Terraform Data Sources Explained: Read Existing Cloud Resources

Terraform Data Sources Explained#

Learn how Terraform data sources read existing infrastructure, lookup cloud resources, and connect reusable Terraform code with real environments.

Quick answer#

Resource#

Creates or manages infrastructure.

Data source#

Reads existing information from a provider.

What is a Terraform data source?#

A data source lets Terraform read information about existing resources without managing their full lifecycle.

Example#

data "aws_ami" "ubuntu" { most_recent = true owners = ["099720109477"] filter { name = "name" values = ["ubuntu/images/hvm-ssd/ubuntu-jammy-22.04-amd64-server-*"] } }

Using the data source#

resource "aws_instance" "web" { ami = data.aws_ami.ubuntu.id instance_type = "t3.micro" }

Common use cases#

• Find latest AMI/image.
• Read existing VPC/subnet.
• Lookup DNS zone.
• Read existing key vault or secret reference.
• Fetch Kubernetes cluster information.

Common mistakes#

• Using data sources when Terraform should manage the resource.
• Using broad filters that match the wrong resource.
• Not understanding that data sources are read during plan/apply.

Best practices#

• Use precise filters.
• Name data sources clearly.
• Document externally managed resources.
• Avoid relying on unstable lookup results for production.

Related guides#

• Terraform Learning Path
• Terraform Providers Explained

Official references#

• Terraform data sources documentation