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Versions: 00 01

Network Working Group                                         J. Michaud
Internet-Draft                                          February 8, 2017
Intended status: Informational
Expires: August 12, 2017


                   XML Hypertext Application Language
                        draft-michaud-xml-hal-01

Abstract

   This document proposes a media type for representing resources and
   their relations with hyperlinks.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   Copyright (c) 2017 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  HAL Documents  . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Resource . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Reserved Elements  . . . . . . . . . . . . . . . . . . . .  4
       4.1.1.  link . . . . . . . . . . . . . . . . . . . . . . . . .  4
       4.1.2.  resource . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     5.1.  rel  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     5.1.  href . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.2.  templated  . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.3.  type . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.4.  deprecation  . . . . . . . . . . . . . . . . . . . . . . .  5
     5.5.  name . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.6.  profile  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.7.  title  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.8.  hreflang . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Media Type Parameters  . . . . . . . . . . . . . . . . . . . .  8
     7.1.  profile  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Self Link  . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.2.  Link relations . . . . . . . . . . . . . . . . . . . . . .  8
     8.3.  Hypertext Cache Pattern  . . . . . . . . . . . . . . . . .  8
     8.4.  Namespace and prefix . . . . . . . . . . . . . . . . . . .  9
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   10.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  9
   11.  Normative References  . . . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 10
   Appendix B.  Frequently Asked Questions  . . . . . . . . . . . . . 11
     B.1.  How should a client know the
           meaning/structure/semantics/type of a  . . . . . . . . . . 11
     B.2.  Where can I find libraries for working with HAL? . . . . . 11
     B.3.  Why does HAL have no forms?  . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11


1.  Introduction

   There is an emergence of non-HTML HTTP applications ("Web APIs")
   which use hyperlinks to direct clients around their resources.

   The XML Hypertext Application Language (HAL) is a standard which
   establishes conventions for expressing hypermedia controls, such as
   links, with XML [W3C.REC-xml-20081126].




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   HAL is a generic media type with which Web APIs can be developed and
   exposed as series of links.  Clients of these APIs can select links
   by their link relation type and traverse them in order to progress
   through the application.

   HAL's conventions result in a uniform interface for serving and
   consuming hypermedia, enabling the creation of general-purpose
   libraries that can be re-used on any API utilizing HAL.

   The primary design goals of HAL are generality and simplicity.  HAL
   can be applied to many different domains, and imposes the minimal
   amount of structure necessary to cover the key requirements of a
   hypermedia Web API.

2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  HAL Documents

   A HAL Document uses the format described in [W3C.REC-xml-20081126]
   and has the media type "application/hal+xml".

   Its root MUST be a resource element.

   For example:

   GET /orders/523 HTTP/1.1
   Host: example.org
   Accept: application/hal+xml

   HTTP/1.1 200 OK
   Content-Type: application/hal+xml

   <resource rel="self" href="/orders/523">
       <link rel="warehouse" href="/warehouse/56"/>
       <link rel="invoice" href="/invoices/873"/>
       <currency>USD</currency>
       <status>shipped</status>
       <total>10.20</total>
   </resource>

   Here, we have a HAL document representing an order resource with the
   URI "/orders/523".  It has "warehouse" and "invoice" links, and its
   own state in the form of "currency", "status", and "total" elements.




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4.  Resource

   The resource element represents a Resource and SHOULD also host the
   set of attributes that describe a Link (as defined by [RFC5988]). See
   Section 5. and Section 8.1. for details.

   It has two reserved elements:

   (1)  "link": a link to another resource.

   (2)  "resource": an embedded resource.

   Everything else MUST be valid XML and represents the current state of
   the Resource.

4.1.  Reserved Elements

4.1.1.  link

   The reserved "link" element is OPTIONAL.

   This element MAY be used to host the set of attributes that describe
   a Link (as defined by [RFC5988]). See Section 5. for details.

4.1.2.  resource

   The reserved "resource" element is OPTIONAL

   If present, this element MUST observe the same constraints defined
   for the root resource but MUST additionally host a Link so that
   embedded resources can more easily be identified when processed.

   Embedded resources MAY be a full, partial, or inconsistent version of
   the representation served from the target URI.

