Module output when you don't know which resource will be created

[B Holmes]

Hi,

I'm writing a module that creates a snapshot-based RDS instance if the snapshot_name variable has a value, otherwise create a plain vanilla Oracle instance.

I'm using the count attribute and conditional replace to pass the with-snapshot resource's count a 1 if there is a snapshot_name, or 0 if snapshot_name is empty.

Likewise I'm using the count attribute and conditional replace to pass the without-snapshot resource's count a 1 if there isn't a snapshot_name, or 0 if there is.

So far, so good, this code is working.  But I want to create an rds_instance_address output that will take either the value of:

aws_db_instance.with_snap.address  or

aws_db_instance.without_snap.address

Things I've tried:

output "rds_instance_address" {

  value = "${aws_db_instance.with_snap.address}"

  value = "${aws_db_instance.without_snap.address}"

}

But when without_snap has no value (because we've provisioned a snapshot-based RDS instead), rds_instance_address has no value.  Same happens with:

output "rds_instance_address" {

   value = "${aws_db_instance.with_snap.address}${aws_db_instance.with_snap.address}"

}

Unf output takes no count attribute, so can't use above boolean trick to assign a 1 if aws_db_instance.with_snap.address has a value, etc.

Is there any way to achieve this?  The reason I need a single output that has one or other value is that I need to output to a text file and if you pass an empty variable to local-exec (that cat's variables to a text file), it errors: "local-exec provisioner command must be a non-empty string".

Many thanks

[Lowe Schmidt]

Have a look at conditionals https://www.terraform.io/docs/configuration/interpolation.html#conditionals

You should be able to do something like

output "rds_instance_address" {

  value = "${aws_db_instance.with_snap.address ? aws_db_instance.with_snap.address :  aws_db_instance.without_snap.address} "

}

--

Lowe Schmidt | +46 723 867 157

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[B Holmes]

output "rds_instance_address" {

  value = "${aws_db_instance.with_snap.address ? aws_db_instance.with_snap.address :  aws_db_instance.without_snap.address} "

}

Doesn't work.  If either trueval or falseval don't exist, it outputs nothing.  Can verify this using a simple example:

module "eips" {

  source = "poth/to/module/eips"

  create_an_eip1 = "true"

}

output "eip_pub_ip" {

  value = "${module.eips.eip_pub_ip}"

 }

where module eips contains:

variable "create_an_eip1" {}

resource "aws_eip" "eip1" {

  count = "${var.create_an_eip1 ? 1 : 0 }"

}

resource "aws_eip" "eip2" {

  count = "${var.create_an_eip1 ? 0 : 1 }"

}

output "eip_pub_ip" {

  value = "${var.create_an_eip1 ? aws_eip.eip1.public_ip : aws_eip.eip2.public_ip }"

}

eip1 is successfully created if create_an_eip1 is true, or eip2 is successfully created if create_an_eip1 is false.  But nothing is output either way.