Module Registry Protocol

The module registry protocol is what OpenTofu CLI uses to discover metadata about modules available for installation and to locate the distribution package for a selected module.

The primary implementation of this protocol is the public OpenTofu Registry at registry.opentofu.org. By writing and deploying your own implementation of this protocol, you can create a separate registry to distribute your own modules, as an alternative to publishing them on the public OpenTofu Registry.

Module Addresses​

Each OpenTofu module has an associated address. A module address has the syntax hostname/namespace/name/system, where:

• hostname is the hostname of the module registry that serves this module.
• namespace is the name of a namespace, unique on a particular hostname, that can contain one or more modules that are somehow related. On the public OpenTofu Registry the "namespace" represents the organization that is packaging and distributing the module.
• name is the module name, which generally names the abstraction that the module is intending to create.
• system is the name of a remote system that the module is primarily written to target. For multi-cloud abstractions, there can be multiple modules with addresses that differ only in "system" to reflect provider-specific implementations of the abstraction, like registry.opentofu.org/hashicorp/consul/aws vs. registry.opentofu.org/hashicorp/consul/azurerm. The system name commonly matches the type portion of the address of an official provider, like aws or azurerm in the above examples, but that is not required and so you can use whichever system keywords make sense for the organization of your particular registry.

The hostname/ portion of a module address (including its slash delimiter) is optional, and if omitted defaults to registry.opentofu.org/.

For example:

• hashicorp/consul/aws is a shorthand for registry.opentofu.org/hashicorp/consul/aws, which is a module on the public registry for deploying Consul clusters in Amazon Web Services.
• example.com/awesomecorp/consul/happycloud is a hypothetical module published on a third-party registry.

If you intend to share a module you've developed for use by all OpenTofu users, please consider publishing it into the public OpenTofu Registry to make your module more discoverable. You only need to implement this module registry protocol if you wish to publish modules whose addresses include a different hostname that is under your control.

Module Versions​

Each distinct module address has associated with it a set of versions, each of which has an associated version number. OpenTofu assumes version numbers follow the Semantic Versioning 2.0 conventions, with the user-facing behavior of the module serving as the "public API".

Each module block may select a distinct version of a module, even if multiple blocks have the same source address.

Service Discovery​

The module registry protocol begins with OpenTofu CLI using OpenTofu's remote service discovery protocol, with the hostname in the module address acting as the "User-facing Hostname".

The service identifier for the module registry protocol is modules.v1. Its associated string value is the base URL for the relative URLs defined in the sections that follow.

For example, the service discovery document for a host that only implements the module registry protocol might contain the following:

{
  "modules.v1": "/tofu/modules/v1/"
}

If the given URL is a relative URL then OpenTofu will interpret it as relative to the discovery document itself. The specific module registry protocol endpoints are defined as URLs relative to the given base URL, and so the specified base URL should generally end with a slash to ensure that those relative paths will be resolved as expected.

The following sections describe the various operations that a module registry must implement to be compatible with OpenTofu CLI's module installer. The indicated URLs are all relative to the URL resulting from service discovery, as described above. We use a hypothetical URL for a provider registry, assuming that the caller already performed service discovery on a hypothetical registry.example.io to learn the base URL.

The URLs are shown with the convention that a path portion with a colon : prefix is a placeholder for a dynamically-selected value, while all other path portions are literal. For example, in :namespace/:type/versions, the first two path portions are placeholders while the third is literally the string "versions".

List Available Versions for a Specific Module​

This is the primary endpoint for resolving module sources, returning the available versions for a given fully-qualified module.

Parameters​

• namespace (string: <required>) - The user or organization the module is owned by. This is required and is specified as part of the URL path.

• name (string: <required>) - The name of the module. This is required and is specified as part of the URL path.

• system (string: <required>) - The name of the target system. This is required and is specified as part of the URL path.

Sample Request​

$ curl 'https://registry.opentofu.org/v1/modules/hashicorp/consul/aws/versions'

Sample Response​

The modules array in the response always includes the requested module as the first element.

OpenTofu does not use the other elements of this list. However, third-party implementations should always use a single-element list for forward compatiblity.

Each returned module has an array of available versions, which OpenTofu matches against any version constraints given in configuration.

{
   "modules": [
      {
         "versions": [
            {"version": "1.0.0"},
            {"version": "1.1.0"},
            {"version": "2.0.0"}
         ]
      }
   ]
}

Return 404 Not Found to indicate that no module is available with the requested namespace, name, and target system.

Download Source Code for a Specific Module Version​

This endpoint downloads the specified version of a module for a single target system.

Parameters​

• namespace (string: <required>) - The user the module is owned by. This is required and is specified as part of the URL path.

• name (string: <required>) - The name of the module. This is required and is specified as part of the URL path.

• system (string: <required>) - The name of the target system. This is required and is specified as part of the URL path.

• version (string: <required>) - The version of the module. This is required and is specified as part of the URL path.

Sample Request​

$ curl -i 'https://registry.opentofu.org/v1/modules/foo/bar/baz/0.0.1/download'

Sample Response​

A successful response contains the location from which the module version's source can be downloaded.

It is expected to be found in the JSON encoded body as the value for the key location:

HTTP/2 200
Content-Length: 81

{"location": "git::https://github.com/foo/terraform-baz-bar?ref=v0.0.1"}

In the absence of a response body, OpenTofu will use the X-Terraform-Get header as the module location:

HTTP/2 204 No Content
Content-Length: 0
X-Terraform-Get: git::https://github.com/foo/terraform-baz-bar?ref=v0.0.1

The module location value accepts the same values as the source argument in a module block in OpenTofu configuration, as described in Module Sources, except that it may not recursively refer to another module registry address. The value of the module location may instead be a relative URL, indicated by beginning with /, ./ or ../, in which case it is resolved relative to the full URL of the download endpoint to produce an HTTP URL module source.