Terraform Basics

Basics

• Terraform is a tool for building and changing infrastructure safely and efficiently.
• Terraform centered around infrastructure-as-code (IAC) principle which makes it well suited for versioned, reproducible infrastructure definition.
• Infrastrucutre is described using .tf files that use the terraform configuration langauge.

Workflow

Project Setup

A terraform project is just a set of files describing the provider, resources, etc.

For example, imagine this is the project setup we have:

+ mars
|--+ main.tf

And the contents of main.tf:

provider "local" {
  // attributes if any
}

resource "local_file" "hello" {
  content = "Hello, Terraform"
  filename = "hello.txt"
}

• The provider block describes the platform where resources will be created - this can include Cloud services (GCP, AWS, etc.), SaaS services, API providers, etc. Local provider allows creating resources locally (e.g., files).
• The resource block describes a resource that needs to be created along with all the properties. The resource type (e.g., local_file) must be supported by the provider.

Init

Running terraform init discovers all the providers that are needed by the project, downloads the provider plugins for them from the Terraform Registry.

$ terraform init
Initializing the backend...

Initializing provider plugins...
- Reusing previous version of hashicorp/local from the dependency lock file
- Using previously-installed hashicorp/local v1.4.0

Terraform has been successfully initialized!

Planning

In this stage, terraform finds the current states of all the resources the terrform module cares about and computes the steps necessary to match them to the desired state.

$ terraform plan
Terraform used the selected providers to generate the following execution plan. Resource
actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # local_file.hello will be created
  + resource "local_file" "hello" {
      + content              = "Hello, Terraform"
      + directory_permission = "0777"
      + file_permission      = "0777"
      + filename             = "hello.txt"
      + id                   = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

───────────────────────────────────────────────────────────────────────────────────────────

Note: You didn't use the -out option to save this plan, so Terraform can't guarantee to
take exactly these actions if you run "terraform apply" now.

Notice the message at the end. Using -out can ensure that apply phase will take the exact steps shown during plan. Without -out, some dependent state might change and terraform might be forced to re-plan changing the overall steps since the last plan call.

Apply

Apply phase is when the plan described by the module is actually executed. For example, running terraform apply inside the above project directory will create a hello.txt file with the “Hello, Terraform” message.

Terraform used the selected providers to generate the following execution
plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # local_file.hello will be created
  + resource "local_file" "hello" {
      + content              = "Hello, Terraform"
      + directory_permission = "0777"
      + file_permission      = "0777"
      + filename             = "hello.txt"
      + id                   = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

local_file.hello: Creating...
local_file.hello: Creation complete after 0s [id=392b5481eae4ab2178340f62b752297f72695d57]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

At this point, if you run terraform plan again, it will be skipped because the real-world state matches the terraform description exactly. But if you modify the hello.txt file to contain something else and then run, terraform will detect the changes and re-plan accordingly.