Explicitly using variable default in Terraform

Question

I have a map of values (populated from Consul), which I use to configure my resources, however if values for optional variables are missing, I would like Terraform to act as it the parameter was not provided. For example:

resource "aws_db_instance" "db" {
  engine = "${lookup(config_map, "db_engine", "postgres")}"
  port   = "${lookup(config_map, "db_port", "<pick default for the engine>")}"
}

If port is not given, Terraform picks a default value depending on the engine. Can I trigger this behavior explicitly?

norbjd · Accepted Answer · 2018-12-26 20:04:45Z

1

The following should do what you expect (syntax is validated, however apply has not been tested : I'll update the answer if it works, or delete it otherwise).

First, you should have somewhere a mapping between engines and default ports (here, this is a variable, but it could be stored in Consul like your config_map) :

variable "default_ports_by_engine" {
  type = "map"

  # key = engine, value = port
  default = {
    "postgres" = "3333"
    "mysql"    = "3334"
    # other engines/ports...
  }
}

Then, you can use this variable in a nested lookup :

resource "aws_db_instance" "db" {
  engine = "${lookup(config_map, "db_engine", "postgres")}"
  port   = "${
    lookup(
      var.default_ports_by_engine,
      "${lookup(config_map, "db_engine", "postgres")}"
    )
  }"
}

Notice that not passing a third argument to lookup function will make Terraform fail if db_engine is not found in default_ports_by_engine.

answered Dec 26, 2018 at 20:04

norbjd

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3 Comments

yurez Over a year ago

I suppose this will work, but I had hoped to reuse the logic performed by Terraform, rather than replicate it.

norbjd Over a year ago

What do you mean by "replicate the logic"?

yurez Over a year ago

In this example, I need to check the default ports used by Terraform for all the engines I want to use and hard-code them.

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