Terraform Up and Running, 3rd Edition, Chapter 4, Modularizing MySQL

[0x1uke]

I'm working through Terraform Up and Running. I'm on chapter 4, and I notice in the example code a module is only created for the webserver-cluster service but not for the MySQL datastore. Is there a reason why one can't modularize MySQL to follow the DRY principle?

I gave it shot, but I'm stuck with a cycle error in my MySQL module between the db_username and dp_password input variables and trying to pass those values to the child modules as output variables. The idea I'm getting at is one would pass the db_username and db_password as environment variables and the child module will have access to them to create the database using the parent module. I suppose I could create a variables.tf in the prod and stage MySQL directories defining the db_username and db_password variables, but that also seems repetitive. I've included a link below to my code for more detail.

https://github.com/0x1uke/terraform-up-and-running

Thank you!

[brikis98]

I'm working through Terraform Up and Running. I'm on chapter 4, and I notice in the example code a module is only created for the webserver-cluster service but not for the MySQL datastore. Is there a reason why one can't modularize MySQL to follow the DRY principle?

The book does turn the MySQL code into a module, but it's a bit later, in chapter 7, under the "Working with Multiple AWS Regions" section.

I gave it shot, but I'm stuck with a cycle error in my MySQL module between the db_username and dp_password input variables and trying to pass those values to the child modules as output variables. The idea I'm getting at is one would pass the db_username and db_password as environment variables and the child module will have access to them to create the database using the parent module. I suppose I could create a variables.tf in the prod and stage MySQL directories defining the db_username and db_password variables, but that also seems repetitive. I've included a link below to my code for more detail.

https://github.com/0x1uke/terraform-up-and-running

Thank you!

From a quick look at your code:

• To pass the username and password to the module you do need another variables.tf in staging and prod, with a copy of the db_username and db_password variables. So instead of https://github.com/0x1uke/terraform-up-and-running/blob/master/stage/data-stores/mysql/main.tf#L25-L26, you'd have in staging and in prod:

  # Define these in variables.tf in staging and in prod
  variable "db_username" {
    description = "The username for the database"
    type        = string
    sensitive   = true
  }
  
  variable "db_password" {
    description = "The password for the database"
    type        = string
    sensitive   = true
  }
  
  module "mysql" {
    # ... (skipping other params for brevity)...
  
    # Pass the variables through to the module
    db_username = var.db_username
    db_password = var.db_password
  }

  Does it feel non-DRY to have to copy/paste those variable definitions? I agree! But that is how Terraform is designed. In some cases, you can reduce this copy/paste by using Terragrunt, as described later in the book.

• You shouldn't be using terraform_remote_state to read the database's state in the database module: https://github.com/0x1uke/terraform-up-and-running/blob/master/modules/data-stores/mysql/main.tf#L27-L35. You should only need that to read the database's state from other modules.

[0x1uke]

Yevgeniy,

Thank you for your reply. In case I get stuck, I'll take a look at the DB module in Chapter 7, but I really appreciate the sanity check that a variables.tf is required in the DB staging and prod because I wasn't sure if I was missing something. I'll also fix the terraform_remote_state piece. I'll close this issue out.

Thank you again, and thank you for the awesome book.

[brikis98]

Glad to help and happy to hear you're enjoying the book!