113 lines
4.8 KiB
Markdown
113 lines
4.8 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "Internals: Remote Service Discovery"
|
|
sidebar_current: "docs-remote-service-discovery"
|
|
description: |-
|
|
Remote service discovery is a protocol used to locate Terraform-native
|
|
services provided at a user-friendly hostname.
|
|
---
|
|
|
|
# Remote Service Discovery
|
|
|
|
Terraform implements much of its functionality in terms of remote services.
|
|
While in many cases these are generic third-party services that are useful
|
|
to many applications, some of these services are tailored specifically to
|
|
Terraform's needs. We call these _Terraform-native services_, and Terraform
|
|
interacts with them via the remote service discovery protocol described below.
|
|
|
|
## User-facing Hostname
|
|
|
|
Terraform-native services are provided, from a user's perspective, at a
|
|
user-facing "friendly hostname" which serves as the key for configuration and
|
|
for any authentication credentials required.
|
|
|
|
The discovery protocol's purpose is to map from a user-provided hostname to
|
|
the base URL of a particular service. Each host can provide different
|
|
combinations of services -- or no services at all! -- and so the discovery
|
|
protocol has a secondary purpose of allowing Terraform to identify _which_
|
|
services are valid for a given hostname.
|
|
|
|
For example, module source strings can include a module registry hostname
|
|
as their first segment, like `example.com/namespace/name/provider`, and
|
|
Terraform uses service discovery to determine whether `example.com` _has_
|
|
a module registry, and if so where its API is available.
|
|
|
|
A user-facing hostname is a fully-specified
|
|
[internationalized domain name](https://en.wikipedia.org/wiki/Internationalized_domain_name)
|
|
expressed in its Unicode form (the corresponding "punycode" form is not allowed)
|
|
which must be resolvable in DNS to an address that has an HTTPS server running
|
|
on port 443.
|
|
|
|
User-facing hostnames are normalized for internal comparison using the
|
|
standard Unicode [Nameprep](https://en.wikipedia.org/wiki/Nameprep) algorithm,
|
|
which includes converting all letters to lowercase, normalizing combining
|
|
diacritics to precomposed form where possible, and various other normalization
|
|
steps.
|
|
|
|
## Discovery Process
|
|
|
|
Given a hostname, discovery begins by forming an initial discovery URL
|
|
using that hostname with the `https:` scheme and the fixed path
|
|
`/.well-known/terraform.json`.
|
|
|
|
For example, given the hostname `example.com` the initial discovery URL
|
|
would be `https://example.com/.well-known/terraform.json`.
|
|
|
|
Terraform then sends a `GET` request to this discovery URL and expects a
|
|
JSON response. If the response does not have status 200, does not have a media
|
|
type of `application/json` or, if the body cannot be parsed as a JSON object,
|
|
then discovery fails and Terraform considers the host to not support _any_
|
|
Terraform-native services.
|
|
|
|
If the response is an HTTP redirect then Terraform repeats this step with the
|
|
new location as its discovery URL. Terraform is guaranteed to follow at least
|
|
one redirect, but nested redirects are not guaranteed nor recommended.
|
|
|
|
If the response is a valid JSON object then its keys are Terraform native
|
|
service identifiers, consisting of a service type name and a version string
|
|
separated by a period. For example, the service identifier for version 1 of
|
|
the module registry protocol is `modules.v1`.
|
|
|
|
The value of each object element is the base URL for the service in question.
|
|
This URL may be either absolute or relative, and if relative it is resolved
|
|
against the final discovery URL (_after_ following redirects).
|
|
|
|
The following is an example discovery document declaring support for
|
|
version 1 of the module registry protocol:
|
|
|
|
```json
|
|
{
|
|
"modules.v1": "https://modules.example.com/v1/"
|
|
}
|
|
```
|
|
|
|
## Supported Services
|
|
|
|
At present, only one service identifier is in use:
|
|
|
|
* `modules.v1`: [module registry API version 1](/docs/registry/api.html)
|
|
|
|
## Authentication
|
|
|
|
If credentials for the given hostname are available in
|
|
[the CLI config](/docs/commands/cli-config.html) then they will be included
|
|
in the request for the discovery document.
|
|
|
|
The credentials may also be provided to endpoints declared in the discovery
|
|
document, depending on the requirements of the service in question.
|
|
|
|
## Non-standard Ports in User-facing Hostnames
|
|
|
|
It is strongly recommended to provide the discovery document for a hostname
|
|
on the standard HTTPS port 443. However, in development environments this is
|
|
not always possible or convenient, so Terraform allows a hostname to end
|
|
with a port specification consisting of a colon followed by one or more
|
|
decimal digits.
|
|
|
|
When a custom port number is present, the service on that port is expected to
|
|
implement HTTPS and respond to the same fixed discovery path.
|
|
|
|
For day-to-day use it is strongly recommended _not_ to rely on this mechanism
|
|
and to instead provide the discovery document on the standard port, since this
|
|
allows use of the most user-friendly hostname form.
|