» Modules

For a hands-on tutorial, try the Organize Configuration with Modules track 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. 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
}

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. Most of the arguments correspond to input variables defined by the module, including the servers argument in the above example. Terraform also defines a few meta-arguments that are reserved by Terraform and used for its own purposes; we will discuss those throughout the rest of this section.

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.

» 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

When refactoring an existing configuration to split code into child modules, moving resource blocks between modules causes Terraform to see the new location as an entirely different resource from the old. Always check the execution plan after moving code across modules to ensure that no resources are deleted by surprise.

If you want to make sure an existing resource is preserved, use the terraform state mv command to inform Terraform that it has moved to a different module.

When passing resource addresses to terraform state mv, resources within child modules must be prefixed with module.<MODULE NAME>.. If a module was called with count or for_each (see below), its resource addresses must be prefixed with module.<MODULE NAME>[<INDEX>]. instead, where <INDEX> matches the count.index or each.key value of a particular module instance.

Full resource addresses for module contents are used within the UI and on the command line, but cannot be used within a Terraform configuration. Only outputs from a module can be referenced from elsewhere in your configuration.

» Other Meta-arguments

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

In addition to the above, the lifecycle argument is not currently used by Terraform but is 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.

» Module Versions

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 attribute in the module block to specify versions:

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

  servers = 3
}

The version attribute 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.

» Multiple Instances of a Module

Use the for_each or the count argument to create multiple instances of a module from a single module block. These arguments have the same syntax and type constraints as for_each and count when used with resources.

# my_buckets.tf
module "bucket" {
  for_each = toset(["assets", "media"])
  source   = "./publish_bucket"
  name     = "${each.key}_bucket"
}
# publish_bucket/bucket-and-cloudfront.tf
variable "name" {} # this is the input parameter of the module

resource "aws_s3_bucket" "example" {
  # Because var.name includes each.key in the calling
  # module block, its value will be different for
  # each instance of this module.
  bucket = var.name

  # ...
}

resource "aws_iam_user" "deploy_user" {
  # ...
}

This example defines a local child module in the ./publish_bucket subdirectory. That module has configuration to create an S3 bucket. The module wraps the bucket and all the other implementation details required to configure a bucket.

We declare multiple module instances by using the for_each attribute, which accepts a map (with string keys) or a set of strings as its value. Additionally, we use the special each.key value in our module block, because the each object is available when we have declared for_each on the module block. When using the count argument, the count object is available.

Resources from child modules are prefixed with module.module_name[module index] when displayed in plan output and elsewhere in the UI. For a module with without count or for_each, the address will not contain the module index as the module's name suffices to reference the module.

In our example, the ./publish_bucket module contains aws_s3_bucket.example, and so the two instances of this module produce S3 bucket resources with resource addresses of module.bucket["assets"].aws_s3_bucket.example and module.bucket["media"].aws_s3_bucket.example respectively.

» Providers Within Modules

In a configuration with multiple modules, there are some special considerations for how resources are associated with provider configurations.

Each resource in the configuration must be associated with one provider configuration. Provider configurations, unlike most other concepts in Terraform, are global to an entire Terraform configuration and can be shared across module boundaries. Provider configurations can be defined only in a root Terraform module.

Providers can be passed down to descendent modules in two ways: either implicitly through inheritance, or explicitly via the providers argument within a module block. These two options are discussed in more detail in the following sections.

A module intended to be called by one or more other modules must not contain any provider blocks, with the exception of the special "proxy provider blocks" discussed under Passing Providers Explicitly below.

For backward compatibility with configurations targeting Terraform v0.10 and earlier Terraform does not produce an error for a provider block in a shared module if the module block only uses features available in Terraform v0.10, but that is a legacy usage pattern that is no longer recommended. A legacy module containing its own provider configurations is not compatible with the for_each, count, and depends_on arguments that were introduced in Terraform v0.13. For more information, see Legacy Shared Modules with Provider Configurations.

Provider configurations are used for all operations on associated resources, including destroying remote objects and refreshing state. Terraform retains, as part of its state, a reference to the provider configuration that was most recently used to apply changes to each resource. When a resource block is removed from the configuration, this record in the state will be used to locate the appropriate configuration because the resource's provider argument (if any) will no longer be present in the configuration.

