Providers, Resources, and Data Sources#

Terraform has three core object types. Providers are plugins that talk to APIs (AWS, Azure, GCP, Kubernetes, GitHub). Resources are the things you create and manage. Data sources read existing objects without managing them.

# providers.tf
terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }
  required_version = ">= 1.5.0"
}

provider "aws" {
  region = var.region
}

# A resource Terraform creates and manages
resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
  tags = { Name = "main-vpc" }
}

# A data source that reads an existing AMI
data "aws_ami" "ubuntu" {
  most_recent = true
  owners      = ["099720109477"]
  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 = var.instance_type
  subnet_id     = aws_vpc.main.id
}

Resources create, update, and delete. Data sources only read. If you need information about something Terraform does not manage, use a data source.

What Modules Are#

A Terraform module is a directory containing .tf files. Every Terraform configuration is already a module (the “root module”). When you call another module from your root module, that is a “child module.” Modules let you encapsulate a set of resources behind a clean interface of input variables and outputs.

Module Structure#

A well-organized module looks like this:

modules/vpc/
  main.tf           # resource definitions
  variables.tf      # input variables
  outputs.tf        # output values
  versions.tf       # required providers and terraform version
  README.md         # usage documentation

The module itself has no backend, no provider configuration, and no hardcoded values. Everything configurable comes in through variables. Everything downstream consumers need comes out through outputs.