Network Working Group M. Nottingham
Internet-Draft Rackspace
Intended status: Informational July 5, 2012
Expires: January 6, 2013
Home Documents for HTTP APIs
draft-nottingham-json-home-01
Abstract
This document proposes a "home document" format for non-browser HTTP
clients.
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 January 6, 2013.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. JSON Home Documents . . . . . . . . . . . . . . . . . . . . . 3
4. Resource Objects . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Resolving Templated Links . . . . . . . . . . . . . . . . 5
5. Resource Hints . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. allow . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. representations . . . . . . . . . . . . . . . . . . . . . 7
5.3. accept-patch . . . . . . . . . . . . . . . . . . . . . . . 7
5.4. accept-post . . . . . . . . . . . . . . . . . . . . . . . 7
5.5. accept-put . . . . . . . . . . . . . . . . . . . . . . . . 7
5.6. accept-ranges . . . . . . . . . . . . . . . . . . . . . . 8
5.7. prefer . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.8. docs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.9. precondition-req . . . . . . . . . . . . . . . . . . . . . 8
5.10. status . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Creating and Serving Home Documents . . . . . . . . . . . . . 8
6.1. Managing Change in Home Documents . . . . . . . . . . . . 9
6.2. Evolving and Mixing APIs with Home Documents . . . . . . . 9
6.3. Documenting APIs that use Home Documents . . . . . . . . . 10
7. Consuming Home Documents . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Appendix B. Frequently Asked Questions . . . . . . . . . . . . . 12
B.1. Why not Microformats? . . . . . . . . . . . . . . . . . . 12
B.2. What about authentication? . . . . . . . . . . . . . . . . 12
B.3. What about 'Faults' (i.e., errors)? . . . . . . . . . . . 12
B.4. How Do I find the XML Schema / JSON Schema / etc. for
a particular media type? . . . . . . . . . . . . . . . . . 13
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
There is an emerging preference for non-browser Web applications
(colloquially, "HTTP APIs") to use a link-driven approach to their
interactions to assure loose coupling, thereby enabling extensibility
and API evolution.
This is based upon experience with previous APIs that specified
static URI paths (such as
"http://api.example.com/v1.0/widgets/abc123/properties") have
resulted in brittle, tight coupling between clients and servers.
Sometimes, these APIs were documented by a document format like
WADL [1] that is used as a design time description; i.e., the URIs
and other information they describe are "baked into" client
implementations.
In contrast, a "follow your nose" API advertises the resources
available to clients using link relations [RFC5988] and the formats
they support using internet media types [RFC4288]. A client can then
decide - at run time - which resources to interact with 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.
As such, the client needs to be able to discover this information
quickly and efficiently use it to interact with the server. Just as
with a human-targeted home page for a site, we can create 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 standard home document format. This
specification suggests one for consideration, using the JSON format
[RFC4627].
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. JSON Home Documents
A JSON Home Document uses the format described in [RFC4627] and has
the media type "application/json-home".
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Its content consists of a root object with a "resources" property,
whose names are link relation types (as defined by [RFC5988]), and
values are Resource Objects, defined below.
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
{
"resources": {
"http://example.org/rel/widgets": {
"href": "/widgets/"
},
"http://example.org/rel/widget": {
"href-template": "/widgets/{widget_id}",
"href-vars": {
"widget_id": "http://example.org/param/widget"
},
"hints": {
"allow": ["GET", "PUT", "DELETE", "PATCH"],
"representations": ["application/json"],
"accept-patch": ["application/json-patch"],
"accept-post": ["application/xml"],
"accept-ranges": ["bytes"]
}
}
}
}
Here, we have a home document that links to a resource, "/widgets/"
with the relation "http://example.org/rel/widgets". It also links to
an undefined number of resources with the relation type
"http://example.org/rel/widget" using a URI Template [RFC6570], along
with a mapping of several identifiers to specific variables 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 formats they accept, and the fact that they support partial
requests [I-D.ietf-httpbis-p5-range] using the "bytes" range-
specifier.
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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.
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.
