XRDS (short for eXtensible Resource Descriptor Sequence) is an XML format to describe metadata about a web resource and make available (English " Discovery " ) - in particular services that are available under this resource (English " Service discovery " ). For example, use an OpenID provider XRDS documents to describe the address and the abilities of his OpenID service.


The XML format used by XRDS was originally developed in 2004 as a description format for XRIs from the OASIS XRI for (Extensible Resource Identifier) ​​. The abbreviation XRDS was coined during subsequent discussions between members of the XRI Technical Committee and OpenID developers when first Internet Identity Workshop in Berkeley (California, USA) in October 2005.

A protocol XRDS documents can be retrieved from a URL with the was called Yadis and in March 2006 under Yadis.org (URL no longer current ) published. Yadis was the description format for OpenID 1.1.

Such a description format turned out subsequently to be as useful for URLs and XRIs out that the URL-based use of XRDS was formally added to the XRI Resolution 2.0 specification in November 2007. This format and protocol was then part of the OpenID Authentication 2.0 specification.

XRDS Simple

At the beginning of 2008 led the work on the OAuth protocol of Eran Hammer - Lahav the development of XRDS Simple, which is a special profile of XRDS, the XRDS restricted to the essential elements and thus makes easier to use. Furthermore, some extensions defined to support OAuth and other HTTP -based protocols. The end of 2008 further work on XRDS Simple were hired. The results to date have been returned to the XRDS project and flow into the XRD a 1.0 specification.


In addition to the main application XRI XRDS is used among others in the following areas:

  • OpenID: Description of authentication services, as well as extensions of some providers.
  • OAuth: OAuth description of services and skills.
  • Higgins Project: Description of Higgins "Context Providers".
  • XDI.org: For I- I- name and number directory and based thereon Identititätsdienste.
  • XDI protocol: Description of XDI services and skills.


The following example shows an XRDS document for XRI i -name = example. As can be seen therein, an outer element acts as a container for one or more (eXtensible Resource Descriptor ) elements. Many XRDS documents contain only a single XRD element. Some applications, such as XRI resolution can lead to several elements in a element to describe a metadata - sequence multiple related resources.

< xrds: XRDS xmlns: xrds = " xri :/ / $ xrds " xmlns = " xri :/ / $ xrd * ($ v * 2.0) " xmlns: openid = " http://openid.net/xmlns/1.0 " >         * Example < / Query >           2008-05 - 05T00: 15:00.000 Z      Xri :/ / =      < - Synonyms ->      ! 4C72.6C81.D78F.90B2      http://example.com/example-user      http://example.net/blog      Xri :/ / =! 4C72.6C81.D78F.90B2      < - Services ->             <-! XRI resolution service ->        Xri :/ / =! F83.62B1.44F.2813        Xri :/ / $ res * auth * ($ v * 2.0) < / Type>        Application / xrds xml        http://resolve.example.com < / URI >        http://resolve2.example.com < / URI >        Https://resolve.example.com < / URI >           < - OpenID 2.0 authentication service ->             Http://specs.openid.net/auth/2.0/signon < / Type>        Http://www.myopenid.com/server < / URI >        Http://example.myopenid.com/           < - OpenID 1.0 authentication service ->             Http://openid.net/server/1.0 < / Type>        Http://www.livejournal.com/openid/server.bml < / URI >        http://www.livejournal.com/users/example/           < - Service type JPEG files ->                    / media / pictures < / Path>        image / jpeg        http://pictures.example.com < / URI >         Synonyms

XRDS documents to support the description of synonyms of a resource. In this connection, a synonym or a URI XRI be denoting the same resource. For example describes the above XRDS document, the following synonyms:


The most important part of the XRDS document contains the description of services (English " Service Endpoints " ) that are associated with the resource example above describes the XRDS document, the following services. :

Service types

In XRDS the type of a service is described by a URI or XRI. The following list shows some of the known types of services. Under http://xrdstype.net Since May 2008, a project is underway to catalog those types of services.

XRI resolution



XDI.org I- Services

I- Services are affiliated with XRIs identity services.


XRDS is a free open source specification from OASIS. The OASIS XRI for working from the beginning of his work on ( in 2003) under a free license, as can be read at the XRI and charter.