Introduction

The DevOps movement includes a series of IT engineering tactics aimed at shortening the amount of time it takes to make changes to software design and deployment. One of these tactics is to leverage technology to write a plan that contains deployment specifications ready to be orchestrated in the cloud or on-premises. Terraform is a tool that allows you to build and manage infrastructure safely and efficiently. This includes low-level components such as compute instances, storage, and networking, as well as high-level components such as DNS entries, SaaS features, etc. Terraform can manage both existing service providers and custom in-house solutions. In this course, you will learn about how to use Terraform to define and to manage your Cloud or on-premise infrastructure

Understanding Infrastructure As Code (IaC)

Infrastructure as code is a concept that allows you to:

Write down what you want to deploy such as virtual machines, disks, virtual networks, etc... as human readable code without having to use the provider Web GUI
Enable DevOps since your infrastructure will be described in a code that can be tracked inside a version control repository. This enables more visibility and collaboration across teams

IaC with Terraform

Terraform is an IaC tool
Why choosing Terraform as IaC tool ?
- Terraform interacts and takes care of communications with control layer APIs with ease
- Terraform supports a vast array of private and public Cloud vendors and so much more (on-premise infrastructures, etc...)
- Terraform tracks the state of each deployed resource
What is the Terraform workflow?
1. Write: starting by writting the code that describes the infrastructure inside a version control repository (best practice) or locally
2. Plan: preview the changes that the writtten code would bring into the real infrastructure
3. Apply: after one last plan/review, you are ready to provision the real infrastructure
Initializing the Terraform working directory

Before you start coding your infrastructure, you need to understand how to initialize Terraform inside your working directory

You have to enter the following command to get started:
```
terraform init
```
This command will:
- Download all of the required components (modules, plugins)
- Sets up the resource tracking mechanism, called backend
Terraform key concepts : Plan, Apply and Destroy
- Plan:
  - Reads the code and shows a "plan" of what will be deployed with with code without actually deploying any resource
  - Allows the user to review the action plan before executing anything
  - Requires authetication credentials, if required
- Apply:
  - Deploys the infrastructure resources according to the descriptions in the code
  - Updates the deployment state mechanism file aka state file or tfstate
- Destroy:
  - Looks at the recorded state file created during the deployment and destroys all of the resources created with the code
  - Updates the deployment state file by removing all of the destroyed resources
  - Non-reversible command
Resource addressing in Terraform: Understanding Terraform Code
- Understanding the provider configuration
  
  The provider block lets you define your provider configurations
- Understanding a resource block definition
  
  A resource block defines a resource that will be created from scratch and will be managed by Terraform code
  
  Resource address is: aws_instance.web
- Understanding a data source block definition
  
  A data source block give the possibility to get informations of resources that already exists in the infrastructure (eg: getting the public IP address of an existing VM, etc..)
  
  Data source address is: data.aws_instance.my-vm
- Terraform configuration files uses .tf extension

Terraform Fundamentals

Installing Terraform
- Method 1: Using the Terraform binary
  - Download zipped binary from Hashicorp website: https://www.terraform.io/downloads.html
  - Unzip Terraform binary
  - Move it to a directory that is included in your $PATH environment variable
- Method 2: Using the Terraform repository (Linux only)
  - Set up Hashicorp Terraform repository on Linux
    - For YUM https://www.terraform.io/docs/cli/install/yum.html
    - For APT https://www.terraform.io/docs/cli/install/apt.html
  - Install the "terraform" through the package manager
    - For YUM: yum install terraform
    - For APT: apt install terraform
Terraform providers
- Providers are Terraform's way of abstracting integrations with API control layer of the infrastructure vendors.
- Terraform, by default, looks for Providers in the Terraform providers registry https://registry.terraform.io/browse/providers
- Providers are plugins. They are released on a separate rhythm from Terraform itself, and each provider has its own series of version numbers.
- You can write your own Terraform provider as well.
- Terraform finds and installs providers when initializing working directory via terraform init command.
- As a best practice Providers should be pegged down to a specific version, so that any changes across provider version doesn't break your Terraform code.
  
  Provider definition example in a file provider.tf . You can choose any other filename.
```
provider "aws" {
  region  = "eu-west-1"
  version = "3.69.0"
}
```
  By executing the command terraform init ,Terraform will download the required files for this provider from the default Terraform registry.
  
  You can find all of the available providers list on the Terraform registry here: https://registry.terraform.io/browse/providers