Terraform Data Sources - How to Query Existing Infrastructure

What Are Data Sources?

Data sources let you fetch information about existing infrastructure that Terraform doesn't manage. They're read-only — they query but never create or modify resources.

Basic Usage

# Look up the latest Ubuntu AMI
data "aws_ami" "ubuntu" {
  most_recent = true
  owners      = ["099720109477"]  # Canonical
 
  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-jammy-22.04-amd64-server-*"]
  }
}
 
resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t3.micro"
}

Common Data Sources

AWS Account Info

data "aws_caller_identity" "current" {}
data "aws_region" "current" {}
 
locals {
  account_id = data.aws_caller_identity.current.account_id
  region     = data.aws_region.current.name
}

Existing VPC

data "aws_vpc" "main" {
  filter {
    name   = "tag:Name"
    values = ["production-vpc"]
  }
}

Availability Zones

data "aws_availability_zones" "available" {
  state = "available"
}

SSM Parameter

data "aws_ssm_parameter" "db_host" {
  name = "/prod/database/host"
}

Remote State

data "terraform_remote_state" "networking" {
  backend = "s3"
  config = {
    bucket = "terraform-state"
    key    = "networking/terraform.tfstate"
    region = "us-east-1"
  }
}
 
# Use: data.terraform_remote_state.networking.outputs.vpc_id

Data Source vs Resource

When to Use Data Sources

• Looking up AMIs — always use data sources for latest AMI
• Cross-stack references — terraform_remote_state for shared outputs
• Existing infrastructure — query resources not managed by your config
• Dynamic values — availability zones, account ID, region

Best Practices

1. Use most_recent = true for AMI lookups to avoid stale IDs
2. Use filters instead of hardcoding IDs — more portable
3. Prefer outputs over remote_state — use Terraform Cloud or parameter store for cross-stack data
4. Data sources refresh every plan — be aware of API rate limits

Learn More

• Terraform for Beginners Course — hands-on data source labs

Related: How to install Terraform on macOS — our most popular installation guide.

• Terraform By Example Book — real-world patterns
• Terraform Cheat Sheet — quick command reference