Network Working Group R. Sayre
Internet-Draft October 25, 2005
Expires: April 28, 2006
The Atom Publishing Protocol (Basic)
draft-sayre-atompub-protocol-basic-05.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo presents a protocol that uses XML and HTTP to publish and
edit Web resources.
Editorial Note
To provide feedback on this Internet-Draft, join the atom-protocol
mailing list <http://www.imc.org/atom-protocol/index.html>.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. The Atom Publishing Protocol Model . . . . . . . . . . . . . 3
4. APP Feeds . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Media Feeds . . . . . . . . . . . . . . . . . . . . . . . . 7
6. APP Outlines . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . 11
A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 11
B. Change History . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . 13
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1. Introduction
The Atom Publishing Protocol (APP) protocol uses HTTP [RFC2616] and
XML [W3C.REC-xml-20040204] to publish and edit Web resources.
2. Notational Conventions
The APP namespace is "http://purl.org/atom/app#". This specification
refers to it by using the prefix "pub", but that prefix is arbitrary.
The terms 'URI' and 'IRI' are shorthand for the identifiers specified
in [RFC3986] and [RFC3987].
3. The Atom Publishing Protocol Model
The APP uses HTTP to operate on collections of Web resources
represented by Atom feeds [AtomFormat]. This section illustrates the
editing cycle for Atom entries.
o GET is used to retrieve a representation of a resource or perform
a read-only query.
o POST is used to create a new, dynamically-named resource.
o PUT is used to update a known resource.
o DELETE is used to remove a resource.
3.1 Discovery
To discover the location of the feeds exposed by an APP service, the
client must locate and request an APP Outline (Section 6). APP
Outlines describe the layout of an APP service.
Client Server
| |
| 1.) GET Outline URI |
|------------------------------->|
| |
| 2.) Service Outline Doc |
|<-------------------------------|
| |
1. The client sends a GET request to the Service Outline Resource.
2. The server responds with a APP Outline Document containing the
locations of feeds provided by the service. The content of this
document can vary based on aspects of the client request,
including, but not limited to, authentication credentials.
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3.2 Listing
Once the client has discovered the location of a feed in the outline,
it can request a listing of the feed's entries. However, a feed
might contain an extremely large number of entries, so servers are
likely to list a small subset of them by default.
Client Server
| |
| 1.) GET to Atom Feed URI |
|------------------------------->|
| |
| 2.) 200 OK, Atom Feed Doc |
|<-------------------------------|
| |
1. The client sends a GET request to the Atom Feed's URI.
2. The server responds with an Atom Feed Document containing a full
or partial listing of the feed's membership.
3.3 Authoring
After locating a feed, a client can add entries by sending a POST
request to the feed; other changes are accomplished by sending HTTP
requests to each entry.
3.3.1 Create
Client Server
| |
| 1.) POST Entry to Feed URI |
|------------------------------->|
| |
| 2.) 201 Created @ Location |
|<-------------------------------|
| |
1. The client sends an Atom Entry to the server via HTTP POST. The
Request URI is that of the Atom Feed.
2. The server responds with a response of "201 Created" and a
"Location" header containing the URI of the newly-created Atom
Entry.
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3.3.2 Read
Client Server
| |
| 1.) GET or HEAD to Entry URI |
|------------------------------->|
| |
| 2.) 200 OK Atom Entry |
|<-------------------------------|
| |
1. The client sends a GET (or HEAD) request to the entry's URI.
2. The server responds with an Atom Entry document.
3.3.3 Update
Client Server
| |
| 1.) PUT to Entry URI |
|------------------------------->|
| |
| 2.) 200 OK |
|<-------------------------------|
| |
1. The client PUTs an updated Atom Entry Document to the entry's
URI.
2. The server responds with a successful status code.
3.3.4 Delete
Client Server
| |
| 1.) DELETE to Entry URI |
|------------------------------->|
| |
| 2.) 204 No Content |
|<-------------------------------|
| |
1. The client sends a DELETE request to the entry's URI.
2. The server responds with successful status code.
3.4 Success and Failure
HTTP defines classes of response. HTTP status codes of the form 2xx
signal that a request was successful. HTTP status codes of the form
4xx or 5xx signal that an error has occurred, and the request has
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failed. Consult the HTTP specification for more detailed definitions
of each status code.
