Home Documents for HTTP Services: XML Syntax
draft-wilde-home-xml-01

Abstract

The current draft for HTTP Home Documents provides a JSON syntax only. This draft provides an XML syntax for the same underlying data model, so that the concept of HTTP Home Documents can be consistently exposed in both JSON- and XML-based HTTP services.

Note to Readers

Please discuss this draft on the apps-discuss mailing list.

Online access to all versions and files is available on github.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on December 13, 2013.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Introduction
2. XML Example
3. XML Schema
4. IANA Considerations
4.1. Media Type application/home+xml
5. Security Considerations
6. Open Issues
7. Change Log
7.1. From -00 to -01
8. References
8.1. Normative References
8.2. Informative References
Appendix A. XML-to-HTML for Home Documents
Author's Address

1. Introduction

An Internet Draft currently under development [I-D.nottingham-json-home] proposes the concept of "Home Documents for HTTP APIs" and described them as follows:

"This document proposes a 'home document' format for non-browser HTTP clients. [...] The goal of home documents is to serve as a starting point for hypermedia APIs, where clients need to have an entry point, and then can use the API by following links. Home documents thus serve the same purpose as home pages on web sites: They are stable entry points that provide starting points for clients with some knowledge of the services linked from them."

While this general concept of a home document is independent of the representation format, the current draft only defines a JSON syntax. In order to make this concept available across representations, this draft defines an XML syntax for the concepts defined in [I-D.nottingham-json-home].

At this point it is undecided whether both drafts will be merged eventually, or whether they will both be published as separate documents. Regardless of the final publication setup, it should be noted that this draft is only defining the XML syntax, whereas all the concepts represented in this syntax are defined by [I-D.nottingham-json-home].

2. XML Example

The following Home Document in XML syntax uses the same data as the Home Document shown in Section 2 of [I-D.nottingham-json-home]:

<resources xmlns="urn:ietf:rfc:XXXX">
 <resource rel="http://example.org/rel/widgets">
  <link href="/widgets/"/>
 </resource>
 <resource rel="http://example.org/rel/widget">
  <template href-template="/widgets/{widget_id}">
   <var name="widget_id" URI="http://example.org/param/widget"/>
  </template>
  <hints>
   <allow>
    <i>GET</i>
    <i>PUT</i>
    <i>DELETE</i>
    <i>PATCH</i>
   </allow>
   <representations>
    <i>application/json</i>
   </representations>
   <accept-patch>
    <i>application/patch+json</i>
   </accept-patch>
   <accept-post>
    <i>application/xml</i>
   </accept-post>
   <accept-ranges>
    <i>bytes</i>
   </accept-ranges>
  </hints>
 </resource>
</resources>

The mapping between JSON arrays and XML uses "item" elements <i/>, where each of those elements represents one array item. For properties that have a single values (i.e., they are not defined as an array of values), this value is directly contained as content in the corresponding element.

Currently, the draft does not specify an extension model (how to represents hints that are not specified in the draft itself), and therefore the extension model for XML is currently undefined as well. The XML syntax will be updated to reflect the extension model once it has been specified for the JSON syntax.

