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Network Working Group                                      M. Nottingham
Internet-Draft                                         February 15, 2017
Intended status: Informational
Expires: August 19, 2017


                      Home Documents for HTTP APIs
                     draft-nottingham-json-home-06

Abstract

   This document proposes a "home document" format for non-browser HTTP
   clients.

Note to Readers

   The issues list for this draft can be found at
   https://github.com/mnot/I-D/labels/json-home .

   The most recent (often, unpublished) draft is at
   https://mnot.github.io/I-D/json-home/ .

   Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
   pages/json-home .

   For information about implementations, see https://github.com/mnot/I-
   D/wiki/json-home .

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 August 19, 2017.







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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  API Home Documents  . . . . . . . . . . . . . . . . . . . . .   4
   3.  API Objects . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Resource Objects  . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Resolving Templated Links . . . . . . . . . . . . . . . .   8
   5.  Resource Hints  . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  allow . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  formats . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  acceptPatch . . . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  acceptPost  . . . . . . . . . . . . . . . . . . . . . . .  10
     5.5.  acceptPut . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.6.  acceptRanges  . . . . . . . . . . . . . . . . . . . . . .  10
     5.7.  acceptPrefer  . . . . . . . . . . . . . . . . . . . . . .  11
     5.8.  docs  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.9.  preconditionRequired  . . . . . . . . . . . . . . . . . .  11
     5.10. authSchemes . . . . . . . . . . . . . . . . . . . . . . .  11
     5.11. status  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  HTTP Resource Hint Registry . . . . . . . . . . . . . . .  12
     7.2.  Media Type Registration . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  15
   Appendix B.  Creating and Serving Home Documents  . . . . . . . .  15
     B.1.  Managing Change in Home Documents . . . . . . . . . . . .  15
     B.2.  Evolving and Mixing APIs with Home Documents  . . . . . .  16
   Appendix C.  Consuming Home Documents . . . . . . . . . . . . . .  16
   Appendix D.  Frequently Asked Questions . . . . . . . . . . . . .  17



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     D.1.  Why not use (insert other service description format)?  .  17
     D.2.  Why doesn't the format allow references or inheritance? .  17
     D.3.  What about "Faults" (i.e., errors)? . . . . . . . . . . .  17
     D.4.  How Do I find the schema for a format?  . . . . . . . . .  17
     D.5.  How do I express complex query arguments? . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   It is becoming increasingly common to use HTTP [RFC7230] for
   applications other than traditional Web browsing.  Such "HTTP APIs"
   are used to integrate processes on disparate systems, make
   information available to machines across the Internet, and as part of
   the implementation of "micro-services."

   By using HTTP, these applications realise a number of benefits, from
   message framing to caching, and well-defined semantics that are
   broadly understood and useful.

   Often, these applications of HTTP are defined by documenting static
   URLs that clients need to know and servers need to implement.  Any
   interaction outside of these bounds is uncharted territory.

   For some applications, this approach brings issues, especially when
   the interface changes, either due to evolution, extension or drift
   between implementations.  Furthermore, implementing more than one
   instance of interface can bring further issues, as different
   environments have different requirements.

   The Web itself offers one way to address these issues, using links
   [RFC3986] to navigate between states.  A link-driven application
   discovers relevant resources at run time, using a shared vocabulary
   of link relations [RFC5988] and internet media types [RFC6838] to
   support a "follow your nose" style of interaction - just as a Web
   browser does to navigate the Web.

   A client can then decide which resources to interact with "on the
   fly" based upon its capabilities (as described by link relations),
   and the server can safely add new resources and formats without
   disturbing clients that are not yet aware of them.

   Doing so can provide any of a number of benefits, including:

   o  Extensibility - Because new server capabilities can be expressed
      as link relations, new features can be layered in without
      introducing a new API version; clients will discover them in the
      home document.  This promotes loose coupling between clients and
      servers.



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   o  Evolvability - Likewise, interfaces can change gradually by
      introducing a new link relation and/or format while still
      supporting the old ones.

   o  Customisation - Home documents can be tailored for the client,
      allowing diffrent classes of service or different client
      permissions to be exposed naturally.

   o  Flexible deployment - Since URLs aren't baked into documentation,
      the server can choose what URLs to use for a given service.

   o  API mixing - Likewise, more than one API can be deployed on a
      given server, without fear of collisions.

