---
description: >-
  Modules are containers for multiple resources that are used together in
  configurations. Learn how to call one module from another and access module
  output.
---

# Module Blocks

A _module_ is a container for multiple resources that are used together.

Every OpenTofu configuration has at least one module, known as its
_root module_, which consists of the resources defined in the `.tf` and `.tofu`
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](../../language/modules/develop/index.mdx).

## 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](../../language/values/variables.mdx). Modules are called
from within other modules using `module` blocks:

```hcl
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](../../language/values/variables.mdx)
  defined by the module. (The `servers` argument in the example above is one of
  these.)

- OpenTofu 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
OpenTofu. Its value is either the path to a local directory containing the
module's configuration files, or a remote module source that OpenTofu should
download and use. For more information on
possible values for this argument, see [Module Sources](../../language/modules/sources.mdx).

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
`tofu init` to allow OpenTofu 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:

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

  servers = 3
}
```

The `version` argument accepts a [version constraint string](../../language/expressions/version-constraints.mdx).
OpenTofu 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 OpenTofu Registry](https://registry.opentofu.org/)
or any [TACOS](../../intro/tacos.mdx) (TF Automation and Collaboration Software) private modules 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`, OpenTofu 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](../../language/meta-arguments/count.mdx)
  for details.

- `for_each` - Creates multiple instances of a module from a single `module`
  block. See
  [the `for_each` page](../../language/meta-arguments/for_each.mdx)
  for details.

- `providers` - Passes provider configurations to a child module. See
  [the `providers` page](../../language/meta-arguments/module-providers.mdx)
  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](../../language/meta-arguments/depends_on.mdx)
  for details.

OpenTofu does not use the `lifecycle` argument. However, the `lifecycle` block is reserved for future versions.

## 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](../../language/values/outputs.mdx) 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`:

```hcl
resource "aws_elb" "example" {
  # ...

  instances = module.servers.instance_ids
}
```

For more information about referring to named values, see
[Expressions](../../language/expressions/index.mdx).

## Transferring Resource State Into Modules

Moving `resource` blocks from one module into several child modules causes
OpenTofu to see the new location as an entirely different resource. As a
result, OpenTofu 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](../../language/modules/develop/refactoring.mdx) to record the old and new
addresses for each resource instance. This directs OpenTofu 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 OpenTofu, such as if a particular virtual
machine is running on degraded underlying hardware. In this case, you can use
[the `-replace=...` planning option](../../cli/commands/plan.mdx#replace-address)
to force OpenTofu 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:

```shellsession
$ tofu 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, OpenTofu only allows selecting
individual resource instances. There is no syntax to force replacing _all_
resource instances belonging to a particular module.