As a consequence, you must ensure that all resources that belong to a particular provider configuration are destroyed before you can remove that provider configuration's block from your configuration. If Terraform finds a resource instance tracked in the state whose provider configuration block is no longer available then it will return an error during planning, prompting you to reintroduce the provider configuration.

» Provider Version Constraints in Modules

Although provider configurations are shared between modules, each module must declare its own provider requirements, so that Terraform can ensure that there is a single version of the provider that is compatible with all modules in the configuration and to specify the source address that serves as the global (module-agnostic) identifier for a provider.

To declare that a module requires particular versions of a specific provider, use a required_providers block inside a terraform block:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = ">= 2.7.0"
    }
  }
}

A provider requirement says, for example, "This module requires version v2.7.0 of the provider hashicorp/aws and will refer to it as aws." It doesn't, however, specify any of the configuration settings that determine what remote endpoints the provider will access, such as an AWS region; configuration settings come from provider configurations, and a particular overall Terraform configuration can potentially have several different configurations for the same provider.

If you are writing a shared Terraform module, constrain only the minimum required provider version using a >= constraint. This should specify the minimum version containing the features your module relies on, and thus allow a user of your module to potentially select a newer provider version if other features are needed by other parts of their overall configuration.

» Implicit Provider Inheritance

For convenience in simple configurations, a child module automatically inherits default (un-aliased) provider configurations from its parent. This means that explicit provider blocks appear only in the root module, and downstream modules can simply declare resources for that provider and have them automatically associated with the root provider configurations.

For example, the root module might contain only a provider block and a module block to instantiate a child module:

provider "aws" {
  region = "us-west-1"
}

module "child" {
  source = "./child"
}

The child module can then use any resource from this provider with no further provider configuration required:

resource "aws_s3_bucket" "example" {
  bucket = "provider-inherit-example"
}

We recommend using this approach when a single configuration for each provider is sufficient for an entire configuration.

In more complex situations there may be multiple provider configurations, or a child module may need to use different provider settings than its parent. For such situations, you must pass providers explicitly.

» Passing Providers Explicitly

When child modules each need a different configuration of a particular provider, or where the child module requires a different provider configuration than its parent, you can use the providers argument within a module block to explicitly define which provider configurations are available to the child module. For example:

# The default "aws" configuration is used for AWS resources in the root
# module where no explicit provider instance is selected.
provider "aws" {
  region = "us-west-1"
}

# An alternate configuration is also defined for a different
# region, using the alias "usw2".
provider "aws" {
  alias  = "usw2"
  region = "us-west-2"
}

# An example child module is instantiated with the alternate configuration,
# so any AWS resources it defines will use the us-west-2 region.
module "example" {
  source    = "./example"
  providers = {
    aws = aws.usw2
  }
}

The providers argument within a module block is similar to the provider argument within a resource, but is a map rather than a single string because a module may contain resources from many different providers.

The keys of the providers map are provider configuration names as expected by the child module, and the values are the names of corresponding configurations in the current module.

Once the providers argument is used in a module block, it overrides all of the default inheritance behavior, so it is necessary to enumerate mappings for all of the required providers. This is to avoid confusion and surprises that may result when mixing both implicit and explicit provider passing.

Additional provider configurations (those with the alias argument set) are never inherited automatically by child modules, and so must always be passed explicitly using the providers map. For example, a module that configures connectivity between networks in two AWS regions is likely to need both a source and a destination region. In that case, the root module may look something like this:

provider "aws" {
  alias  = "usw1"
  region = "us-west-1"
}

provider "aws" {
  alias  = "usw2"
  region = "us-west-2"
}

module "tunnel" {
  source    = "./tunnel"
  providers = {
    aws.src = aws.usw1
    aws.dst = aws.usw2
  }
}

The subdirectory ./tunnel must then contain proxy configuration blocks like the following, to declare that it requires its calling module to pass configurations with these names in its providers argument:

provider "aws" {
  alias = "src"
}

provider "aws" {
  alias = "dst"
}

Each resource should then have its own provider attribute set to either aws.src or aws.dst to choose which of the two provider configurations to use.