   Whether an application ought to use links in this fashion depends on
   how it is deployed; generally, the most benefit will be received when
   multiple instances of the service are deployed, possibly with
   different versions, and they are consumed by clients with different
   capabilities.  In particular, Internet Standards that use HTTP as a
   substrate are likely to require the attributes described above.

   This document defines a "home page" format using the JSON format
   [RFC7159] for APIs to use as a launching point for the interactions
   they offer, using links.  Having a well-defined format for this
   purpose promotes good practice and tooling.

1.1.  Notational Conventions

   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].

2.  API Home Documents

   An API Home Document (or, interchangeably, "home document") uses the
   format described in [RFC7159] and has the media type "application/
   json-home".

   *Note: this media type is not final, and will change before final
   publication.*

   Its content consists of a root object with:

   o  A "resources" member, whose value is an object that describes the
      resources associated with the API.  Its member names are link
      relation types (as defined by [RFC5988]), and their values are
      Resource Objects (Section 4).




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   o  Optionally, a "api" member, whose value is an API Object
      (Section 3) that contains information about the API as a whole.

   For example:

     GET / HTTP/1.1
     Host: example.org
     Accept: application/json-home

     HTTP/1.1 200 OK
     Content-Type: application/json-home
     Cache-Control: max-age=3600
     Connection: close

     {
       "api": {
         "title": "Example API",
         "links": {
           "author": "mailto:api-admin@example.com",
           "describedBy": "https://example.com/api-docs/"
         }
       }
       "resources": {
         "tag:me@example.com,2016:widgets": {
           "href": "/widgets/"
         },
         "tag:me@example.com,2016:widget": {
           "hrefTemplate": "/widgets/{widget_id}",
           "hrefVars": {
             "widget_id": "https://example.org/param/widget"
           },
           "hints": {
             "allow": ["GET", "PUT", "DELETE", "PATCH"],
             "formats": {
               "application/json": {}
             },
             "acceptPatch": ["application/json-patch+json"],
             "acceptRanges": ["bytes"]
           }
         }
       }
     }

   Here, we have a home document for the API "Example API", whose author
   can be contacted at the e-mail address "api-admin@example.com", and
   whose documentation is at "https://example.com/api-docs/".





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   It links to a resource "/widgets/" with the relation
   "tag:me@example.com,2016:widgets".  It also links to an unknown
   number of resources with the relation type
   "tag:me@example.com,2016:widget" using a URI Template [RFC6570],
   along with a mapping of identifiers to a variable for use in that
   template.

   It also gives several hints about interacting with the latter
   "widget" resources, including the HTTP methods usable with them, the
   PATCH and POST formats they accept, and the fact that they support
   partial requests [RFC7233] using the "bytes" range-specifier.

   It gives no such hints about the "widgets" resource.  This does not
   mean that it (for example) doesn't support any HTTP methods; it means
   that the client will need to discover this by interacting with the
   resource, and/or examining the documentation for its link relation
   type.

   Effectively, this names a set of behaviors, as described by a
   resource object, with a link relation type.  This means that several
   link relations might apply to a common base URL; e.g.:

   {
     "resources": {
       "tag:me@example.com,2016:search-by-id": {
         "hrefTemplate": "/search?id={widget_id}",
         "hrefVars": {
           "widget_id": "https://example.org/param/widget_id"
         }
       },
       "tag:me@example.com,2016:search-by-name": {
         "hrefTemplate": "/search?name={widget_name}",
         "hrefVars": {
           "widget_name": "https://example.org/param/widget_name"
         }
       }
     }
   }

   Note that the examples above use both tag [RFC4151] and https
   [RFC7230] URIs; any URI scheme can be used to identify link relations
   and other artefacts in home documents.  Typically, these are not
   links to be followed; they are only used to identify things.








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3.  API Objects

   An API Object contains links to information about the API itself.

   Two optional members are defined:

   o  "title" has a string value indicating the name of the API;

   o  "links" has an object value, whose member names are link relation
      types [RFC5988], and values are URLs [RFC3986].  The context of
      these links is the API home document as a whole.

   No links are required to be conveyed, but APIs might benefit from
   setting the following:

   o  author - a suitable URL (e.g., mailto: or https:) for the
      author(s) of the API

   o  describedBy - a link to documentation for the API

   o  license - a link to the legal terms for using the API

   Future members of the API Object MAY be defined by specifications
   that update this document.

4.  Resource Objects

   A Resource Object links to resources of the type indicated in their
   name using one of two mechanisms; either a direct link (in which case
   there is exactly one resource of that relation type associated with
   the API), or a templated link, in which case there are zero to many
   such resources.

