Terraform: ignore_changes conditionally? (temporary EC2 instance public IP address)

Question

I've got a couple of EC2 instances managed by Terraform, with public IP addresses in a VPC.

With each Terraform run, I'm updating DNS records based on those public IPs.

I don't need these instances on all the time though, and when I shut them down, they of course lose their public IP address.

At first this was causing associate_public_ip_address to become false, and Terraform wanted to recreate the instances. lifecycle.ignore_changes solves this:

resource "aws_instance" "instance1" {
  associate_public_ip_address = true
  ...

  lifecycle {
    ignore_changes = ["associate_public_ip_address"]
  }
}

resource "cloudflare_record" "record1" {
  domain = "example.com"
  name   = "server1.example.com"
  value  = "${aws_instance.instance1.public_ip}"
  type   = "A"
}

But... the public_ip attribute also becomes blank ;). This causes an attempted update to the A record, which fails because you can't set a blank record.

So, I'm trying to work out a solution. I could use elastic IPs, which I might end up resorting to, but I'd ideally like to avoid that if I can. My next thought was... is there any way I could use ignore_changes conditionally, and thus ignore changes to the DNS record's value if it is blank?

Are there any other solutions you can recommend?

Answer 1

Maybe you could use "count" attribute for this?

resource "cloudflare_record" "record1" {
    count = ${aws_instance.instance1.public_ip == "" ? 0 : 1}"
    ...
}