Day 3: Resources and Data Sources | Terraform in 5 Days

Real Resources: VPC + EC2

vpc.tf

# Create a VPC
resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
  tags = { Name = "main-vpc" }
}

# Public subnet
resource "aws_subnet" "public" {
  vpc_id            = aws_vpc.main.id  # reference another resource
  cidr_block        = "10.0.1.0/24"
  availability_zone = "us-east-1a"
  map_public_ip_on_launch = true
}

# Security group
resource "aws_security_group" "web" {
  vpc_id = aws_vpc.main.id

  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

ec2.tf

data "aws_ami" "amazon_linux" {
  most_recent = true
  owners      = ["amazon"]
  filter {
    name   = "name"
    values = ["al2023-ami-*-x86_64"]
  }
}

resource "aws_instance" "web" {
  ami                    = data.aws_ami.amazon_linux.id
  instance_type          = "t3.micro"
  subnet_id              = aws_subnet.public.id
  vpc_security_group_ids = [aws_security_group.web.id]

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

  tags = { Name = "web-server" }
}

ℹ️

References like aws_vpc.main.id create implicit dependencies. Terraform builds a dependency graph and creates resources in the right order. The subnet waits for the VPC. The EC2 waits for the subnet. You don't have to specify the order.

📝 Day 3 Exercise

Build a VPC + EC2 Setup

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Day 3 Summary

Resources reference each other: resource_type.name.attribute. Terraform infers dependency order.
Data sources query existing resources. data.aws_ami.amazon_linux.id finds the latest Amazon Linux AMI.
user_data runs shell commands when the instance first boots.
depends_on for explicit dependencies when the implicit graph isn't enough.

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Day 2: Variables, Outputs, and State

Day 4: Modules: Reusable Infrastructure

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