   Direct links are indicated with an "href" property, whose value is a
   URI [RFC3986].

   Templated links are indicated with an "hrefTemplate" property, whose
   value is a URI Template [RFC6570].  When "hrefTemplate" is present,
   the Resource Object MUST have a "hrefVars" property; see "Resolving
   Templated Links".

   Resource Objects MUST have exactly one of the "href" or "href-vars"
   properties.

   In both forms, the links that "href" and "hrefTemplate" refer to are
   URI-references [RFC3986] whose base URI is that of the API Home
   Document itself.




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   Resource Objects MAY also have a "hints" property, whose value is an
   object that uses named Resource Hints (see Section 5) as its
   properties.

4.1.  Resolving Templated Links

   A URI can be derived from a Templated Link by treating the
   "hrefTemplate" value as a Level 3 URI Template [RFC6570], using the
   "hrefVars" property to fill the template.

   The "hrefVars" property, in turn, is an object that acts as a mapping
   between variable names available to the template and absolute URIs
   that are used as global identifiers for the semantics and syntax of
   those variables.

   For example, given the following Resource Object:

     "https://example.org/rel/widget": {
       "hrefTemplate": "/widgets/{widget_id}",
       "hrefVars": {
         "widget_id": "https://example.org/param/widget"
       },
       "hints": {
         "allow": ["GET", "PUT", "DELETE", "PATCH"],
         "formats": {
           "application/json": {}
         },
         "acceptPatch": ["application/json-patch+json"],
         "acceptRanges": ["bytes"]
       }
     }

   If you understand that "https://example.org/param/widget" is an
   numeric identifier for a widget, you can then find the resource
   corresponding to widget number 12345 at "https://example.org/
   widgets/12345" (assuming that the Home Document is located at
   "https://example.org/").

5.  Resource Hints

   Resource hints allow clients to find relevant information about
   interacting with a resource beforehand, as a means of optimizing
   communications, as well as advertising available behaviors (e.g., to
   aid in laying out a user interface for consuming the API).

   Hints are just that - they are not a "contract", and are to only be
   taken as advisory.  The runtime behavior of the resource always
   overrides hinted information.



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   For example, a resource might hint that the PUT method is allowed on
   all "widget" resources.  This means that generally, the user has the
   ability to PUT to a particular resource, but a specific resource
   might reject a PUT based upon access control or other considerations.
   More fine-grained information might be gathered by interacting with
   the resource (e.g., via a GET), or by another resource "containing"
   it (such as a "widgets" collection) or describing it (e.g., one
   linked to it with a "describedBy" link relation).

   This specification defines a set of common hints, based upon
   information that's discoverable by directly interacting with
   resources.  See Section 7.1 for information on defining new hints.

5.1.  allow

   o  Resource Hint Name: allow

   o  Description: Hints the HTTP methods that the current client will
      be able to use to interact with the resource; equivalent to the
      Allow HTTP response header.

   o  Specification: [this document]

   Content MUST be an array of strings, containing HTTP methods.  As per
   HTTP, when GET is supported, a client MAY assume that HEAD is
   supported.

5.2.  formats

   o  Resource Hint Name: formats

   o  Description: Hints the representation types that the resource
      makes available, using the GET method.

   o  Specification: [this document]

   Content MUST be an object, whose keys are media types, and values are
   objects, currently empty.

5.3.  acceptPatch

   o  Resource Hint Name: accept-Patch

   o  Description: Hints the PATCH [RFC5789] request formats accepted by
      the resource for this client; equivalent to the Accept-Patch HTTP
      response header.

   o  Specification: [this document]



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   Content MUST be an array of strings, containing media types.

   When this hint is present, "PATCH" SHOULD be listed in the "allow"
   hint.

5.4.  acceptPost

   o  Resource Hint Name: acceptPost

   o  Description: Hints the POST request formats accepted by the
      resource for this client.

   o  Specification: [this document]

   Content MUST be an array of strings, containing media types.

   When this hint is present, "POST" SHOULD be listed in the "allow"
   hint.

5.5.  acceptPut

   o  Resource Hint Name: acceptPut

   o  Description: Hints the PUT request formats accepted by the
      resource for this client.

   o  Specification: [this document]

   Content MUST be an array of strings, containing media types.

   When this hint is present, "PUT" SHOULD be listed in the "allow"
   hint.

