terraform

Table of ContentsClose

1. Basic commands
2. Other notes
- 2.1. Terraform AWS ubuntu AMI
3. FAQ

1. Basic commands

terraform init
terraform plan
terraform plan -destroy # dry run
terraform apply
terraform destroy

# using tfvars
terraform plan -var-file=development.tfvars

# state
terraform show
terraform state list

# format
terraform fmt
terraform validate

# debugging
terraform console

2. Other notes

After including a new module, you need to do terraform init
state can contain sensitive info, so it usually needs some kind of credentials for access if using a remote backend. Better to use credentials as env vars.
.terraform directory not supposed to be version controlled
Ideally the infrastructure that is used by Terraform should exist outside of the infrastructure that Terraform manages.

2.1. Terraform AWS ubuntu AMI

data "aws_ami" "ubuntu" {
  most_recent = true
  filter {
     name   = "name"
     # https://cloud-images.ubuntu.com/locator/ec2/
     # NOTE: I found the value for this name by manually trying to creating an
     #       EC2 instance from the AWS console, there seems to be no other
     #       proper way
     values = ["ubuntu/images/hvm-ssd-gp3/ubuntu-noble-24.04-amd64-server-*"]
  }

  filter {
     name   = "root-device-type"
     values = ["ebs"]
   }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }
}

3. FAQ

3.1. Variable substitution in resource names?

Because the resource name is used only within the module, Terraform requires it to be a literal.
Resource names is that they are scoped only to the module where you declare them, and so there is generally no need for them to be dynamic
See this thread

3.2. Local variable vs Input variables

Input variables
- Are usually declared in variables.tf using variable blocks
  - Think of these variable declarations needed to make the module work (we pass these "into" the module)
  - Naming the file variables.tf is merely a convention. (It just needs to have the variable block setup)
- Can be accessed in the module using var
- Can be passed using various mechanism of which *.tfvars and -var cli option are two ways.

3.3. Parent and Child module and Input variables

*.tfvars afaik only works for the root module
If you want to pass input variables into a child module you do it in the module block. However, you can source them from the root module's *.tfvars

3.4. Modules vs Data sources and Outputs & Inputs(Variables)

Name	Description
Module	Collection of tf files (our envs are modules, our things inside modules directory, basically any number of tf files inside a directory)
Root Module	Caller module
Child Module	Callee module
Variables (Input)	This is the way we can interact with a module and alter its behavior (You'd use `variables.tf` and `variable` blocks for this)
Output	These are things we explicitly decide to export out of the module, once exported, the outputs get stored in `tf state`
Data Source	Use information defined outside of Terraform in your tf config
State Data Source	See remote state data source , this basically allows you to use `data` blocks to access `tf state` data of from some `backend`

3.5. Calling of a module

To call a module means to "include" the contents of that module into the configuration with specific values for its input variables.
The "calling" happens via the module block