How to iterate over all aws_instances in terraform? - amazon-ec2

Question

I'm relatively new to terraform and I'm trying to iterate over all aws_instances to apply a null_resource. Can you use multiple splats to access all instances, regardless of their names?
The EC2 instances are broken down by three types:
aws_instance.web.* (3 instances)
aws_instance.app.* (3 instances)
aws_instance.db.* (2 instances)
Here's my attempt to apply a null_resource to all eight aws_instances:
resource "null_resource" "install_security_package" {
#count = "${length(aws_instance)}" #terraform error: resource count can't reference variable: aws_instance
#count = "${length(aws_instance.*)}" #terraform error: resource variables must be three parts: TYPE.NAME.ATTR
count = "${length(aws_instance.*.*)}" #terraform error: unknown resource 'aws_instance.*'
connection {
type = "ssh"
host = "${element(aws_instance.*.private_ip, count.index)}"
user = "${lookup(var.user, var.platform)}"
private_key = "${file("${var.private_key_path}")}"
timeout = "2m"
}
provisioner "remote-exec" {
inline = [
"sudo rpm -Uvh http://www.example.com/security/repo/security_baseline.rpm",
]
}
}

Answer 1

It is not currently possible to match all resources of a given type. The "splat" syntax, as you've seen, only allows selecting all of the instances created from a particular resource block.
The closest you can get to this with Terraform today is to concatenate together the different resources:
concat(aws_instance.web.*.private_ip, aws_instance.app.*.private_ip, aws_instance.db.*.private_ip)
In the current version of Terraform as of this answer it is necessary to use some of the workarounds shared in github issue #4084 in order to avoid duplicating that complex expression in multiple places. A forthcoming feature called Local Values will make this simpler in the near future, allowing the list to be given an name to be re-used in multiple places:
# Won't work until Terraform PR#15449 is merged and released
locals {
aws_instance_addrs = "${concat(aws_instance.web.*.private_ip, aws_instance.app.*.private_ip, aws_instance.db.*.private_ip)}"
}
resource "null_resource" "install_security_package" {
count = "${length(local.aws_instance_addrs)}"
connection {
type = "ssh"
host = "${local.aws_instance_addrs[count.index]}"
user = "${lookup(var.user, var.platform)}"
private_key = "${file("${var.private_key_path}")}"
timeout = "2m"
}
provisioner "remote-exec" {
inline = [
"sudo rpm -Uvh http://www.example.com/security/repo/security_baseline.rpm",
]
}
}