5.6.  acceptRanges

   o  Resource Hint Name: acceptRanges

   o  Description: Hints the range-specifiers available to the client
      for this resource; equivalent to the Accept-Ranges HTTP response
      header [RFC7233].

   o  Specification: [this document]

   Content MUST be an array of strings, containing HTTP range-specifiers
   (typically, "bytes").






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5.7.  acceptPrefer

   o  Resource Hint Name: acceptPrefer

   o  Description: Hints the preferences [RFC7240] supported by the
      resource.  Note that, as per that specifications, a preference can
      be ignored by the server.

   o  Specification: [this document]

   Content MUST be an array of strings, containing preferences.

5.8.  docs

   o  Resource Hint Name: docs

   o  Description: Hints the location for human-readable documentation
      for the relation type of the resource.

   o  Specification: [this document]

   Content MUST be a string containing an absolute-URI [RFC3986]
   referring to documentation that SHOULD be in HTML format.

5.9.  preconditionRequired

   o  Resource Hint Name: preconditionRequired

   o  Description: Hints that the resource requires state-changing
      requests (e.g., PUT, PATCH) to include a precondition, as per
      [RFC7232], to avoid conflicts due to concurrent updates.

   o  Specification: [this document]

   Content MUST be an array of strings, with possible values "etag" and
   "last-modified" indicating type of precondition expected.

5.10.  authSchemes

   o  Resource Hint Name: authSchemes

   o  Description: Hints that the resource requires authentication using
      the HTTP Authentication Framework [RFC7235].

   o  Specification: [this document]

   Content MUST be an array of objects, each with a "scheme" property
   containing a string that corresponds to a HTTP authentication scheme,



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   and optionally a "realms" property containing an array of zero to
   many strings that identify protection spaces that the resource is a
   member of.

   For example, a Resource Object might contain the following hint:

     {
       "authSchemes": [
         {
           "scheme": "Basic",
           "realms": ["private"]
         }
       ]
     }

5.11.  status

   o  Resource Hint Name: status

   o  Description: Hints the status of the resource.

   o  Specification: [this document]

   Content MUST be a string; possible values are:

   o  "deprecated" - indicates that use of the resource is not
      recommended, but it is still available.

   o  "gone" - indicates that the resource is no longer available; i.e.,
      it will return a 404 (Not Found) or 410 (Gone) HTTP status code if
      accessed.

6.  Security Considerations

   Clients need to exercise care when using hints.  For example, a naive
   client might send credentials to a server that uses the auth-req
   hint, without checking to see if those credentials are appropriate
   for that server.

7.  IANA Considerations

7.1.  HTTP Resource Hint Registry

   This specification defines the HTTP Resource Hint Registry.  See
   Section 5 for a general description of the function of resource
   hints.





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   In particular, resource hints are generic; that is, they are
   potentially applicable to any resource, not specific to one
   application of HTTP, nor to one particular format.  Generally, they
   ought to be information that would otherwise be discoverable by
   interacting with the resource.

   Hint names MUST be composed of the lowercase letters (a-z), digits
   (0-9), underscores ("_") and hyphens ("-"), and MUST begin with a
   lowercase letter.

   Hint content SHOULD be described in terms of JSON [RFC7159]
   constructs.

   New hints are registered using the Expert Review process described in
   [RFC5226] to enforce the criteria above.  Requests for registration
   of new resource hints are to use the following template:

   o  Resource Hint Name: [hint name]

   o  Description: [a short description of the hint's semantics]

   o  Specification: [reference to specification document]

   Initial registrations are enumerated in Section 5.

7.2.  Media Type Registration

   TBD

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.




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   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <http://www.rfc-editor.org/info/rfc6570>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <http://www.rfc-editor.org/info/rfc7234>.

8.2.  Informative References

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, DOI 10.17487/RFC4151, October 2005,
              <http://www.rfc-editor.org/info/rfc4151>.

   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, DOI 10.17487/RFC5789, March 2010,
              <http://www.rfc-editor.org/info/rfc5789>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <http://www.rfc-editor.org/info/rfc7232>.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,
              <http://www.rfc-editor.org/info/rfc7233>.




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   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

   [RFC7240]  Snell, J., "Prefer Header for HTTP", RFC 7240,
              DOI 10.17487/RFC7240, June 2014,
              <http://www.rfc-editor.org/info/rfc7240>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <http://www.rfc-editor.org/info/rfc7807>.

