Terraform Note

1. Terraform module

Any set of Terraform configuration files in a folder is a module.
Different from regular Terraform .tf file, the provider is removed since providers should be configured by the user of the module and not by the module itself.

Syntax of using module

module "<NAME>" {
  source = "<SOURCE>"
  [CONFIG ...]
}

Example:

a module that is responsible for creating EC2 cluster

zw@zw-dev:~/code/terraform-projects/intro-to-terraform/modules$ tree .
.
└── services
    └── webserver-cluster
        ├── main.tf
        ├── outputs.tf
        └── variables.tf

2 directories, 3 files

You can create a new file in stage/services/webserver-cluster/main.tf and use the webserver-cluster module in it as follow:
```
provider "aws" {
  region = "us-east-2"
}
module "webserver_cluster" {
  source = "<path>/modules/services/webserver-cluster"
}
```
- Notice the source specifies the folder contains the .tf files.
- The module will be refered will be webserver_cluster.

1.1. Module inputs

As function in general programming language which has input variables, you can add input parameters in Terraform.

First, define variables in modules, such as cluster_name

variable "cluster_name" {
  description = "The name to use for all the cluster resources"
  type        = string
}

Second, set the new input variable accordingly

module "webserver_cluster" {
  source = "<path>/modules/services/webserver-cluster"
  cluster_name = "webservers-prod"
}

When use the module, we configure the cluster_name.

1.2. Module output

First, define output in modules,

Second, refer the output as same as refering resources outputs

# module.<MODULE_NAME>.<OUTPUT_NAME>
autoscaling_group_name = module.webserver_cluster.asg_name

(optinal), you could “pass through” output

output "clb_dns_name" {
  value       = module.webserver_cluster.clb_dns_name
  description = "The domain name of the load balancer"
}

The clb_dns_name is the output from module webserver_cluster and it is passed through from its usages place.