {
    name     = "app-group"
}
resource "aws_alb_listener" "front_end" {
   # Association of default_action takes some time and
   # if this action is required by you module, it's creation
   # might fail due to async provisioning of the
   # resources by terraform
   default_action {
        target_group_arn = "${aws_alb_target_group.app.id}"
        type             = "forward"
    }
}