attach security group created in another app - terraform

Question

folder structure.
I am creating the following for 2 seperate applications using same modules in terragrunt
LB
Instances
Security Groups
my question is how do I reference a security group created for app1 in app2?
eg.
in app1
I can references it as
security_groups = ["${aws_security_group.sec_group_A.id}"]
how can I refer the same security group in app2?
resource "aws_security_group" "sec_group_A" {
name = "sec_group_A"
...
...
}
resource "aws_elb" "bar" {
name = "foobar-terraform-elb"
security_groups = ["${aws_security_group.sec_group_A.id}"]
...
...
}

Answer 1

In app2, you can:
data "aws_security_group" "other" {
name = "sec_group_A"
}
and then use the ID:
resource "aws_elb" "bar" {
name = "foobar-terraform-elb"
security_groups = ["${data.aws_security_group.other.id}"]
...
...
}
(caveat for using data is that you are running two separate terraform applys - one configuration creates the group, and other configuration references the group).

Answer 2

I have no experience of using terragrunt, but normally I would be calling my modules from a "main.tf" file in the root of the project. An example folder structure is below
.
├── main.tf
└── modules
├── app1
│ ├── main.tf
│ ├── outputs.tf
│ └── variables.tf
└── app2
├── main.tf
├── outputs.tf
└── variables.tf
My app1 outputs.tf declares a security group A output
output "sec_group_a" { value = "${aws_security_group.sec_group_A}" }
I can then call this output in my main.tf file in the root of the project. This would look something like the below
module "app1" {
source = "./modules/app1"
...
// Pass in my variables
}
module "app2" {
source = "./modules/app2"
sec_group_A = "${module.app1.sec_group_A}"
...
//Pass in the rest of my variables
}
Finally inside of the app2 module you can call this as you would any other variable.
resource "aws_elb" "bar" {
name = "foobar-terraform-elb"
security_groups = ["${var.sec_group_A.id}"]
...
...
}
I'd read up on modules here https://www.terraform.io/docs/modules/index.html to get a better understanding of how they fit together.
Alternatively you can grab the data from your remote state (if you have one configured) as long as sec_group_A declared as an output in app1. See https://www.terraform.io/docs/providers/terraform/d/remote_state.html