5.  Link

   A Link is defined as a set of attributes that represent a hyperlink
   from the containing resource to a URI. The attributes are as follows:

5.1.  rel

   The "rel" attribute is REQUIRED.

   It is an attribute whose relation values are link relation types (as
   defined by [RFC5988]).





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5.1.  href

   The "href" attribute is REQUIRED.

   Its value is either a URI [RFC3986] or a URI Template [RFC6570].

   If the value is a URI Template then the Link SHOULD have a
   "templated" attribute whose value is true.

5.2.  templated

   The "templated" attribute is OPTIONAL.

   Its value is boolean (as defined by [W3C.REC-xmlschema-2-20041028])
   and SHOULD be true when the Link's "href" attribute is a URI
   Template.

   Its value SHOULD be considered false if the attribute is absent.

5.3.  type

   The "type" attribute is OPTIONAL.

   Its value is a string used as a hint to indicate the media type
   expected when dereferencing the target resource.

5.4.  deprecation

   The "deprecation" attribute is OPTIONAL.

   Its presence indicates that the link is to be deprecated (i.e.
   removed) at a future date.  Its value is a URL that SHOULD provide
   further information about the deprecation.

   A client SHOULD provide some notification (e.g. by logging a warning
   message) whenever it traverses over a link that has this attribute.
   The notification SHOULD include the deprecation attribute's value so
   that a client maintainer can easily find information about the
   deprecation.

5.5.  name

   The "name" attribute is OPTIONAL.

   Its value MAY be used as a secondary key for selecting Links which
   share the same relation type.





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5.6.  profile

   The "profile" attribute is OPTIONAL.

   Its value is a string which is a URI that hints about the profile (as
   defined by [RFC6906]) of the target resource.

5.7.  title

   The "title" attribute is OPTIONAL.

   Its value is a string and is intended for labeling the link with a
   human-readable identifier (as defined by [RFC5988]).

5.8.  hreflang

   The "hreflang" attribute is OPTIONAL.

   Its value is a string and is intended for indicating the language of
   the target resource (as defined by [RFC5988]).































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6.  Example

   The following is an example representing a list of orders

   GET /orders HTTP/1.1
   Host: example.org
   Accept: application/hal+xml

   HTTP/1.1 200 OK
   Content-Type: application/hal+xml

   <resource rel="self" href="/orders">
       <link rel="next" href="/orders?page=2"/>
       <link rel="find" href="/orders/{?id}" templated="true"/>
       <resource rel="order" href="/orders/123">
           <link rel="basket" href="/baskets/98712"/>
           <link rel="customer" href="/customers/7809"/>
           <total>30.00</total>
           <currency>USD</currency>
           <status>shipped</status>
       </resource>
       <resource rel="order" href="/orders/124">
           <link rel="basket" href="/baskets/97213"/>
           <link rel="customer" href="/customers/12369"/>
           <total>20.00</total>
           <currency>USD</currency>
           <status>processing</status>
       </resource>
       <currentlyProcessing>14</currentlyProcessing>
       <shippedToday>20</shippedToday>
   </resource>

   Here, the order list document provides a "next" link directing to the
   next page, and a "find" link containing a URI Template which can be
   expanded with an 'id' variable to go directly to a specific order.

   It also has two embedded resources, "order".  Each of these has its
   own links to the associated "basket" and "customer" resources, and
   properties showing their "total", "currency" and "status".

   Additionally, the order list resource has its own properties
   "currentlyProcessing" and "shippedToday".









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7.  Media Type Parameters

7.1.  profile

   The media type identifier application/hal+xml MAY also include an
   additional "profile" parameter (as defined by [RFC6906])

   HAL documents that are served with the "profile" parameter still
   SHOULD include a "profile" link belonging to the root resource.

8.  Recommendations

8.1.  Self Link

   Each Resource SHOULD have a 'self' link that corresponds with the
   IANA registered 'self' relation (as defined by [RFC5988]) and whose
   target is the resource's URI.

8.2.  Link relations

   Custom link relation types (Extension Relation Types in [RFC5988])
   SHOULD be URIs that when dereferenced in a web browser provide
   relevant documentation, in the form of an HTML page, about the
   meaning and/or behavior of the target Resource.  This will improve
   the discoverability of the API.

