CODEWITHSUNDEEP
amazon-web-servicesamazon-rdsterraform
SnazzyBootMan
SnazzyBootMan

Reputation: 739

Terraform Interpolation in outputs

I create an AWS RDS instance with different KMS CMKs depending on whether or not the environment is Production or Non-Production. So I have two resources that use the terraform count if:

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

This spins up an RDS instance with different KMS keys with different addresses. I need to capture that endpoint (which I do with terraform show after the build finishes) so why doesn't this work in Terraform?

output "rds_endpoint" {
    value = "${var.bluegreen == "nonprod" ? aws_db_instance.rds_nonprod.address : aws_db_instance.rds_prod.address}"
}

Upvotes: 2

Views: 911

Answers (1)

Martin Atkins
Martin Atkins

Reputation: 74684

It is an error to access attributes of a resource that has count = 0, and unfortunately Terraform currently checks both "sides" of a conditional during its check step, so expressions like this can fail. Along with this, there is a current behavior that errors in outputs are not explicitly shown since outputs can get populated when the state isn't yet complete (e.g. as a result of using -target). These annoyances all sum up to a lot of confusion in this case.

Instead of using a conditional expression in this case, it works better to use "splat expressions", which evaluate to an empty list in the case where count = 0. This would looks something like the following:

output "rds_endpoint" {
    value = "${element(concat(aws_db_instance.rds_nonprod.*.address, aws_db_instance.rds_prod.*.address), 0)}"
}

This takes the first element of a list created by concatenating together all of the nonprod addresses and all of the prod addresses. Due to how you've configured count on these resource blocks, the resulting list will only ever have one element and so it will just take that element.

In general, to debug issues with outputs it can be helpful to evaluate the expressions in terraform console, or somewhere else in a config, to bypass the limitation that errors are silently ignored on outputs.

Upvotes: 2

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