Resource Objects MUST have only and exactly one of the "href" and
"href-template" properties.
Direct links are indicated with an "href" property, whose value is a
URI [RFC3986].
Templated links are indicated with an "href-template" property, whose
value is a URI Template [RFC6570]. When "href-template" is present,
the Resource Object MUST have a "href-vars" property; see "Resolving
Templated Links".
In both forms, the links that "href" and "href-template" refer to are
URI-references [RFC3986] whose base URI is that of the JSON Home
Document itself.
Resource Objects MAY also have a "hints" property, whose value is an
object that uses named Resource Hints as its properties.
4.1. Resolving Templated Links
A URI can be derived from a Templated Link by treating the "href-
template" value as a Level 3 URI Template [RFC6570], using the "href-
vars" property to fill the template.
The "href-vars" 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:
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"http://example.org/rel/widget": {
"href-template": "/widgets/{widget_id}",
"href-vars": {
"widget_id": "http://example.org/param/widget"
},
"hints": {
"allow": ["GET", "PUT", "DELETE", "PATCH"],
"representations": ["application/json"],
"accept-patch": ["application/json-patch"],
"accept-post": ["application/xml"],
"accept-ranges": ["bytes"]
}
}
If you understand that "http://example.org/param/widget" is an
numeric identifier for a widget (perhaps by dereferencing that URL
and reading the documentation), you can then find the resource
corresponding to widget number 12345 at
"http://example.org/widgets/12345" (assuming that the Home Document
is located at "http://example.org/").
5. Resource Hints
Resource hints allow clients to find relevant information about
interacting with a resource beforehand, as a means of optimising
communications, as well as advertising available behaviours (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 behaviour 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
could 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).
This specification defines a set of common hints, based upon
information that's discoverable by directly interacting with
resources. A future draft will explain how to define new hints.
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5.1. allow
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.
Content MUST be an array of strings, containing HTTP methods.
5.2. representations
Hints the representation types that the resource produces and
consumes, using the GET and PUT methods respectively, subject to the
'allow' hint.
Content MUST be an array of strings, containing media types.
5.3. accept-patch
Hints the PATCH request formats [RFC5789] accepted by the resource
for this client; equivalent to the Accept-Patch HTTP response header.
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. accept-post
Hints the POST request formats accepted by the resource for this
client.
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. accept-put
Hints the PUT request formats accepted by the resource for this
client.
Content MUST be an array of strings, containing media types. If
absent, a client MAY assume that any format indicated by the
'representations' hint is acceptable in a PUT.
When this hint is present, "PUT" SHOULD be listed in the "allow"
hint.
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5.6. accept-ranges
Hints the range-specifiers available to the client for this resource;
equivalent to the Accept-Ranges HTTP response header
[I-D.ietf-httpbis-p5-range].
Content MUST be an array of strings, containing HTTP range-
specifiers.
5.7. prefer
Hints the preferences [I-D.snell-http-prefer] supported by the
resource. Note that, as per that specifications, a preference can be
ignored by the server.
Content MUST be an array of strings, contain preferences.
5.8. docs
Hints the location for human-readable documentation for the relation
type of the resource.
Content MUST be a string containing an absolute-URI [RFC3986]
referring to documentation that SHOULD be in HTML format.
5.9. precondition-req
Hints that the resource requires state-changing requests (e.g., PUT,
PATCH) to include a precondition, as per
[I-D.ietf-httpbis-p4-conditional], to avoid conflicts due to
concurrent updates.
Content MUST be an array of strings, with possible values "etag" and
"last-modified" indicating type type of precondition expected.
5.10. status
Hints the status of the resource.
Content MUST be a string; possible values are:
o "deprecated" - indicates that use of the resource is not
recommended, but it can still be used.
6. Creating and Serving Home Documents
When making a home document available, there are a few things to keep
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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 personalised, 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 SHOULD 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.
6.1. Managing Change in Home Documents
The URIs used in 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 these risks, servers changing links SHOULD 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 Assure that they handle requests for the "old" URIs appropriately;
e.g., with a 404 Not Found, or by redirecting the client to the
new URI.
o Alternatively, considering the "old" and "new" URIs as equally
valid references for an "overlap" period.