Appendix A.  Acknowledgements

   Thanks to Jan Algermissen, Mike Amundsen, Bill Burke, Sven Dietze,
   Graham Klyne, Leif Hedstrom, Joe Hildebrand, Jeni Tennison, Erik
   Wilde and Jorge Williams for their suggestions and feedback.

Appendix B.  Creating and Serving Home Documents

   When making an API home document available, there are a few things to
   keep in mind:

   o  A home document is best located at a memorable URI, because its
      URI will effectively become the URI for the API itself to clients.

   o  Home documents can be personalized, just as "normal" home pages
      can.  For example, you might advertise different URIs, and/or
      different kinds of link relations, depending on the client's
      identity.

   o  Home documents ought to be assigned a freshness lifetime (e.g.,
      "Cache-Control: max-age=3600") so that clients can cache them, to
      avoid having to fetch it every time the client interacts with the
      service.

   o  Custom link relation types, as well as the URIs for variables,
      should lead to documentation for those constructs.

B.1.  Managing Change in Home Documents

   The URIs used in API home documents MAY change over time.  However,
   changing them can cause issues for clients that are relying on cached
   home documents containing old links.

   To mitigate the impact of such changes, servers ought to consider:




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   o  Reducing the freshness lifetime of home documents before a link
      change, so that clients are less likely to refer to an "old"
      document.

   o  Regarding the "old" and "new" URIs as equally valid references for
      an "overlap" period.

   o  After that period, handling requests for the "old" URIs
      appropriately; e.g., with a 404 Not Found, or by redirecting the
      client to the new URI.

B.2.  Evolving and Mixing APIs with Home Documents

   Using home documents affords the opportunity to change the "shape" of
   the API over time, without breaking old clients.

   This includes introducing new functions alongside the old ones - by
   adding new link relation types with corresponding resource objects -
   as well as adding new template variables, media types, and so on.

   It's important to realise that a home document can serve more than
   one "API" at a time; by listing all relevant relation types, it can
   effectively "mix" different APIs, allowing clients to work with
   different resources as they see fit.

Appendix C.  Consuming Home Documents

   Clients might use home documents in a variety of ways.

   In the most common case - actually consuming the API - the client
   will scan the Resources Object for the link relation(s) that it is
   interested in, and then to interact with the resource(s) referred to.
   Resource Hints can be used to optimize communication with the client,
   as well as to inform as to the permissible actions (e.g., whether PUT
   is likely to be supported).

   Note that the home document is a "living" document; it does not
   represent a "contract", but rather is expected to be inspected before
   each interaction.  In particular, links from the home document MUST
   NOT be assumed to be valid beyond the freshness lifetime of the home
   document, as per HTTP's caching model [RFC7234].

   As a result, clients ought to cache the home document (as per
   [RFC7234]), to avoid fetching it before every interaction (which
   would otherwise be required).






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   Likewise, a client encountering a 404 (Not Found) on a link is
   encouraged obtain a fresh copy of the home document, to assure that
   it is up-to-date.

Appendix D.  Frequently Asked Questions

D.1.  Why not use (insert other service description format)?

   There are a fair number of existing service description formats,
   including those that specialise in "RESTful" interactions.  However,
   these formats generally are optimised for pairwise integration, or
   one-server-to-many-client integration, and less capable of describing
   protocols where both the server and client can evolve and be
   extended.

D.2.  Why doesn't the format allow references or inheritance?

   Adding inheritance or references would allow more modularity in the
   format and make it more compact, at the cost of considerable
   complexity and the associated potential for errors (both in the
   specification and by its users).

   Since good tools and compression are effective ways to achieve the
   same ends, this specification doesn't attempt them.

D.3.  What about "Faults" (i.e., errors)?

   In HTTP, errors are conveyed by HTTP status codes.  While this
   specification could (and even may) allow enumeration of possible
   error conditions, there's a concern that this will encourage
   applications to define many such "faults", leading to tight coupling
   between the application and its clients.  See [RFC7807] for further
   considerations.

D.4.  How Do I find the schema for a format?

   That isn't addressed by home documents.  Ultimately, it's up to the
   media type accepted and generated by resources to define and
   constrain (or not) their syntax.

D.5.  How do I express complex query arguments?

   Complex queries - i.e., those that exceed the expressive power of
   Link Templates or would require ambiguous properties of a "resources"
   object - aren't intended to be defined by a home document.  The
   appropriate way to do this is with a "form" language, much as HTML
   defines.




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   Note that it is possible to support multiple query syntaxes on the
   same base URL, using more than one link relation type; see the
   example at the start of the document.

Author's Address

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/









































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