How to parse instance id in Terraform - amazon-web-services

Question

We want to get away from using incremental numbers for the host names of our instances. These machines are not in an auto-scaling group, hence we could keep the prod-webserver-001, prod-webserver-002 etc convention if we wanted but it just doesn't make sense with tags being available.
What we've been doing with auto-scaling groups, we want to utilize parts of the instance id. I'm able to accomplish this with a post script with our ASG servers but unlike that, I want Terraform to keep track of the DNS record so that they get destroyed when I issue a terraform destroy.
Ideally we want the first six characters after the hyphen in the instance_id. For example if the instance_id is i-0876cr2456 we want to use 0876cr within the name.
prod-webserver-0876cr
prod-webserver-09a24i
Terraform code:
resource "aws_instance" "instance" {
...
...
}
resource "aws_route53_record" "instance_dns_a" {
count = "${var.num_instances}"
zone_id = "${var.internal_zone_id}"
# New line but we want the parsed version
name = "${aws_instance.instance.id}"
# old that works
name = "${format(prod-${service_name}-%03d", count.index + 1)}"
}

Answer 1

You could extract the first 6 characters from the instance id by using substr:
$ terraform console
> substr("i-123456adgcgabsadh", 2, 6)
123456
So to use it in your Route53 record you'd want to use something like:
resource "aws_instance" "instance" {
...
...
}
resource "aws_route53_record" "instance_dns_a" {
count = "${var.num_instances}"
zone_id = "${var.internal_zone_id}"
name = "prod-${var.service_name}-${substr(aws_instance.instance.*.id[count.index], 2, 6}"
}
I would be worried about truncating the instance ID for something that needs to be unique though because you will then inevitably end up with instances with ids of i-123456a... and i-123456b and then you'll end up overwriting the prod-webserver-123456 record with the IP address of the second instance and losing the first record. Any reason you need to truncate here? You have 63 characters to play with for the first label in DNS which should be enough to leave this untruncated no?