3. XML Schema

The following XML Schema is describing the XML shown in Section 2. Since there currently is no extension model, the XML Schema does currently not contain any extension points.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="urn:ietf:rfc:XXXX"
 elementFormDefault="qualified" xmlns:home="urn:ietf:rfc:XXXX">
 <xs:element name="resources">
  <xs:complexType>
   <xs:sequence maxOccurs="unbounded" minOccurs="0">
    <xs:element name="resource">
     <xs:complexType>
      <xs:sequence>
       <xs:choice>
        <xs:element name="link">
         <xs:complexType>
          <xs:attribute name="href" type="xs:anyURI" use="required"/>
         </xs:complexType>
        </xs:element>
        <xs:element name="template">
         <xs:complexType>
          <xs:sequence maxOccurs="unbounded" minOccurs="0">
           <xs:element name="var">
            <xs:complexType>
             <xs:attribute name="name" use="required"/>
             <xs:attribute name="URI" type="xs:anyURI" use="required"/>
            </xs:complexType>
           </xs:element>
          </xs:sequence>
          <xs:attribute name="href-template" use="required"/>
         </xs:complexType>
        </xs:element>
       </xs:choice>
       <xs:element minOccurs="0" name="hints">
        <xs:complexType>
         <xs:choice maxOccurs="unbounded" minOccurs="0">
          <xs:element name="allow" type="home:arrayType"/>
          <xs:element name="representations" type="home:arrayType"/>
          <xs:element name="accept-patch" type="home:arrayType"/>
          <xs:element name="accept-post" type="home:arrayType"/>
          <xs:element name="accept-put" type="home:arrayType"/>
          <xs:element name="accept-ranges" type="home:arrayType"/>
          <xs:element name="prefer" type="home:arrayType"/>
          <xs:element name="docs" type="xs:anyURI"/>
          <xs:element name="precondition-req" type="home:arrayType"/>
          <xs:element name="auth-req">
           <xs:complexType>
            <xs:sequence maxOccurs="unbounded" minOccurs="0">
             <xs:element name="scheme">
              <xs:complexType>
               <xs:sequence maxOccurs="unbounded" minOccurs="0">
                <xs:element name="realm"/>
               </xs:sequence>
               <xs:attribute name="name" type="xs:token"/>
              </xs:complexType>
             </xs:element>
            </xs:sequence>
           </xs:complexType>
          </xs:element>
          <xs:element name="status">
           <xs:simpleType>
            <xs:restriction base="xs:token">
             <xs:enumeration value="deprecated"/>
             <xs:enumeration value="gone"/>
            </xs:restriction>
           </xs:simpleType>
          </xs:element>
         </xs:choice>
        </xs:complexType>
       </xs:element>
      </xs:sequence>
      <xs:attribute name="rel" type="xs:anyURI" use="required"/>
     </xs:complexType>
    </xs:element>
   </xs:sequence>
  </xs:complexType>
 </xs:element>
 <xs:complexType name="arrayType">
  <xs:sequence maxOccurs="unbounded" minOccurs="0">
   <xs:element name="i"/>
  </xs:sequence>
 </xs:complexType>
</xs:schema>

4. IANA Considerations

This specification registers a media type for the XML syntax of Home Documents (as defined in [I-D.nottingham-json-home]).

4.1. Media Type application/home+xml

The Internet media type [RFC6838] for a Home Document in XML syntax is application/home+xml.

Type name: application

Subtype name: home+xml

Required parameters: none

Optional parameters: Same as charset parameter for the media type "application/xml" as specified in RFC 3023 [RFC3023].

Encoding considerations: Same as encoding considerations of media type "application/xml" as specified in RFC 3023 [RFC3023].

Security considerations: This media type has all of the security considerations described in RFC 3023 [RFC3023] and [I-D.nottingham-json-home].

Interoperability considerations: N/A

Published specification: RFC XXXX

Applications that use this media type: Applications that publish Home Documents for HTTP services using XML syntax.

Additional information:

Magic number(s): N/A
File extension(s): XML documents should use ".xml" as the file extension.
Macintosh file type code(s): TEXT

Person & email address to contact for further information: Erik Wilde <erik.wilde@emc.com>

Intended usage: COMMON

Restrictions on usage: none

Author: Erik Wilde <erik.wilde@emc.com>

Change controller: IETF

5. Security Considerations

The general security considerations for XML home documents are the same as those for JSON home documents, as described in the "Security Considerations" of [I-D.nottingham-json-home]. The specific security considerations introduced by XML as a representation format are described in the "Security Considerations" of Section 4.1.

6. Open Issues

The move from a predefined list of hints to a hint registry is not yet reflected in the XML; in particular because the latest draft defines the hint data model to be JSON-specific.
What is the extension model for the XML syntax? Should processing of other namespaces be defined as "should ignore", so that same-namespace extensions are encouraged?
Should the XML syntax provide support for embedded human-readable documentation? This would probably not be supported in the JSON syntax, but could be marked as strictly optional and XML-specific.

7. Change Log

Note to RFC Editor: Please remove this section before publication.