   The CURIE Syntax [W3C.NOTE-curie-20101216] MAY be used for brevity
   for these URIs.  CURIEs are established within a HAL document via XML
   namespace on the root Resource.

   <resource rel="self" href="/orders" xmlns:acme="http://a.com/rels/">
     <link rel="acme:widgets" href="/widgets"/>
   </resource>

   The above demonstrates the relation "http://a.com/rels/widgets" being
   abbreviated to "acme:widgets" via CURIE syntax.

8.3.  Hypertext Cache Pattern

   The "hypertext cache pattern" allows servers to use embedded
   resources to dynamically reduce the number of requests a client
   makes, improving the efficiency and performance of the application.

   Clients MAY be automated for this purpose so that, for any given link
   relation, they will read from an embedded resource (if present) in
   preference to traversing a link.

   To activate this client behavior for a given link, servers SHOULD add



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   an embedded resource into the representation with the same relation.

   Servers SHOULD NOT entirely "swap out" a link for an embedded
   resource (or vice versa) because client support for this technique is
   OPTIONAL.

   The following examples shows the hypertext cache pattern applied to
   an "author" link:

   Before:

   <resource rel="self" href="/books/the-way-of-zen">
       <link rel="author" href="/people/alan-watts"/>
   </resource>

   After:

   <resource rel="self" href="/books/the-way-of-zen">
       <link rel="author" href="/people/alan-watts"/>
       <resource rel="author" href="/people/alan-watts">
           <name>Alan Watts</name>
           <born>January 6, 1915</born>
           <died>November 16, 1973</died>
       <resource>
   </resource>

8.4.  Namespace and prefix

   The default namespace of "http://stateless.co/hal/ns" SHOULD be used
   to allow for mixed, and potentially conflicting, xml vocabularies to
   coexist in the same xml instance (e.g. embedding an
   application/hal+xml payload in another xml).

   If a namespace prefix is needed, "hal" MAY be used by default.

9.  Security Considerations

   TBD

10.  IANA Considerations

   TBD

11.  Normative References

   [RFC6906]  Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
              March 2013.




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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [W3C.REC-xml-20081126]

              Bray, T., Paoli, J., Sperberg-McQueen, C. M., Maler, E.,
              Yergeau, F., "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium REC REC-xml-20081126,
              November 2008, <http://www.w3.org/TR/2008/REC-xml-
              20081126/>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570, March 2012.

   [W3C.NOTE-curie-20101216]

              McCarron, S. and M. Birbeck, "CURIE Syntax 1.0", World
              Wide Web Consortium NOTE NOTE-curie-20101216, December
              2010, <http://www.w3.org/TR/2010/NOTE-curie-20101216>.

   [W3C.REC-xmlschema-2-20041028]

              Biron, P. V., Malhotra, A., "XML Schema Part 2: Datatypes
              Second Edition", World Wide Web Consortium REC REC-
              xmlschema-2-20041028, October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/>.


Appendix A.  Acknowledgements

   Thanks to Mike Kelly for making this specification possible through
   the creation of HAL and the hal+json specification.

   I would also like to thank folks on the Hal Discuss Google group for
   providing feedback for the draft.

   The author takes all responsibility for errors and omissions.








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Appendix B.  Frequently Asked Questions

B.1.  How should a client know the meaning/structure/semantics/type of a
   resource?

   There are two main approaches to solving this problem.  Both involve
   exposing additional documentation describing the resource which may
   be human and/or machine readable (e.g.  an HTML page, an XML Schema,
   an ALPS profile, etc.).  The difference between the two approaches is
   in where that URI is shared with the client, which is either:

   (1)  The relation documentation associated to a Link relation type.

   (2)  A 'profile' (via a Link's profile attribute or via a Link with a
   'profile' relation type).

B.2.  Where can I find libraries for working with HAL?

   A list of libraries is maintained here:
   http://stateless.co/hal_specification.html

B.3.  Why does HAL have no forms?

   Omitting forms from HAL was an intentional design decision that was
   made to keep it focused on linking for APIs.  HAL is therefore a good
   candidate for use as a base media type on which to build more complex
   capabilities.

Author's Address

   Jeff Michaud

   Email: cometaj2@comcast.net
   Twitter: @cometaj2

















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