Terraform share common variables for modules-CodePudding

We have a common variables in our infrastructure on AWS, planned to be used by several modules. For example subnet ids, vpc id and so on.

To avoid duplication those variable in each module in *.tfvars files. Is that possible make them available from any terraform modules? While modules itself can be isolated from each other.

I think about kind of core module, which can be imported where that common variables needs. But in doubts that module is a right way to do that, as modules intended to have only resources in them, but we need expose only variables. Is it right way to use modules to share variables? Or how you guys cope with this problem? Think it's common or it's bad approach in terraform?)

Thanks.

CodePudding user response：

If you have a set of expressions (including hard-coded literal values) that you want to reuse then it is valid to write a module which only contains input variable declarations, local values, and output values as a way to model that.

The simplest form of this would be a module that only contains output blocks whose values are hard-coded literal values, like this:

output "example" {
  value = "result"
}

The official module hashicorp/subnets/cidr is an example of that: it doesn't declare any resources of its own, and instead it just encapsulates some logic for calculating a set of subnets based on some input variables.

This is a special case of Data-only Modules where the data comes from inside the module itself, rather than from data sources. A nice thing about modelling shared data in this way is that if you later decide to calculate those results automatically based on data sources then you'll be able to do so, while keeping the details encapsulated. For example, if you define a module which takes an environment name as an input variable and returns derived settings about that environment, the module could contain local logic to calculate those results today but could later determine some of those settings by fetching them from a prescribed remote location, such as AWS SSM Parameter store, if the need arises.