OpenStack API Special Interest Group
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api-sig/guidelines/discoverability.rst

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API Discoverability

This topic document serves to provide guidance on how to have a public REST API expose the URIs and resources to end users in a machine-readable way.

See also the topic document on consuming-catalog.

See also the topic document on consuming-catalog/version-discovery.

Versioned and Unversioned Endpoints

Each service should have a base endpoint which is referred to as the "Unversioned" endpoint for the service.

Note

It is highly recommended that Cloud Operators register the Unversioned Endpoint for a service in the Keystone Catalog. If they do, the process described in the version-discovery-algorithm will be able to be both the most featureful for the API Consumer and the most efficient.

Each service must have at least one Major API version.

If a service has, or expects to have, more than one Major API version, each of those versions should have a base endpoint which is referred to as the "Versioned" endpoint for that version of the service.

All version discovery documents must be accessible via unauthenticated connection.

If a service only uses microversions and only has one Major API version, that service should not have any additional "Versioned" endpoints.

For instance, the Glance service at cloud example.com might have an Unversioned Endpoint at:

https://image.example.com

That Glance service may then also provide three major versions, v1, v2, and `v3`:

https://image.example.com/v1
https://image.example.com/v2
https://image.example.com/v3

Additionally, the Placement service at cloud example.com might have an Unversioned Endpoint at:

https://placement.example.com

The Placement service only uses microversions, so there are no additional Versioned Endpoints.

In both cases, the Unversioned Endpoint is the endpoint recommended to be registered in the Service Catalog.

Preference for Subpaths

Historically each of the OpenStack services were also given a port. It is strongly recommended to not use those ports and instead use the normal port 443 for https. If multiple API services are to be installed on a single machine, it is highly recommended to use subpaths:

https://api.example.com/compute
https://api.example.com/image

The rationale behind this recommendation is to ease use for users who may have restrictive port-blocking firewalls in their operating environment. Since the traffic is HTTP traffic and not a different protocol, it is not necessary to distinguish it by port number, and doing so increases the chances that users will have problems connecting to individual API endpoints.

Version Discovery

Each service should provide a Version Discovery API on both the Unversioned Endpoint and each of Versioned Endpoints of the service to be used by clients to discover the supported API versions.

Unversioned Discovery

Each service should provide a Version Discovery API at the Unversioned Endpoint of the service. It should be exposed to all users without authentication.

Note

It is recommended that the Version Discovery API not be protected by authentication requirements. The information returned is not specific to a user, but is, instead, a fundamental characteristic of the base API of the service running. Additionally, the v2 and v3 authentication API are different, so requiring authentication before version discovery makes it harder to determine reliably whether v2 or v3 authentication should be used.

The Unversioned Version Discovery API for each service should return a list of Version Information for all of the Base Endpoints the service provides, along with that version's minimum and maximum microversions. These values are used by the client to discover the supported API versions.

Version Information Schema <version-information-schema.json>

version-information-schema.json

Version Discovery Schema <version-discovery-schema.json>

version-discovery-schema.json

An Unversioned Version Discovery response would look as follows:

GET /
{
     "versions": [
          {
              "id": "v2.1",
              "links": [
                  {
                      "href": "https://compute.example.com/v2/",
                      "rel": "self"
                  },
                  {
                      "href": "https://compute.example.com/",
                      "rel": "collection"
                  }
              ],
              "status": "CURRENT",
              "max_version": "5.2",
              "min_version": "2.1"
          },
   ]
}

Each Version Information in the list should contain the following information:

id

The major API version. Follows format outlined in versioning, preceeded by a "v". Required.

links

Contains information about where to find the actual versioned endpoint. See version-links below. Required.

status

Support and lifecycle status of the versioned endpoint. Required. See endpoint-status

max_version

The maximum microversion available if the version of the service supports microversions. Optional. See microversion_specification

min_version

The minimum microversion available if the version of the service supports microversions. Optional. See microversion_specification

If a service has no Versioned Endpoints, it should simply list its Base Endpoint in the document, like so:

GET /
{
     "versions": [
          {
              "id": "v1.0",
              "links": [
                  {
                      "href": "https://placement.example.com/",
                      "rel": "self"
                  },
                  {
                      "href": "https://placement.example.com/",
                      "rel": "collection"
                  }
              ],
              "status": "CURRENT",
              "max_version": "1.25",
              "min_version": "1.0"
          }
     ]
}

Versioned Discovery

Each service should provide a Version Discovery API at each Versioned Endpoint of the service. It should be exposed to all users without authentication.

The document returned from the Version Discovery API for Versioned Endpoint should be identical to the document returned from the Unversioned Endpoint. In this way, a client that is looking for a version of an API can always get the complete information in one step, rather than a sequence of attempts, failures and re-attempts.

However, in service of getting to a perfect future from amidst an imperfect past, services that already deliver a different document on their Versioned Endpoints who are concerned with API breakage resulting from changing the payload of their Versioned Version Discovery Document from a single object named version to a list of objects named versions, it can be nonetheless a step forward to add a link to the list of links provided that points to the Unversioned Discovery Endpoint.

For services that do not return the Versioned Version Discovery Document inside of an object named version but instead with the information directly in the root object, it is similarly suggested to add the collection link. (see https://www.iana.org/assignments/link-relations/link-relations.xhtml for the list of defined relation types)

Versioned Discovery Schema <versioned-discovery-schema.json>

versioned-discovery-schema.json

For example:

GET /v2
{
     "version": {
          "id": "v2.0",
          "links": [
              {
                  "href": "https://image.example.com/v2",
                  "rel": "self"
              },
              {
                  "href": "https://image.example.com/",
                  "rel": "collection"
              }
          ],
          "status": "CURRENT"
      }
}

Endpoint Status

While it is typical for there to only be a single versioned API for a given service, it is also sometimes useful to be able to offer more than one version of a given API. Common examples of this are when an older version is still made available in order to support clients that have not yet upgraded to the current version, or when a new API version is being tested before it is released. To distinguish these different APIs for the same service, the status value is used. The following values can be returned for status:

CURRENT

The newest API that is currently being developed and improved. Unless you need to support old code, use this API. One and only one API must be marked as CURRENT.

SUPPORTED

An older version of the API. No new features will be added to this version, but any bugs discovered in the code may be fixed.

DEPRECATED

This API will be removed in the foreseeable future. You should start planning on using alternatives.

EXPERIMENTAL

This API is under development ('alpha'), and you can expect it to change or even be removed.

Note

The links conform to the links guideline.

The links field of the endpoint description should contain at least two entries, listed here by the value of their rel field.

self

The location of the base endpoint for the given version of the service.

collection

The location of the base endpoint for the Unversioned Version Discovery Endpoint. The collection entry provides guidance to allow a client to navigate from a Versioned Endpoint that may have been listed in the Service Catalog to the Unversioned Endpoint if they are looking for a different version without resorting to attempting to guess at URL schemes and performing URL manipulation.

Guidance

TODO Add sections that describe a best practice for API discoverability, possibly using JSON-Home, JSONSchema documents, and JSON-HAL.