03._Variables.txt

Content-Type: text/x-zim-wiki
Wiki-Format: zim 0.6
Creation-Date: 2023-10-25T11:38:27+02:00

====== 03. Variables ======
===== Variables =====
'''
variable "image_id" {
  type        = string
  description = "Image ID to use when creating an instance"
  default     = "id-xxxx"
  sensitive   = false
  nullable    = false

  validation {
    condition     = length(var.image_id) > 3 && substr(var.image_id, 0, 3) == "id-"
    error_message = "The image_id value must be a valid Image id, starting with \"id-\"."
  }
}
'''

Terraform CLI defines the following optional arguments for variable declarations:
* ''type'' This argument specifies what value types are accepted for the variable.
	* ''string''
	* ''number''
	* ''bool'': can be ''true'' or ''false''
	* ''list'': for instance, ''type = list(string)''
	* ''map''
* ''default'' A default value which then makes the variable optional.
* ''description'' This specifies the input variable's documentation.
* ''validation'' A block to define validation rules, usually in addition to type constraints.
* ''sensitive'' Limits Terraform UI output when the variable is used in configuration. **IMPORTANT: the variable remains plain text readable inside the state.**
* ''nullable'' Specify if the variable can be ''null'' within the module.

**Examples**

'''
# this is valid!
variable "image_id" {}
'''

'''
variable "image_id" {
  type = string
}
'''

'''
variable "availability_zone_names" {
  type    = list(string)
  default = ["us-west-1a"]
}
'''

'''
variable "docker_ports" {
  type = list(object({
    internal = number
    external = number
    protocol = string
  }))
  default = [
    {
      internal = 8300
      external = 8300
      protocol = "tcp"
    }
  ]
}
'''


===== Reference attributes =====
'''
resource "aws_eip" "lb" {
  domain = "vpc"
}

resource "aws_security_group" "allow_tls" {
  name        = "allow_tls"
  description = "Allow TLS inbound traffic"

  ingress {
    description = "TLS from VPC"
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["${aws_eip.lb.public_ip}/32"]
  }
}
'''

**Syntax**
* ''<resourcetype>.<resourcename>.<attribute>''
* ''data.<name>''
* ''var.<name>''
* ''local.<name>''
* ''module.<instancename>.<outputname>''

===== Output values =====
**They can be used to**
* Display information
* Register data inside the state
* Send information back to a caller module

'''
output "public_url" {
  value = "https://${aws_eip.lb.public_ip}:8080"
}
'''

**Output can be referenced from a caller module**

'''
# Here, we are in the root module
module "my_endpoint" {
  source = ../module/endpoint
}

# module.my_endpoint.public_url
'''


===== Ordre des précédance des variables =====
1. Default values
2. Environnement ''export TF_VAR_str="env"'' → defines str=env
3. ''terraform.tfvars'' file with lines such as ''key=value''
4. ''terraform.tfvars.json'' file
5. ''*.auto.tfvars'' and ''*.auto.tfvars.json'' files
6. CLI using ''-var'' or ''-var-file''

→ Storing credentials as part of environment variables is also a much better approach than hard coding it in the source files.


===== Basic map usage =====
'''
resource "aws_instance" "myec2" {
  ami           = "ami-082b5a644766e0e6f"
  instance_type = var.types["us-west-2"]
}

variable "types" {
  type = map(any)
  default = {
    us-east-1  = "t2.micro"
    us-west-2  = "t2.nano"
    ap-south-1 = "t2.small"
  }
}
'''

**Access**
* ''var.my_map.my_key''
* ''var.my_map["my_key"]''
* ''lookup(var.my_map, "my_key", <default_value>)''
→ Note: access using zero based index is NOT possible.

**Map**
* [[https://developer.hashicorp.com/terraform/language/functions/merge|merge]] takes an arbitrary number of maps or objects, and returns a single map or object that contains a merged set of elements from all arguments.
* [[https://developer.hashicorp.com/terraform/language/functions/lookup|lookup]] retrieves the value of a single element from a map, given its key. If the given key does not exist, the given default value is returned instead.


===== Basic list usage =====
''list(...)'': a sequence of values identified by consecutive whole numbers starting with zero. The keyword ''list'' is a shorthand for ''list(any)'', which accepts any element type as long as every element is **the same type**.

'''
resource "aws_instance" "myec2" {
  ami           = "ami-082b5a644766e0e6f"
  instance_type = var.list[0]
}
'''

'''
variable "list" {
  type    = list(string)
  default = ["m5.large", "m5.xlarge", "t2.medium"]
}
'''

**Access**
* ''var.my_list[<index>]''
* ''element(var.my_list, <index>)''

**Key functions**
* [[https://developer.hashicorp.com/terraform/language/functions/concat|concat]] takes two or more lists and combines them into a single list.
* [[https://developer.hashicorp.com/terraform/language/functions/contains|contains]] determines whether a given list or set contains a given single value as one of its elements.
* [[https://developer.hashicorp.com/terraform/language/functions/formatlist|formatlist]] produces a list of strings using a string template and a list of variables
* [[https://developer.hashicorp.com/terraform/language/functions/index_function|index]] finds the element index for a given value in a list.


===== list + list → map : use zipmap ! =====
See: [[https://developer.hashicorp.com/terraform/language/functions/zipmap|zipmap]]

'''
zipmap(["a", "b"], [1, 2])
{
  "a" = 1
  "b" = 2
}
'''


===== Local Values =====
Constants that are defined within a module.
Local values can be helpful to avoid repeating the same values or expressions multiple times in a configuration.

'''
locals {
  common_tags = {
    Owner   = "DevOps Team"
    service = "backend"
  }
}
resource "aws_instance" "app-dev" {
  ami           = "ami-082b5a644766e0e6f"
  instance_type = "t2.micro"
  tags          = local.common_tags
}
'''

Locals can help defining helpers
'''
locals {
  name_prefix = "${var.name != "" ? var.name : var.default_name}"
}
'''


===== For expression =====
See [[https://developer.hashicorp.com/terraform/language/expressions/for|documentation]]
Have a deep look at it since there are questions on the exam about this statement.