Day 07 – Using Data Sources in Terraform (VPC, Security Groups & AMI)

One of the best practices in Terraform is to avoid hardcoding IDs (like VPCs, Subnets, Security Groups, or AMIs). Instead, we can use Terraform Data Sources to dynamically fetch existing infrastructure or latest AMIs.

Today, I explored how to:

Fetch an existing VPC
Fetch an existing Security Group
Fetch the latest Amazon Linux 2 AMI
Deploy an EC2 instance using these data sources

🔹 What are Data Sources?

In Terraform, a Data Source allows you to query AWS for existing resources instead of creating new ones.

They are read-only (don’t modify infra).
Super useful when you need to reference existing infra (like a shared VPC).
Makes your Terraform DRY, reusable, and cloud-friendly.

🔹 Terraform Code

`main.tf`

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "6.9.0"
    }
  }
}

provider "aws" {
  region = "ap-south-1"
}

# 1. Fetch existing VPC
data "aws_vpc" "default" {
  default = true
}

# 2. Fetch existing Security Group by name
data "aws_security_group" "default_sg" {
  filter {
    name   = "group-name"
    values = ["default"]
  }

  vpc_id = data.aws_vpc.default.id
}

# 3. Fetch latest Amazon Linux 2 AMI
data "aws_ami" "amazon_linux" {
  most_recent = true

  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }

  owners = ["amazon"]
}

# 4. Launch EC2 using the data sources
resource "aws_instance" "example" {
  ami                         = data.aws_ami.amazon_linux.id
  instance_type               = "t2.micro"
  subnet_id                   = data.aws_vpc.default.id
  vpc_security_group_ids      = [data.aws_security_group.default_sg.id]
  associate_public_ip_address = true

  user_data = <<-EOF
              #!/bin/bash
              sudo yum install nginx -y
              sudo systemctl start nginx
              EOF

  tags = {
    Name = "Day07-DataSource-EC2"
  }
}

# Outputs
output "instance_ip" {
  value = aws_instance.example.public_ip
}

output "instance_url" {
  value = "http://${aws_instance.example.public_ip}"
}