Table of contents

Inheritance

There is no implicit inheritance. Things do not inherit from each other. In essence, imagine a bunch of blocks and those blocks can only communicate but attaching them to each other.

Module Definition

Consider the following snippet”

module "this" {
  selfdefined = "hi"
  enabled = var.enabled
  ...
}

We have a defined a variable here; selfdefined. Defined locally within the module.

But we can also define it separately enabled in a variable file as is convention. The variables.tf using the keyword variable adds variables to the variable namespace which is scoped to the module the file belongs to.

Variable Resolution Order

1. -var CLI flags

terraform apply -var="enabled=false"

terraform apply -var-file=prod.tfvars

2. Auto-loaded tfvars

• terraform.tfvars <- only applicable to the directory that terraform is running in (working directory)
• *.auto.tfvars

3. Environmental variables

export TF_VAR_enabled=false

4. Default value In the terraform code; if not present, an error will be thrown

Module Intercommunication

The major thing to understand is that implicit sharing is impossible. A child modules can NEVER access a variable, resource, or state from its parents or sibling modules without explicitly beging passed.

That means

• Variables MUST flow downwards. Declare a variable at the root module and even child must pass it to the next child to propagate it to the bottom.

• Outputs MUST flow downwards. A child module can expose values via output. Only the direct parent can access these output values

• Sibling modules can never communicate without a parent module that calls both of them.