Day 13: Terraform Data Sources

Source: Dev.to

Data Source

Think of a data source like a phone directory with a username and phone number as key‑value pairs accessed via an API. Instead of hard‑coding values, you can retrieve them using a key.

In short, Terraform data sources for AWS allow you to retrieve information about existing AWS resources or external data that can then be referenced within your Terraform configurations.

Example: When creating an EC2 instance you need an AMI ID. Rather than manually looking up the latest Amazon Linux 2 AMI, you can use a data source to fetch the AMI ID automatically.

Data sources allow Terraform to read information about existing infrastructure. They:

• Do not create, update, or delete resources
• Allow you to reference resources managed elsewhere
• Enable sharing infrastructure between teams
• Are defined with data blocks instead of resource blocks

How to Use AWS Data Sources

You define a data source using the data block in your Terraform configuration:

data "provider_type" "name" {
  # Configuration settings for filtering or identifying the data source
}

• provider_type – The type of AWS data source (e.g., aws_ami, aws_vpc, aws_s3_bucket).
• name – A local identifier used to reference the data source.
• Configuration settings – Vary by data source and typically include id, name, or filter blocks with name and values arguments.

Examples: Data vs. Resource Block

# Resource Block – Terraform MANAGES this
resource "aws_vpc" "my_vpc" {
  cidr_block = "10.0.0.0/16"
}

# Data Block – Terraform READS this
data "aws_vpc" "existing_vpc" {
  filter {
    name   = "tag:Name"
    values = ["shared-network-vpc"]
  }
}

In the resource block, Terraform creates and manages the VPC. In the data block, Terraform only reads information about an existing VPC that matches the provided filter.

Task

Create a VPC using Terraform, then use a data source to fetch an AMI ID and launch an EC2 instance within the VPC you just created.

Creation of VPC

# Simulated infrastructure created by another team
provider "aws" {
  region = "us-east-1"
}

resource "aws_vpc" "shared" {
  cidr_block = "10.0.0.0/16"
  tags = {
    Name = "shared-network-vpc"
  }
}

resource "aws_subnet" "shared" {
  vpc_id     = aws_vpc.shared.id
  cidr_block = "10.0.1.0/24"
  tags = {
    Name = "shared-primary-subnet"  # ← This tag is important!
  }
}

Run the plan to see the actions Terraform will take:

terraform plan

Typical output (truncated for brevity):

# aws_subnet.shared will be created
+ resource "aws_subnet" "shared" {
    + cidr_block = "10.0.1.0/24"
    + tags = {
        + "Name" = "shared-primary-subnet"
      }
    + vpc_id = (known after apply)
  }

# aws_vpc.shared will be created
+ resource "aws_vpc" "shared" {
    + cidr_block = "10.0.0.0/16"
    + enable_dns_support = true
    + tags = {
        + "Name" = "shared-network-vpc"
      }
  }