» Proxy Configuration Blocks

A proxy configuration block is one that contains only the alias argument. It serves as a placeholder for provider configurations passed between modules, and declares that a module expects to be explicitly passed an additional (aliased) provider configuration.

» Legacy Shared Modules with Provider Configurations

In Terraform v0.10 and earlier there was no explicit way to use different configurations of a provider in different modules in the same configuration, and so module authors commonly worked around this by writing provider blocks directly inside their modules, making the module have its own separate provider configurations separate from those declared in the root module.

However, that pattern had a significant drawback: because a provider configuration is required to destroy the remote object associated with a resource instance as well as to create or update it, a provider configuration must always stay present in the overall Terraform configuration for longer than all of the resources it manages. If a particular module includes both resources and the provider configurations for those resources then removing the module from its caller would violate that constraint: both the resources and their associated providers would, in effect, be removed simultaneously.

Terraform v0.11 introduced the mechanisms described in earlier sections to allow passing provider configurations between modules in a structured way, and thus we explicitly recommended against writing a child module with its own provider configuration blocks. However, that legacy pattern continued to work for compatibility purposes -- though with the same drawback -- until Terraform v0.13.

Terraform v0.13 introduced the possibility for a module itself to use the for_each, count, and depends_on arguments, but the implementation of those unfortunately conflicted with the support for the legacy pattern.

To retain the backward compatibility as much as possible, Terraform v0.13 continues to support the legacy pattern for module blocks that do not use these new features, but a module with its own provider configurations is not compatible with for_each, count, or depends_on. Terraform will produce an error if you attempt to combine these features. For example:

Error: Module does not support count

  on main.tf line 15, in module "child":
  15:   count = 2

Module "child" cannot be used with count because it contains a nested provider
configuration for "aws", at child/main.tf:2,10-15.

This module can be made compatible with count by changing it to receive all of
its provider configurations from the calling module, by using the "providers"
argument in the calling module block.

To make a module compatible with the new features, you must either remove all of the provider blocks from its definition or, if you need multiple configurations for the same provider, replace them with proxy configuration blocks as described in Passing Providers Explicitly.

If the new version of the module uses proxy configuration blocks, or if the calling module needs the child module to use different provider configurations than its own default provider configurations, the calling module must then include an explicit providers argument to describe which provider configurations the child module will use:

provider "aws" {
  region = "us-west-1"
}

provider "aws" {
  region = "us-east-1"
  alias  = "east"
}

module "child" {
  count = 2
  providers = {
    # By default, the child module would use the
    # default (unaliased) AWS provider configuration
    # using us-west-1, but this will override it
    # to use the additional "east" configuration
    # for its resources instead.
    aws = aws.east
  }
}

Since the association between resources and provider configurations is static, module calls using for_each or count cannot pass different provider configurations to different instances. If you need different instances of your module to use different provider configurations then you must use a separate module block for each distinct set of provider configurations:

provider "aws" {
  alias  = "usw1"
  region = "us-west-1"
}

provider "aws" {
  alias  = "usw2"
  region = "us-west-2"
}

provider "google" {
  alias       = "usw1"
  credentials = "${file("account.json")}"
  project     = "my-project-id"
  region      = "us-west1"
  zone        = "us-west1-a"
}

provider "google" {
  alias       = "usw2"
  credentials = "${file("account.json")}"
  project     = "my-project-id"
  region      = "us-west2"
  zone        = "us-west2-a"
}

module "bucket_w1" {
  source    = "./publish_bucket"
  providers = {
    aws.src    = aws.usw1
    google.src = google.usw2
  }
}

module "bucket_w2" {
  source    = "./publish_bucket"
  providers = {
    aws.src    = aws.usw2
    google.src = google.usw2
  }
}

» Tainting resources within a module

The taint command can be used to taint specific resources within a module:

$ terraform taint module.salt_master.aws_instance.salt_master

It is not possible to taint an entire module. Instead, each resource within the module must be tainted separately.