Network Working Group M. Nottingham
Internet-Draft November 24, 2016
Intended status: Informational
Expires: May 28, 2017
Home Documents for HTTP APIs
draft-nottingham-json-home-05
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The most recent (often, unpublished) draft is at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 May 28, 2017.
Nottingham Expires May 28, 2017 [Page 1]
Internet-Draft Home Documents for HTTP APIs November 2016
Copyright Notice
Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Resource Objects . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Resolving Templated Links . . . . . . . . . . . . . . . . 7
5. Resource Hints . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. allow . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. formats . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. acceptPatch . . . . . . . . . . . . . . . . . . . . . . . 9
5.4. acceptPost . . . . . . . . . . . . . . . . . . . . . . . 9
5.5. acceptRanges . . . . . . . . . . . . . . . . . . . . . . 10
5.6. acceptPrefer . . . . . . . . . . . . . . . . . . . . . . 10
5.7. docs . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.8. preconditionRequired . . . . . . . . . . . . . . . . . . 10
5.9. authSchemes . . . . . . . . . . . . . . . . . . . . . . . 11
5.10. status . . . . . . . . . . . . . . . . . . . . . . . . . 11
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 . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Appendix B. Considerations for Creating and Serving Home
Documents . . . . . . . . . . . . . . . . . . . . . 14
B.1. Managing Change in Home Documents . . . . . . . . . . . . 15
B.2. Evolving and Mixing APIs with Home Documents . . . . . . 15
Appendix C. Considerations for Consuming Home Documents . . . . 16
Appendix D. Frequently Asked Questions . . . . . . . . . . . . . 16
Nottingham Expires May 28, 2017 [Page 2]
Internet-Draft Home Documents for HTTP APIs November 2016
D.1. Why doesn't the format allow references or inheritance? . 16
D.2. What about "Faults" (i.e., errors)? . . . . . . . . . . . 16
D.3. How Do I find the schema for a format? . . . . . . . . . 17
D.4. How do I express complex query arguments? . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
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.
However, one of the core architectural tenants of the Web is the use
of links [RFC3986] to navigate between states; typically, these
applications document static URLs that clients need to know and
servers need to implement, and any interaction outside of these
bounds is uncharted territory.
In contrast, 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.
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.
o Evolvability - Likewise, interfaces can change gradually by
introducing a new link relation and/or format while still
supporting the old ones.
Nottingham Expires May 28, 2017 [Page 3]
Internet-Draft Home Documents for HTTP APIs November 2016
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.
Clients need to be able to discover information about these
applications to use it efficiently; just as with a human-targeted
"home page" for a site, there is a need for a "home document" for a
HTTP API that describes it to non-browser clients.
Of course, an HTTP API might use any format to do so; however, there
are advantages to having a common home document format. This
specification defines one, using the JSON format [RFC7159].
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".
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).
o Optionally, a "api" member, whose value is an API Object
(Section 3) that contains information about the API as a whole.
Nottingham Expires May 28, 2017 [Page 4]
Internet-Draft Home Documents for HTTP APIs November 2016
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/".
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],
Nottingham Expires May 28, 2017 [Page 5]
Internet-Draft Home Documents for HTTP APIs November 2016
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.
3. API Objects
An API Object contains links to information about the API itself.
Two members are defined:
o "title" has a string value indicating the name of the API;
Nottingham Expires May 28, 2017 [Page 6]
Internet-Draft Home Documents for HTTP APIs November 2016
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.
Future members MAY be defined by specifications that update this
document.
4. Resource Objects
A Resource Object links to resources of the defined type 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" and "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.
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:
Nottingham Expires May 28, 2017 [Page 7]
Internet-Draft Home Documents for HTTP APIs November 2016
"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.
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.
Nottingham Expires May 28, 2017 [Page 8]
Internet-Draft Home Documents for HTTP APIs November 2016
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.
5.2. formats
o Resource Hint Name: formats
o Description: Hints the representation types that the resource
produces and consumes, using the GET and PUT methods respectively,
subject to the 'allow' hint.
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]
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]
Nottingham Expires May 28, 2017 [Page 9]
Internet-Draft Home Documents for HTTP APIs November 2016
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. 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").
5.6. 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.
5.7. 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.
5.8. preconditionRequired
o Resource Hint Name: preconditionRequired
Nottingham Expires May 28, 2017 [Page 10]
Internet-Draft Home Documents for HTTP APIs November 2016
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.
5.9. 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,
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"]
}
]
}
5.10. 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.
Nottingham Expires May 28, 2017 [Page 11]
Internet-Draft Home Documents for HTTP APIs November 2016
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.
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.
Nottingham Expires May 28, 2017 [Page 12]
Internet-Draft Home Documents for HTTP APIs November 2016
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>.
[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>.
Nottingham Expires May 28, 2017 [Page 13]
Internet-Draft Home Documents for HTTP APIs November 2016
[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>.
[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>.
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. Considerations for 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.
Nottingham Expires May 28, 2017 [Page 14]
Internet-Draft Home Documents for HTTP APIs November 2016
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:
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.
Nottingham Expires May 28, 2017 [Page 15]
Internet-Draft Home Documents for HTTP APIs November 2016
Appendix C. Considerations for 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).
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 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.2. 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.
Nottingham Expires May 28, 2017 [Page 16]
Internet-Draft Home Documents for HTTP APIs November 2016
D.3. 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.4. 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.
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/
Nottingham Expires May 28, 2017 [Page 17]