Data Sources

OpenTofu lets you provision infrastructure and reference attributes between resources using expressions. But what if a resource already exists—created manually, via another tool, or in a different configuration? Data sources solve this by fetching read-only information about existing infrastructure.

Why Use Data Sources?

• Read attributes of existing resources without managing their lifecycle.
• Integrate with resources created by CloudFormation, Ansible, Terraform, or manually.
• Avoid duplicating state in multiple configurations.

Referencing an Existing AWS Key Pair

Suppose you already have an AWS Key Pair named alpha. You can fetch its key_name for use in an EC2 instance:

data "aws_key_pair" "cerberus_key" {
  key_name = "alpha"
}

resource "aws_instance" "cerberus" {
  ami           = var.ami
  instance_type = var.instance_type
  key_name      = data.aws_key_pair.cerberus_key.key_name
}

• data "aws_key_pair" "cerberus_key" declares a data source.
• The argument key_name = "alpha" locates the existing key pair.
• In the EC2 resource, data.aws_key_pair.cerberus_key.key_name provides the fetched value.

Filtering Data Sources by Tags

When you can’t identify a resource by a single attribute, use filters. For example, locate the key pair tagged with project = cerberus:

data "aws_key_pair" "cerberus_key" {
  filter {
    name   = "tag:project"
    values = ["cerberus"]
  }
}

resource "aws_instance" "cerberus" {
  ami           = var.ami
  instance_type = var.instance_type
  key_name      = data.aws_key_pair.cerberus_key.key_name
}

• The filter block matches key pairs with the given tag.
• Multiple filters can be combined to narrow the search.

Resources vs. Data Sources

Links and References

• OpenTofu Documentation
• Terraform AWS Provider Data Sources
• AWS CloudFormation
• Ansible