--- page_title: Modules - Configuration Language 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 > **Hands-on:** Try the [Reuse Configuration with Modules](https://learn.hashicorp.com/collections/terraform/modules?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) collection on HashiCorp Learn. 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](/language/modules/develop). ## 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). 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) 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](/language/modules/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: ```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). 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](https://registry.terraform.io/) or [Terraform Cloud's private module registry](/cloud-docs/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](/language/meta-arguments/count) 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) for details. - `providers` - Passes provider configurations to a child module. See [the `providers` page](/language/meta-arguments/module-providers) 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) 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](/language/values/outputs) 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). ## 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](/language/modules/develop/refactoring) 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](/cli/commands/plan#replace-address) 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: ```shellsession $ 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.