Network Working Group E. Wilde
Internet-Draft UC Berkeley
Intended status: Informational February 12, 2014
Expires: August 16, 2014

Home Documents for HTTP Services: XML Syntax
draft-wilde-home-xml-03

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.

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This Internet-Draft will expire on August 16, 2014.

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Table of Contents

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].

One notable difference is the handling of links found in "href" or "href-template" attributes on "resource" elements: In the JSON syntax, these are (if they are relative URIs) resolved against the URI of the home document itself. The XML syntax adds support for an optional "xml:base" attribute [W3C.REC-xmlbase-20090128] on the "resources" document element. If this attribute is present, it has to be used according to its specification, and thus becomes part of a possible resolution chain of relative URIs.

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] (but adding xml:base="http://example.com" to demonstrate the mechanism):

<resources xmlns="urn:ietf:rfc:XXXX" xml:base="http://example.com">
 <resource rel="http://example.org/rel/widgets">
  <link href="/"/>
 </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.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xml="http://www.w3.org/XML/1998/namespace" targetNamespace="urn:ietf:rfc:XXXX" elementFormDefault="qualified" xmlns:home="urn:ietf:rfc:XXXX">
 <xs:import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="http://www.w3.org/2001/xml.xsd"/>
 <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:attribute ref="xml:base" use="optional"/>
  </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:

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. Implementation Status

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

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in RFC 6982 [RFC6982]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.

According to RFC 6982, "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

6. 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.

7. Open Issues

8. Change Log

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

8.1. From -02 to -03

8.2. From -01 to -02

8.3. From -00 to -01

9. References

9.1. Normative References

[I-D.nottingham-json-home] Nottingham, M., "Home Documents for HTTP APIs", Internet-Draft draft-nottingham-json-home-03, May 2013.
[RFC3023] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC 3023, January 2001.
[RFC6838] Freed, N., Klensin, J. and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, January 2013.
[W3C.REC-xml-20081126] 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.
[W3C.REC-xmlbase-20090128] Marsh, J. and R. Tobin, "XML Base (Second Edition)", World Wide Web Consortium Recommendation REC-xmlbase-20090128, January 2009.

9.2. Informative References

[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, July 2013.
[W3C.REC-xml-stylesheet-20101028] 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.
[W3C.REC-xslt-19991116] Clark, J., "XSL Transformations (XSLT) Version 1.0", World Wide Web Consortium Recommendation REC-xslt-19991116, November 1999.

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.

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xml="http://www.w3.org/XML/1998/namespace" 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>
       <xsl:text>Link Relation </xsl:text>
       <code><xsl:value-of select="@rel"/></code>
       <xsl:text>: Link </xsl:text>
       <code>
        <xsl:choose>
         <xsl:when test="home:link">
          <xsl:variable name="link-URI">
           <xsl:call-template name="resolve-URI">
            <xsl:with-param name="URI" select="home:link/@href"/>
           </xsl:call-template>
          </xsl:variable>
          <a href="{$link-URI}" title="URI"><xsl:value-of select="$link-URI"/></a>
         </xsl:when>
         <xsl:otherwise>
          <xsl:variable name="template-URI">
           <xsl:call-template name="resolve-URI">
            <xsl:with-param name="URI" select="home:template/@href-template"/>
           </xsl:call-template>
          </xsl:variable>
          <a href="{$template-URI}" title="URI template"><xsl:value-of select="$template-URI"/></a>
         </xsl:otherwise>
        </xsl:choose>
       </code>
       <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:template name="resolve-URI">
  <xsl:param name="URI"/>
  <!-- This is not how URI resolution should be done; http://tools.ietf.org/html/rfc3986#section-5 describes the correct process. -->
  <xsl:value-of select="concat(/home:resources/@xml:base, $URI)"/>
 </xsl:template>
</xsl:stylesheet>

Author's Address

Erik Wilde UC Berkeley EMail: dret@berkeley.edu URI: http://dret.net/netdret/