Use JSON Home for Version/Extension discovery

bp json-home

The Keystone server provides version discovery as a response to the GET /, GET /v2.0, and GET /v3 operations. The format of this response is a JSON document which is Keystone-specific (it’s defined in the Identity API spec). Keystone should support JSON Home, since this is a more standard way to do REST API discovery. This will help to standardize version discovery across the OpenStack ecosystem.

This was discussed on the openstack-dev mailing list.

Problem Description

An application writer has a need to interact with different OpenStack services, so she starts by writing some code to talk to the servers to discover the version of the APIs supported by each server. She finds that all the servers provide a different response and so has to write custom code for each service. This is harder than it should be, since all the services should use the same format for their version document. The format chosen for the version document should provide a link-driven approach, such as JSON Home.

With this change, Keystone will provide a JSON Home document so that it’s consistent with other services that use JSON Home.

Proposed Change

The V3 Identity API spec will be changed to document that the server can respond to a GET /v3 request with a JSON Home document if the Accept header is application/json-home, and also provide an example of what the response will look like. The WADLs for the Identity API v2 will be updated similarly.

The Keystone server will be changed to check the Accept header on GET /, GET /v2.0, and GET /v3 request, and if the header indicates that application/json-home is the preferred format over application/json, then the server will generate a JSON Home document rather than the normal JSON response.

Here’s an example of a minimal document that only has /v3/users and /v3/users/{user_id}:

  "resources": {
    "": {
      "href": "/v3/users"
    "": {
      "href-template": "/v3/users/{user_id}",
      "href-vars": {
        "user_id": ""
    // ...

The keys of the resources property are link relation types. A relationship type needs to be chosen for the key. There are several relationship types registered with IANA, but there’s none designated for Identity API resources. If a group doesn’t want to register a relation with IANA (see section 4.2 “Extension Relation Types” in RFC 5988), they can use some unique URL instead. An application could potentially fetch this URL to get information about the relationship, so we should pick one that could potentially be used to serve up some info about what the relationship is and describe the resource. The Nova project publishes their XSD files at so Keystone should publish its files in a similar location for consistency. For v3 resources, the relation type link will be like<type>, where <type> is like users, user, projects, etc.

A relationship URL also has to be chosen for the parameters. For v3 parameters, these will be like<parameter-type>, where <parameter-type> is like user_id, project_id, etc.

The JSON Home document that the server returns will change depending on which extensions are enabled. The enabled extensions (that are in the pipeline) will intercept the GET request and update the response.

There will be a resource in resources for every resource template in the v3 API (for V3), V3 extensions that are enabled, the public and admin v2 APIs and v2 extensions that are enabled.

Here’s an example of a resource for an extension, OS-EP-FILTER:

  "resources": {
    "": {
      "href": "/v3/OS-EP-FILTER/projects/{project_id}/endpoints/{endpoint_id}",
      "href-vars": {
        "project_id": "",
        "endpoint_id": ""

The relationship type for extensions will be like<api-version>/ext/<extension-name>/<extension-version>/rel/<resource>.

The relationship type for a parameter used by an extension will be like<api-version>/ext/<extension-name>/<extension-version>/param/<param-id>.

The extensions provided by Keystone have routers that implement the keystone.common.wsgi.ExtensionRouter class. A new V3ExtensionRouter will be created from ExtensionRouter that has code to intercept the GET /v3 request and update the response with the info for the extension. The info for the extension will be a property that’s overridden by the extension implementation.



Data Model Impact

None. This will not require database changes.



If the Accept header is application/json-home, the server will respond with a 200 OK and the JSON Home document describing the REST API, as described in the “Proposed Change” section above.

Note that if the client is making the request with Accept: application/json-home, an old server will return the old JSON response with Content-Type: application/json, so clients will have to verify that the Content-Type in the response is actually application/json-home as expected before using the result. A server that conforms to the HTTP 1.1 specification would respond with a 406 Not Acceptable error in the case where it doesn’t support the provided Accept header, so a client should also be able to handle that response.

A client will be able to set the Accept header to a value like application/json; q=0.2, application/json-home and the server will return JSON if it doesn’t support JSON Home. Note that the Keystone server doesn’t support this today. The WebOb library that Keystone uses has support for Accept header handling, but Keystone doesn’t use it for the Accept header (it’s used for Accept-Language handling).

GET /v2.0

Similar to GET /, but returns the JSON Home document for only the V2 API and extensions.

GET /v3

Similar to GET /, but returns the JSON Home document for only the V3 API and extensions.

Security Impact

None. The API is public info.

Notifications Impact


Other End User Impact

python-keystoneclient should be changed to support fetching and using the JSON Home document for discovery.

Performance Impact


Other Deployer Impact


Developer Impact

When adding a new resource, or changing a resource with new arguments, the JSON Home document will have to be updated. Extensions will have to update the JSON Home document.



Primary assignee:

blk-u <Brant Knudson>

Other contributors:


Work Items

  1. Update the Identity V3 spec and other specs with the new Accept header and sample response.

  2. Enhance the Keystone server so that it can process the Accept header, in that application/json; q=0.2, application/json-home could result in a JSON Home response by passing on the requested Accept header to the controller.

  3. Change Keystone server to respond with JSON Home for /, /v2.0, /v3 when the accept header is application/json-home.

  4. Change the v2 and v3 extensions to update the JSON Home response.

  5. Write a Tempest test to verify requests for /, /v2.0, and /v3 with Accept set to application/json-home.

  6. Update python-keystoneclient to be able to use JSON Home for /, /v2.0, and /v3.




Tempest will be changed to validate the response for GET /, GET /v2.0, and GET /v3 with Accept: application/json-home.

Documentation Impact

The documentation will need to be changed to say that Keystone supports JSON Home.


[0] JSON Home

[1] Nottingham, M. “Web Linking”, RFC 5988, October 2010

[2] Discoverable home document for APIs discusson on the openstack-dev mailing list.

[3] HTTP 1.1, section 14.1 Accept Request Header