4. APP Feeds
4.1 GET
Feeds can contain extremely large numbers of resources. A naive
client such as a web spider or web browser would be overwhelmed if
the response to a GET contained every entry in the feed, and the
server would waste large amounts of bandwidth and processing time on
clients unable to handle the response. As a result, responses to a
simple GET request represent a server-determined subset of the
entries in the feed.
An example APP feed:
<feed xmlns="http://www.w3.org/2005/Atom"
xmlns:pub="http://purl.org/atom/app#">
<title>My Posts1</title>
<id>urn:uuid:ce61592c-14e2-4557-978e-dfbd444aefa6</id>
<updated>2005-12-21T04:11:00-08:00</updated>
<!-- 0 or more atom:entry elements follow -->
<entry>
<title type="text">title 25</title>
<updated>2005-12-21T04:11:00-08:00</updated>
<author>
<name>Foo</name>
</author>
<summary>It started out looking simple enough...</summary>
<id>urn:uuid:941e12b4-6eeb-4753-959d-0cbc51875387</id>
<pub:edit href="./entry7.atom"/>
<link href="/permalink7.html" />
</entry>
...
</feed>
Each member entry is represented by an atom:entry element, but those
entries are not an editable representation of the entry. To retrieve
the source representation of the entry, clients send a GET request to
the URI found in each entry's pub:edit element (see Section 4.3.1).
Derived resources are located by examining an entry's atom:link
elements.
4.2 POST
An APP feed also accepts POST requests. The client POSTs a
representation of the desired resource to the APP feed. Some feeds
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only accept POST requests with certain media-types, so a POST MAY
generate a response with a status code of 415 ("Unsupported Media
Type"). In the case of a successful creation, the status code is 201
("Created").
Example request creating a new entry in a feed:
POST /collection HTTP/1.1
Host: example.org
User-Agent: Cosimo/1.0
Content-Type: application/atom+xml
Content-Length: nnnn
...data...
Example response.
HTTP/1.1 201 Created
Date: Mon, 21 Mar 2005 19:20:19 GMT
Server: CountBasic/2.0
ETag: "4c083-268-423f1dc6"
Location: http://example.org/stuff/foo13241234.atom
APP feeds contain primarily textual content. Examples include
weblogs, online journals, and wikis. Clients add entries by sending
POST requests containing Atom Entry Documents. Existing entries are
edited by sending HTTP requests to the URI found in an individual
entry's pub:edit element. Servers can determine the processing
necessary to interpret a request by examining the request's HTTP
method. It is probably unwise to change an existing entry's atom:id
value when issuing a PUT request.
4.3 Entry Extensions
4.3.1 The 'pub:edit' Element
The pub:edit element is a child of atom:entry and has one attribute,
'href'. The value of this attribute is an IRI reference interpreted
relative to xml:base.
4.3.2 The 'pub:control' Element
The pub:control element is a child of atom:entry and is used to
persist editing information and contains arbitrary markup.
5. Media Feeds
The entries within Media Feeds are feeds do not represent uniform
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types of content. For example, they might contain JPEG images, text
documents, MPEG movies, and any other type of resource the server
allows.
5.1 GET
Media Feeds return an Atom feed much like Text Feeds, but with a few
additions. The entries also contain an atom:content element with a
'src' attribute pointing to the media resource. This URI can be used
to edit the uploaded media resource, using PUT and DELETE. Such
entries may contain pub:edit elements used to edit the entry
metadata. As with other entries, derived resources can be located by
inspecting an entry's atom:link elements.
An example Media Feed:
<feed xmlns="http://www.w3.org/2005/Atom"
xmlns:pub="http://purl.org/atom/app#">
<title>My Posts1</title>
<author>
<name>Foo</name>
</author>
<id>urn:uuid:ce61592c-14e2-4557-978e-dfbd444aefa6</id>
<updated>2005-12-21T04:11:00-08:00</updated>
<!-- 0 or more atom:entry elements follow -->
<entry>
<title type="text">Title25</title>
<updated>2005-12-21T04:11:00-08:00</updated>
<id>urn:uuid:941e12b4-6eeb-4753-959d-0cbc51875387</id>
<link href="/permalink7.html" type="text/html" />
<link href="/stuff/public/beach.jpg" type="image/jpg"
title="Low res public version" />
<summary>This was awesome.</summary>
<content src="http://example.org/asdf.jpg" />
</entry>
...
