What're terraform modules?

Module is an abstraction for multiple resources which can be reused.

Let's say we define our website service with the following layers in AWS:

  • s3 for static resources (html, css, js, images, etc.)
  • cloudfront for cdn
  • application load balancer for receiving backend calls
  • Target group with autoscaling group behind application load balancer.
  • ec2 instances behind target group.
  • RDS mysql for database hosting.
  • Appropriate security group and other networking layers.

If we write all resources in terraform generally, we'll create a bunch of resources.

After a few days, we've decided the same tech stack is needed for 4 more websites. Now, we've to copy paste the code of all the resources and have different resource names like rds-website-2, rds-website-3, etc.

What if we want to abstract it all and give a way to just let website owner define variables and launch infrastructure layers?

We put a generic structure of all the resources mentioned above in a folder called modules. Let's call our module as website_infra

Thus, a new website can be launched simply by using:

module 'website_infra' {
 name_of_website = "acsrujan.net" 
 s3_bucket_name = "acsrujan"
 ssl_enabled   = true
}

And rest everything is handled by the module. We can give finer controls in the module definition and flags like if cdn is needed or not, etc.

How to write a module?

The best examples are in open source. Here's something one can begin with:

https://github.com/terraform-aws-modules/terraform-aws-vpc