7.1. From -00 to -01

Updated references from draft-nottingham-json-home-02 to draft-nottingham-json-home-03
Added "Security Considerations" section.
Added XSLT for transforming an XML Home Document to a simple HTML representation.

8. References

8.1. Normative References

[1]	Nottingham, M., "Home Documents for HTTP APIs", Internet-Draft draft-nottingham-json-home-03, May 2013.
[2]	Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC 3023, January 2001.
[3]	Freed, N., Klensin, J. and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, January 2013.
[4]	Sperberg-McQueen, C., Yergeau, F., Paoli, J., Maler, E. and T. Bray, "Extensible Markup Language (XML) 1.0 (Fifth Edition)", World Wide Web Consortium Recommendation REC-xml-20081126, November 2008.

8.2. Informative References

[1]	Clark, J., "XSL Transformations (XSLT) Version 1.0", World Wide Web Consortium Recommendation REC-xslt-19991116, November 1999.
[2]	Pieters, S., Clark, J. and H. Thompson, "Associating Style Sheets with XML documents 1.0 (Second Edition)", World Wide Web Consortium Recommendation REC-xml-stylesheet-20101028, October 2010.

Appendix A. XML-to-HTML for Home Documents

The following XSLT 1.0 stylesheet [W3C.REC-xslt-19991116] transforms XML Home Documents to very simple HTML renditions. By associating this stylesheet [W3C.REC-xml-stylesheet-20101028] with an XML Home Document, it is possible to serve XML Home Documents that will be rendered in a human-friendly way when viewed in a browser.

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0" xmlns:home="urn:ietf:rfc:XXXX" exclude-result-prefixes="home">
 <xsl:output method="html"/>
 <xsl:template match="home:resources">
  <html>
   <head>
    <title>Home Document</title>
   </head>
   <body>
    <h1>Home Document</h1>
    <ul>
     <xsl:for-each select="home:resource">
      <li>
       <span title="Link Relation"><xsl:value-of select="@rel"/></span>
       <xsl:text>: </xsl:text>
       <xsl:choose>
        <xsl:when test="home:link">
         <a href="{home:link/@href}" title="URI"><xsl:value-of select="home:link/@href"/></a>
        </xsl:when>
        <xsl:otherwise>
         <a href="{home:template/@href-template}" title="URI template"><xsl:value-of select="home:template/@href-template"/></a>
        </xsl:otherwise>
       </xsl:choose>
       <dl>
        <xsl:if test="home:template/home:var">
         <dt>Variables: </dt>
         <dd>
          <ul>
           <xsl:for-each select="home:template/home:var">
            <xsl:sort select="@name"/>
            <li>
             <span title="Variable Name"><xsl:value-of select="@name"/></span>
             <xsl:text>: </xsl:text>
             <span title="Associated URI"><xsl:value-of select="@URI"/></span>
            </li>
           </xsl:for-each>
          </ul>
         </dd>
        </xsl:if>
        <xsl:if test="home:hints">
         <dt>Hints: </dt>
         <dd>
          <ul>
           <xsl:for-each select="home:hints/home:*">
            <xsl:sort select="local-name()"/>
            <li>
             <span title="Hint Name"><xsl:value-of select="local-name()"/></span>
             <xsl:text>: </xsl:text>
             <xsl:choose>
              <xsl:when test="home:i">
               <xsl:for-each select="home:i">
                <xsl:value-of select="."/>
                <xsl:if test="position() != last()">
                 <xsl:text>, </xsl:text>
                </xsl:if>
               </xsl:for-each>
              </xsl:when>
              <xsl:otherwise>
               <xsl:value-of select="."/>
              </xsl:otherwise>
             </xsl:choose>
            </li>
           </xsl:for-each>
          </ul>
         </dd>
        </xsl:if>
       </dl>
      </li>
     </xsl:for-each>
    </ul>
   </body>
  </html>
 </xsl:template>
</xsl:stylesheet>

Author's Address

Erik Wilde EMC 6801 Koll Center Parkway Pleasanton, CA 94566, U.S.A. Phone: +1-925-6006244 EMail: erik.wilde@emc.com URI: http://dret.net/netdret/