</feed>
The Atom Syndication Format requires that each such entry contain an
atom:title and atom:summary element. This requirement can be
challenging to meet without requiring users to enter tedious
metadata, but servers should attempt to provide textual data about
the resource in the interests of accessibility. The atom:title
element will likely be provided by the client, as a way for users to
associate their local resources with those they have uploaded to the
server (see POST below).
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5.2 POST
To add an entry to a Media Feed, clients POST the resource to the
Media Feed's URI. Clients should provide a 'Title' request header to
provide the server with a short string identifying the resource to
users. Clients may include a 'Content-Description' header [RFC2045]
providing a more complete description of the content. In addition,
servers may inspect the POSTed entity for additional metadata to be
exposed in an atom:entry element when listed in a Media Feed. For
example, the server might inspect a JPEG file for EXIF headers
containing creator data.
An example request:
POST /collection HTTP/1.1
Host: example.org
User-Agent: Cosimo/1.0
Content-Type: image/jpg
Content-Length: nnnn
Title: A Trip to the beach
Content-Description: It was so fun.
...binary data...
An example response:
HTTP/1.1 201 Created
Date: Mon, 21 Mar 2005 19:20:19 GMT
Server: CountBasic/2.0
ETag: "4c083-268-423f1dc6"
Location: http://example.org/stuff/beach.jpg
6. APP Outlines
In order for authoring to commence, a client must first discover the
Atom Feeds offered by the server.
The outline is an APP Outline Document [APPO]. The top level outline
elements describe distinct groups of resources available on the
server. For example, a user with an account containing three blogs
would have 3 <outline> elements under the root <app> element. There
is no requirement that servers support multiple top-level outlines,
and a feed may appear in more than one location in the document.
Clients read APP feeds by visiting the URI located in the 'href'
attribute of an <outline> element. This URI also serves as the
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location a client POSTs new entries to. The 'class' attribute of the
<outline> element indicates the type of feed. This specification
defines two values for the 'class' attribute:
o feed
o media feed
These values correspond to standard feeds and media feeds,
respectively.
An example APP Outline:
<app>
<outline class="feed" text="Main Blog" href="/entries">
<outline class="media feed" text="Photos" href="/photos"/>
<outline class="feed" text="Drafts" href="/drafts">
</outline>
<outline class="feed" text="Side Bar Blog" href="/2/entries">
<outline class="feed" text="Stuff" href="/2/stuff">
</outline>
<outline class="media feed" text="Moblog" href="/moblog"/>
</app>
7. Security Considerations
APP relies on HTTP Authentication. See [RFC2617] for a more detailed
description of the security properties of HTTP Authentication.
8. References
[APPO] Sayre, R., "APP Outline Format", work-in-progress,
draft-sayre-atompub-protocol-outline-00, October 2005.
[AtomFormat]
Nottingham, M. and R. Sayre, "The Atom Syndication
Format", work-in-progress, August 2005.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
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Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[W3C.REC-xml-20040204]
Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T.,
and E. Maler, "Extensible Markup Language (XML) 1.0 (Third
Edition)", W3C REC REC-xml-20040204, February 2004.
Author's Address
Robert Sayre
Email: rfsayre@boswijck.com
URI: http://boswijck.com
Appendix A. Contributors
This draft is a variant of the in-progress Atom Publishing Protocol
specification from the IETF Atompub WG, and owes a debt to the WG's
members.
Appendix B. Change History
-05: Death to collections!
Switch APPO instead of XOXO.
State the obvious about the extension elements.
Remove RFC2119 reference.
Change "Normative References" to "References".
-04: Add pub:control element.
Reword collection POST.
Prophesize about atom:id.
-03: Remove search/query capabilities added in -02
Drop round-tripping. Most of them were writable, some folks
wanted to edit atom:updated, that leaves atom:id, and that seems
foolish to try and edit, so go ahead and try it if you think you
can.
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Drop ordering... let the server pop things up if it wants to.
-02: Add search/query capabilities.
-01: Split from WG draft, cut SOAP, and much other cruft.
-interlude: Becomes WG draft.
-00: Split from WG draft
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