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A module is a container for multiple resources that are used together. Modules can be used to create lightweight abstractions, so that you can describe your infrastructure in terms of its architecture, rather than directly in terms of physical objects.
.tf files in your working directory when you run
terraform 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
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
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
files inside just as you would do for a root module. Terraform 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 Terraform 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.
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. Terraform 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.