terraform/website/docs/configuration/modules.html.md

layout	page_title	sidebar_current	description
docs	Configuring Modules	docs-config-modules	Modules allow multiple resources to be grouped together and encapsulated.

Modules

A module is a container for multiple resources that are used together. Every Terraform configuration has at least one module, known as its root module, which consists of the resources defined in the .tf files in the main working directory.

A module can call other modules, allowing the suite of resources within the child module to be included into the configuration in a concise way. Modules can also be called multiple times, either within the same configuration or in separate configurations, allowing resource configurations to be packaged and re-used.

This page describes how to call one module from another. Other pages in this section of the documentation describe the different elements that make up modules, and there is further information about how modules can be used, created, and published in the dedicated Modules section.

Calling a Child Module

To call a module means to include the contents of that module into the configuration with specific values for its input variables. Modules are called from within other modules using module blocks:

module "servers" {
  source = "./app-cluster"

  servers = 5
}

The label immediately after the module keyword is a name that will be used to refer to this instance of the module within the calling module. The calling module is the one that includes the module block shown above.

Within the block body (between { and }) are the arguments for the module. Most of the arguments correspond to input variables defined by the module, including the servers argument in the above example.

The source argument is a meta-argument defined and processed by Terraform itself. Its value is the path to a local directory containing the module's configuration files, or optionally a remote module source that Terraform should download and use. For more information on possible values for this argument, see Module Sources.

The same source address can be specified in multiple module blocks to create multiple copies of the resources defined within, possibly with different variable values.

Accessing Module Output Values

The resources defined in a module are encapsulated, so the calling module cannot access their attributes directly. However, the child module can declare output values to selectively export certain values to be accessed by the calling module.

For example, if the ./app-cluster module referenced in the example above exported an output value named instance_ids then the calling module can reference that result using the expression module.servers.instance_ids:

resource "aws_elb" "example" {
  # ...

  instances = module.servers.instance_ids
}

Other Meta-arguments

Along with the source meta-argument described above, module blocks have some more meta-arguments that have special meaning across all modules, described in more detail in other sections:

• version - (Optional) A version constraint string that specifies which versions of the referenced module are acceptable. The newest version matching the constraint will be used. version is supported only for modules retrieved from module registries.

• providers - (Optional) A map whose keys are provider configuration names that are expected by child module and whose values are corresponding provider names in the calling module. This allows provider configurations to be passed explicitly to child modules. If not specified, the child module inherits all of the default (un-aliased) provider configurations from the calling module.

In addition to the above, the argument names count, for_each and lifecycle are not currently used by Terraform but are reserved for planned future features.

Since modules are a complex feature in their own right, further detail about how modules can be used, created, and published is included in the dedicated section on modules.