Terraform Infrastructure As Code

Terraform Infrastructure as Code - Comprehensive Guide

Table of Contents

1. Introduction to Terraform
2. Core Concepts
3. Resources
4. Data Sources
5. Variables and Outputs
6. Modules
7. State Management
8. Workspaces
9. Provider Configuration
10. Advanced Features
11. Dependencies
12. Provisioners
13. Best Practices
14. CI/CD Integration

Introduction to Terraform

Terraform is an open-source Infrastructure as Code (IaC) tool created by HashiCorp that enables you to define and provision infrastructure using a declarative configuration language called HashiCorp Configuration Language (HCL). Terraform manages external resources such as public cloud infrastructure, private cloud infrastructure, network appliances, and software as a service.

Key Benefits

• Declarative Configuration: Define what your infrastructure should look like, not how to create it
• Cloud-Agnostic: Works with multiple cloud providers and services
• Version Control: Infrastructure code can be versioned and reviewed
• Plan Before Apply: Preview changes before applying them
• Resource Graph: Automatically manages dependencies between resources
• State Management: Tracks the current state of your infrastructure

Terraform Workflow

# Initialize Terraform working directory
terraform init

# Initialize and upgrade provider versions
terraform init -upgrade

# Initialize with backend configuration
terraform init -backend-config="bucket=my-state-bucket"

# Generate a plan
terraform plan

# Save plan to file for later apply
terraform plan -out=tfplan

# Plan with specific variable values
terraform plan -var="region=us-west-2" -var="instance_type=t2.micro"

# Apply with interactive approval
terraform apply

# Auto-approve without confirmation
terraform apply -auto-approve

# Apply a saved plan file
terraform apply tfplan

# Destroy with confirmation prompt
terraform destroy

# Auto-approve destruction
terraform destroy -auto-approve

Core Concepts

1. Resources

Resources are the most fundamental elements in Terraform. They represent infrastructure objects like virtual machines, networks, databases, or DNS records.

# Basic resource declaration
resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  tags = {
    Name = "WebServer"
    Environment = "production"
  }
}

2. Providers

Providers are plugins that allow Terraform to interact with cloud platforms, SaaS providers, and other APIs.

# AWS provider configuration
provider "aws" {
  region = "us-west-2"

  default_tags {
    tags = {
      ManagedBy = "Terraform"
      Project   = "MyApp"
    }
  }
}

3. State

Terraform stores information about your infrastructure in a state file. This state is used to map real-world resources to your configuration and track metadata.

4. Configuration Language (HCL)

HCL is designed to be both human-readable and machine-friendly, making it ideal for infrastructure configuration.

Resources

Resources are the building blocks of Terraform configurations. Each resource block describes one or more infrastructure objects.

Basic Resource Syntax

resource "resource_type" "resource_name" {
  argument1 = "value1"
  argument2 = "value2"

  nested_block {
    nested_argument = "nested_value"
  }
}

Resource Examples

AWS EC2 Instance

resource "aws_instance" "app_server" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  key_name      = "my-keypair"

  vpc_security_group_ids = [aws_security_group.app.id]
  subnet_id              = aws_subnet.public.id

  user_data = <<-EOF
              #!/bin/bash
              echo "Hello, World!" > index.html
              nohup busybox httpd -f -p 8080 &
              EOF

  tags = {
    Name        = "AppServer"
    Environment = "production"
    ManagedBy   = "Terraform"
  }
}

AWS VPC

resource "aws_vpc" "main" {
  cidr_block           = "10.0.0.0/16"
  enable_dns_hostnames = true
  enable_dns_support   = true

  tags = {
    Name = "main-vpc"
  }
}

resource "aws_subnet" "public" {
  vpc_id                  = aws_vpc.main.id
  cidr_block              = "10.0.1.0/24"
  availability_zone       = "us-west-2a"
  map_public_ip_on_launch = true

  tags = {
    Name = "public-subnet"
  }
}

resource "aws_subnet" "private" {
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.0.2.0/24"
  availability_zone = "us-west-2a"

  tags = {
    Name = "private-subnet"
  }
}

AWS S3 Bucket

resource "aws_s3_bucket" "data" {
  bucket = "my-app-data-bucket-12345"

  tags = {
    Name        = "Data Bucket"
    Environment = "production"
  }
}

resource "aws_s3_bucket_versioning" "data" {
  bucket = aws_s3_bucket.data.id

  versioning_configuration {
    status = "Enabled"
  }
}

resource "aws_s3_bucket_server_side_encryption_configuration" "data" {
  bucket = aws_s3_bucket.data.id

  rule {
    apply_server_side_encryption_by_default {
      sse_algorithm = "AES256"
    }
  }
}

AWS Security Group

resource "aws_security_group" "web" {
  name        = "web-sg"
  description = "Security group for web servers"
  vpc_id      = aws_vpc.main.id

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

  ingress {
    description = "HTTPS from anywhere"
    from_port   = 443
    to_port     = 443
    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"]
  }

  tags = {
    Name = "web-security-group"
  }
}

Resource Lifecycle

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  lifecycle {
    create_before_destroy = true
    prevent_destroy       = true
    ignore_changes        = [tags]
  }
}

Lifecycle options:

• create_before_destroy: Create new resource before destroying the old one
• prevent_destroy: Prevent accidental destruction of resources
• ignore_changes: Ignore changes to specified attributes
• replace_triggered_by: Force replacement when specific resources change

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  lifecycle {
    replace_triggered_by = [
      aws_iam_policy.example.id
    ]
  }
}

Data Sources

Data sources allow Terraform to use information defined outside of Terraform, or defined by another separate Terraform configuration.

Data Source Syntax

data "resource_type" "name" {
  # Query parameters
}

Data Source Examples

# Query existing AWS resources
data "aws_ami" "ubuntu" {
  most_recent = true
  owners      = ["099720109477"] # Canonical

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*"]
  }
}

# Use data source in resource
resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t2.micro"
}

# Query availability zones
data "aws_availability_zones" "available" {
  state = "available"
}

# Query VPC
data "aws_vpc" "default" {
  default = true
}

# Query remote state
data "terraform_remote_state" "network" {
  backend = "s3"

  config = {
    bucket = "terraform-state"
    key    = "network/terraform.tfstate"
    region = "us-west-2"
  }
}

# Use remote state outputs
resource "aws_instance" "app" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t2.micro"
  subnet_id     = data.terraform_remote_state.network.outputs.subnet_id