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Module Blocks

Hands-on: Try the Reuse Configuration with Modules tutorials.

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, which lets you include the child module's resources 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. For more information about creating re-usable child modules, see Module Development.

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
}

A module that includes a module block like this is the calling module of the child module.

The label immediately after the module keyword is a local name, which the calling module can use to refer to this instance of the module.

Within the block body (between { and }) are the arguments for the module. Module calls use the following kinds of arguments:

  • The source argument is mandatory for all modules.

  • The version argument is recommended for modules from a registry.

  • Most other arguments correspond to input variables defined by the module. (The servers argument in the example above is one of these.)

  • Terraform defines a few other meta-arguments that can be used with all modules, including for_each and depends_on.

Source

All modules require a source argument, which is a meta-argument defined by Terraform. Its value is either the path to a local directory containing the module's configuration files, or a remote module source that Terraform should download and use. This value must be a literal string with no template sequences; arbitrary expressions are not allowed. 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.

After adding, removing, or modifying module blocks, you must re-run terraform init to allow Terraform the opportunity to adjust the installed modules. By default this command will not upgrade an already-installed module; use the -upgrade option to instead upgrade to the newest available version.

Version

When using modules installed from a module registry, we recommend explicitly constraining the acceptable version numbers to avoid unexpected or unwanted changes.

Use the version argument in the module block to specify versions:

module "consul" {
  source  = "hashicorp/consul/aws"
  version = "0.0.5"

  servers = 3
}

The version argument accepts a version constraint string. Terraform will use the newest installed version of the module that meets the constraint; if no acceptable versions are installed, it will download the newest version that meets the constraint.

Version constraints are supported only for modules installed from a module registry, such as the public Terraform Registry or Terraform Cloud's private module registry. Other module sources can provide their own versioning mechanisms within the source string itself, or might not support versions at all. In particular, modules sourced from local file paths do not support version; since they're loaded from the same source repository, they always share the same version as their caller.

Meta-arguments

Along with source and version, Terraform defines a few more optional meta-arguments that have special meaning across all modules, described in more detail in the following pages:

  • count - Creates multiple instances of a module from a single module block. See the count page for details.

  • for_each - Creates multiple instances of a module from a single module block. See the for_each page for details.

  • providers - Passes provider configurations to a child module. See the providers page for details. If not specified, the child module inherits all of the default (un-aliased) provider configurations from the calling module.

  • depends_on - Creates explicit dependencies between the entire module and the listed targets. See the depends_on page for details.

In addition to the above, the lifecycle argument is not currently used by Terraform but is reserved for planned future features.

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
}

For more information about referring to named values, see Expressions.

Transferring Resource State Into Modules

Moving resource blocks from one module into several child modules causes Terraform to see the new location as an entirely different resource. As a result, Terraform plans to destroy all resource instances at the old address and create new instances at the new address.

To preserve existing objects, you can use refactoring blocks to record the old and new addresses for each resource instance. This directs Terraform to treat existing objects at the old addresses as if they had originally been created at the corresponding new addresses.

Replacing resources within a module

You may have an object that needs to be replaced with a new object for a reason that isn't automatically visible to Terraform, such as if a particular virtual machine is running on degraded underlying hardware. In this case, you can use the -replace=... planning option to force Terraform to propose replacing that object.

If the object belongs to a resource within a nested module, specify the full path to that resource including all of the nested module steps leading to it. For example:

$ terraform plan -replace=module.example.aws_instance.example

The above selects a resource "aws_instance" "example" declared inside a module "example" child module declared inside your root module.

Because replacing is a very disruptive action, Terraform only allows selecting individual resource instances. There is no syntax to force replacing all resource instances belonging to a particular module.