HCL: providers, resources & references

The core building blocks of a config.

You write Terraform in HCL (HashiCorp Configuration Language) — a readable, declarative language purpose-built for describing resources. Three concepts are the whole foundation: providers (plugins that talk to a platform’s API), resources (the things you want to exist), and the .tf files that declare them. Get these three and you can read most Terraform in the wild.

terraform {
  required_providers {
    aws = { source = "hashicorp/aws", version = "~> 5.0" }   # which provider, pinned
  }
}
 
provider "aws" {
  region = "us-east-1"              # configure the provider
}
 
resource "aws_instance" "web" {     # resource TYPE "local name"
  ami           = "ami-0abc123"
  instance_type = "t3.micro"
  tags = { Name = "web-server" }
}

Providers: plugins for platforms

A provider is a plugin that knows how to talk to one platform’s API — AWS, Azure, Google Cloud, Kubernetes, GitHub, and hundreds more. You declare which providers you need and pin their versions, Terraform downloads them on terraform init, and each provider exposes a set of resource types you can then declare. The provider is what makes Terraform universal: the same workflow manages anything with a provider.

Resources, arguments & references

A resource block declares one thing that should exist. Inside, arguments set its properties. The powerful part is that resources reference each other’s attributes — aws_security_group.web_sg.id — and from those references Terraform builds a dependency graph and creates things in the correct order automatically. You describe the resources and how they relate; Terraform works out the order of operations.

resource "aws_security_group" "web_sg" {
  name = "web-sg"
  ingress { from_port = 443, to_port = 443, protocol = "tcp", cidr_blocks = ["0.0.0.0/0"] }
}
 
resource "aws_instance" "web" {
  ami                    = "ami-0abc123"
  instance_type          = "t3.micro"
  vpc_security_group_ids = [aws_security_group.web_sg.id]   # reference → SG created first
}