CODEWITHSUNDEEP
amazon-web-servicesterraform
nickackerman42
nickackerman42

Reputation: 515

How to conditionally use existing resource in Terraform?

In my Terraform (0.14.10) module, I'd like to provide the option of deploying into an existing AWS VPC. I'm varying the count operator based on a boolean input use_existing_network.

If use_existing_network is false, I create a VPC network. If it's true, I retrieve the existing resource with a data block.

resource "aws_vpc" "cluster_network" {
  count = var.use_existing_network ? 0 : 1

  cidr_block = "10.0.0.0/16"

  tags = {
    Name = var.network_name
  }
}

data "aws_vpc" "cluster_network" {
  count = var.use_existing_network ? 1 : 0

  filter {
    name   = "tag:Name"
    values = [var.network_name]
  }
}

However, I get the following error when I attempt to reference the network (a tuple, since it's created with the count operator) later:

Error: Invalid index

  on ../../modules/network/main.tf line 61, in resource "aws_internet_gateway" "igw":
  61:   vpc_id = aws_vpc.cluster_network[0].id
    |----------------
    | aws_vpc.cluster_network is empty tuple

The given key does not identify an element in this collection value.

Why doesn't this work, and how can I achieve this?

Upvotes: 1

Views: 303

Answers (1)

Marcin
Marcin

Reputation: 239000

In your case cluster_network is either a resource or data source, depending on the condition. They are referred to differently. You could use local to unify that. For example with try:

locals {
   cluster_network = try(aws_vpc.cluster_network[0], data.aws_vpc.cluster_network[0])
}

Then use local.cluster_network.id in the rest of the code.

Upvotes: 3

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