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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.

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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.
Data sources are read-only. They cannot create, update, or destroy resources. For full lifecycle management, use resource blocks instead.

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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.
The image shows a section of AWS documentation related to EC2 key pairs, including a search result for "aws_key_pair" and an argument reference detailing optional and required parameters for querying key pairs.
Check the AWS Provider Data Sources documentation for all available arguments and attributes: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/key_pair

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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.

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Resources vs. Data Sources

AspectResource BlocksData Source Blocks
LifecycleCreate, Read, Update, DeleteRead only
Keywordresourcedata
Terraform StateManagedNot managed
Use CaseProvision infrastructureQuery existing infrastructure
The image compares Terraform "Resource" and "Data Source," highlighting their differences in keywords, functionality, and terminology. It shows that resources create, update, and destroy infrastructure, while data sources only read infrastructure.

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