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Creating Modules

A module is a container for multiple resources that are used together. You can use modules to create lightweight abstractions, so that you can describe your infrastructure in terms of its architecture, rather than directly in terms of physical objects.

The .tf files in your working directory when you run opentf plan or opentf apply together form the root module. That module may call other modules and connect them together by passing output values from one to input values of another.

To learn how to use modules, see the Modules configuration section. This section is about creating re-usable modules that other configurations can include using module blocks.

Module structure​

Re-usable modules are defined using all of the same configuration language concepts we use in root modules. Most commonly, modules use:

  • Input variables to accept values from the calling module.
  • Output values to return results to the calling module, which it can then use to populate arguments elsewhere.
  • Resources to define one or more infrastructure objects that the module will manage.

To define a module, create a new directory for it and place one or more .tf files inside just as you would do for a root module. OpenTF can load modules either from local relative paths or from remote repositories; if a module will be re-used by lots of configurations you may wish to place it in its own version control repository.

Modules can also call other modules using a module block, but we recommend keeping the module tree relatively flat and using module composition as an alternative to a deeply-nested tree of modules, because this makes the individual modules easier to re-use in different combinations.

When to write a module​

In principle any combination of resources and other constructs can be factored out into a module, but over-using modules can make your overall OpenTF configuration harder to understand and maintain, so we recommend moderation.

A good module should raise the level of abstraction by describing a new concept in your architecture that is constructed from resource types offered by providers.

For example, aws_instance and aws_elb are both resource types belonging to the AWS provider. You might use a module to represent the higher-level concept "HashiCorp Consul cluster running in AWS" which happens to be constructed from these and other AWS provider resources.

We do not recommend writing modules that are just thin wrappers around single other resource types. If you have trouble finding a name for your module that isn't the same as the main resource type inside it, that may be a sign that your module is not creating any new abstraction and so the module is adding unnecessary complexity. Just use the resource type directly in the calling module instead.

Refactoring module resources​

You can include refactoring blocks to record how resource names and module structure have changed from previous module versions. OpenTF uses that information during planning to reinterpret existing objects as if they had been created at the corresponding new addresses, eliminating a separate workflow step to replace or migrate existing objects.