# modules

This directory contains modules that are included from the environment directories.

Modules called from other modules should be placed under the `submodules` subdirectory.
Modules called from other modules should be placed under the `thirdparty` subdirectory.

## Variables
Added 2022-06-10

Variables that affect configuration and are subject to change exist with the modules. There are a few important things to know:

1. If the variable only affects the module itself, just put it in the module directory.
2. If the variable is used by more than one module, put it in the `variables/` directory and create symbolic links into the modules that need it.
3. If a variable is added to `xdr-terraform-live`, it should be one that is set once and never changes. These variables can be found in the `constants.tf` file under `variables/`, and is linked into all modules.
4. When creating a new module, be sure to use `cp -a` instead of `cp -r`. `cp -a` will preserve symbolic links.

## Variable Differentiation
Since we still want different settings base on things like environment or account, there are a few "tropes" used to provide this differentiation.

From easiest to most complex, the 3 methods most often employed are as follows. Note that in these examples, all 3 produce the same result:

### 1. Directly during assignment
This is straightforward:

e.g.:
```
  instance_type = var.environment == "prod" ? "m5a.xlarge" : "t3a.micro"
```

### 2. Map with differentiator
This method creates a map of values, and selects the appropriate one immediately. This is good for things that will always have a value.

e.g.:
```
locals {
  instance_type = {
    prod = "m5a.xlarge",
    test = "t3a.micro",
  }[var.environment]
}
```

### 3. Default with Exceptions
This method sets a default value, creates a map of exceptions, and then does a `lookup` to determine the final value. This is useful when the same value will be used most of the time, but it can be overridden in certain cases.

e.g.
```
locals {
  instance_type_default = "m5a.xlarge"
  instance_type_exceptions = {
    afs-mdr-test-c2-gov = "t3a.micro"
    afs-mdr-test-malware-gov = "t3a.micro"
  }
  instance_type = lookup(local.instance_type_exceptions, var.account_name, local.instance_type_default
}
```