Does Terraform perform interpolation in provider declarations? - terraform

Question

I am trying to declare the following Terraform provider:
provider "mysql" {
endpoint = "${aws_db_instance.main.endpoint}:3306"
username = "root"
password = "root"
}
I get the following error:
Error refreshing state: 1 error(s) occurred:
* dial tcp: lookup ${aws_db_instance.main.endpoint}: invalid domain name
It seems that Terraform is not performing interpolation on my endpoint string, yet I don't see anything in the documentation about this -- what gives?

Answer 1

Yes, it does. There's an example in the docs at https://www.terraform.io/docs/providers/mysql/
# Configure the MySQL provider based on the outcome of
# creating the aws_db_instance.
provider "mysql" {
endpoint = "${aws_db_instance.default.endpoint}"
username = "${aws_db_instance.default.username}"
password = "${aws_db_instance.default.password}"
}

Answer 2

I ran into a similar set of error messages ("connect failed," "invalid domain lookup") and looked into this a bit. I hope this helps you or someone else working across cloud and database providers in Terraform.
This seems to come down to the MySQL provider attempting to establish a database connection as soon as it's initialized, which could be a problem if you're trying to build a database server and configure the database / grants on it as part of the same Terraform run. Providers get initialized based on Terraform finding a resource owned by that provider in your Terraform code, and since this connection attempt happens when the provider gets initialized, you can't work around this with -target=<SPECIFIC RESOURCE>.
The workarounds I can think of would be to have a codebase for setting up the database server and a different codebase for setting up the database grants and suchlike ... or to have Terraform kick off a script that does that work for you (with dynamic parameters, of course!). Either way, you're effectively removing mysql_* resources from your initial Terraform run and that's what fixes this.
There are a couple of code changes that probably need to happen here - the Terraform MySQL provider would need to delay connecting to the database until Terraform tells it to run an operation on a resource, and it may be necessary to look at how Terraform handles dependencies across providers. I tried hacking in deferred connection logic just for the mysql_database resource to see if that solved all my problems and Terraform still complained about a dependency loop in the graph.
You can track the MySQL provider issue here:
https://github.com/terraform-providers/terraform-provider-mysql/issues/2
And the comments from before providers were split into their own releasable codebases:
https://github.com/hashicorp/terraform/issues/5687