• #single line comment
• /* ... */ multiline comment.

• Input variables
• output variables
• local values

• allowing customaization without changing the modules source code.

• provide when prompted during plan or apply

• override default values using the cli arg -var

• override default values using environment variables TF_var_aa

• using terraform.tvars file

• using -var-file <ANY_NAME>.tvars

• using auto.tfvars

• complex type constructors like list and map in input variables

• custom validation rules

• protect sensitive input variables.

• Allow data to be fetched or computed for use elsewhere in the tf conf.
  • Allow tf conf to make us or information
    • defined outside tf
    • defined by another separate tf conf
• Each data resource is associated with a single data source
• Data resources have the same dependnecy dresolution behavior as defined for managed resources.
  • Settin the depends_on meta-argument within a data block, defers reading ot he dat source until after all changes to the dependencies have been applied.
• Support
  • count
  • for_each

• are like return values of a terraform module
  • A root module can use outputs to print specified values in the CLI output after running terraform apply.
  • A sub module can use outputs to expose a subset of its resource attributes to a parent module.
  • When using remote state, root module outputs can be accessed by other configurations via a terraform_remote_state data source.

variable "instance_type_list" {
    description = "EC2 Instance type"
    type = list(string)
    default = [ "t3.micro", "t3.small" ]
}

e.g. use: instance_type = var.instance_type_list[0]

variable "instance_type_map" {
    description = "EC2 instance type"
    type = map(string)
    default = { 
        "dev" = "t3.micro"
        "qa" = "t3.small"
        "prod" = "t3.large"
    }
}

e.g. use: instance_type = var.instance_type_map["prod"]

output "for_output_list" {
    value = [for something in aws_instance.myec2vm: something.public_dns]
}

TODO what is going on here?

output "for_output_map1" {
    value = {for something in aws_instance.myec2vm: something.id => something.public_dns}
}

TODO what is going on here

output "for_output_map_advance" {
    value = {for x, something in aws_instance.myec2vm: x => something.public_dns}
}

output "legacy_splat_instance" {
    description = "Legacy splat operator"
    value = aws_instance.myec2vm.*.public_dns}
}

output "genetalized_splat_instance" {
    description = "genetalized splat operator"
    value = aws_instance.myec2vm.[*].public_dns}
}

• meta-argument accepts a map or a set of strings, and creates an instance for each item.
• string
  • each.key == each.value
• map
  • eack.key != each.value

• converts its arguments to a set value
  • all elements must be of the same type.
  • duplicates are coalesced.

> toset(["a", "b", 3])
[
    "a",
    "b",
    "3",
]