Generally, servers ought not to change URIs without good cause.
6.2. 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.
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6.3. Documenting APIs that use Home Documents
Another use case for "static" API description formats like WSDL and
WADL is to generate documentation for the API from them.
An API that uses the home document format correctly won't have a need
to do so, provided that the link relation types and media types it
uses are well-documented already.
7. 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 optimise 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 [I-D.ietf-httpbis-p6-cache].
As a result, clients SHOULD cache the home document (as per
[I-D.ietf-httpbis-p6-cache]), to avoid fetching it before every
interaction (which would otherwise be required).
Likewise, a client encountering a 404 Not Found on a link SHOULD
obtain a fresh copy of the home document, to assure that it is up-to-
date.
8. Security Considerations
TBD
9. IANA Considerations
TBD
10. References
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10.1. Normative References
[I-D.ietf-httpbis-p6-cache]
Fielding, R., Lafon, Y., Nottingham, M., and J. Reschke,
"HTTP/1.1, part 6: Caching",
draft-ietf-httpbis-p6-cache-19 (work in progress),
March 2012.
[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.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[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.
10.2. Informative References
[I-D.ietf-httpbis-p4-conditional]
Fielding, R., Lafon, Y., and J. Reschke, "HTTP/1.1, part
4: Conditional Requests",
draft-ietf-httpbis-p4-conditional-19 (work in progress),
March 2012.
[I-D.ietf-httpbis-p5-range]
Fielding, R., Lafon, Y., and J. Reschke, "HTTP/1.1, part
5: Range Requests and Partial Responses",
draft-ietf-httpbis-p5-range-19 (work in progress),
March 2012.
[I-D.snell-http-prefer]
Snell, J., "Prefer Header for HTTP",
draft-snell-http-prefer-12 (work in progress),
February 2012.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, March 2010.
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URIs
[1]
[2]
Appendix A. Acknowledgements
Thanks to Mike Amundsen, Bill Burke, Graham Klyne, Leif Hedstrom, and
Jeni Tennison for their suggestions and feedback.
Appendix B. Frequently Asked Questions
B.1. Why not Microformats?
Browser-centric Web applications use HTML as their representation
format of choice. While it is possible to augment HTML for non-
browser clients (using techniques like Microformats [2] ), a few
issues become evident when doing so:
o HTML has a very forgiving syntax. While this is appropriate for
browsers (especially considering that there are many million HTML
authors in the world), it makes for a less-than-precise language
for machines, resulting in both overhead (parsing and
transmission) as well as lack of precision.
o HTML is presentation-centric, making it tempting to reformat it
from time to time, to improve the "look and feel" of a page.
However, doing so can cause comparatively brittle non-browser
clients to lose their understanding of the content's semantics,
unless very careful controls are in place.
Because of this, it's most practical to define a separate format, and
JSON is easily machine-readable, precise, and has a better chance of
being managed for stability.
B.2. What about authentication?
In HTTP, authentication is discoverable by interacting with the
resource (usually, by getting a 401 Unauthorized response status
code, along with one or more challenges). While the home document
could hint it, this isn't yet done, to avoid possible security
complications.
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B.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.
So, this is an area of possible future development; if any such
mechanism appears here, it's likely to be quite restricted.
B.4. How Do I find the XML Schema / JSON Schema / etc. for a particular
media type?
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.
Appendix C. Open Issues
The following is a list of placeholders for open issues.
o Refining and extending representation formats - "application/xml",
for example, isn't enough. While a media type for every
representation is one answer, something like 'profile' might be
good too.
o Object for contact details - do we need an object that describes
who's running the API, etc?
o Defining new hints - guidance is needed on minting new hints.
Possibly a registry.
o Defining new top-level properties - how new ones are minted,
registry, etc.
o Defining new Resource Object properties - how new ones are minted,
registry, etc.
Author's Address
Mark Nottingham
Rackspace
Email: mnot@mnot.net
URI: http://www.mnot.net/
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