Network Working Group R. Sayre
Internet-Draft October 3, 2005
Expires: April 6, 2006
The Atom Publishing Protocol (Basic)
draft-sayre-atompub-protocol-basic-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo presents a protocol for using XML (Extensible Markup
Language) and HTTP (HyperText Transport Protocol) to edit content.
The Atom Publishing Protocol is an application-level protocol for
publishing and editing Web resources belonging to periodically
updated websites. The Atom format is documented in the Atom
Syndication Format (draft-ietf-atompub-format-11.txt).
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Editorial Note
To provide feedback on this Internet-Draft, join the atom-protocol
mailing list <http://www.imc.org/atom-protocol/index.html>.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
3. The Atom Publishing Protocol Model . . . . . . . . . . . . . 5
4. Collections . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Media Collections . . . . . . . . . . . . . . . . . . . . . 12
6. Service Outlines . . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . 16
8. Normative References . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . 17
A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . 19
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1. Introduction
The Atom Publishing Protocol (APP) is an application-level protocol
for publishing and editing Web resources using HTTP [RFC2616] and XML
[W3C.REC-xml-20040204].
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2. 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].
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].
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3. The Atom Publishing Protocol Model
The Atom Publishing Protocol operates on collections of Web
resources. All collections support the same basic interactions, as
do the resources within the collections. The patterns of interaction
are based on the common HTTP verbs. 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 Collections
The APP groups resources into "Collections", which are analogous to
the "folders" or "directories" found in many file systems.
3.2 Discovery
To discover the location of the collections exposed by an APP
service, the client must locate and request a Service Outline
(Section 6). Service 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 Service Outline Document containing
the locations of collections 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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3.3 Listing
Once the client has discovered the location of a collection, it can
request a listing of the collection's membership. However,
collections might be extremely large, so servers are likely to list a
small subset of the collection by default.
Client Server
| |
| 1.) GET to Collection URI |
|------------------------------->|
| |
| 2.) 200 OK, Atom Feed Doc |
|<-------------------------------|
| |
1. The client sends a GET request to the Collection's URI.
2. The server responds with an Atom Feed Document containing a full
or partial listing of the collection's membership.
3.4 Authoring
After locating a collection, a client can add entries by sending a
request to the collection; other changes are accomplished by sending
HTTP requests to its member resources.
3.4.1 Create
Client Server
| |
| 1.) POST to Collection URI |
|------------------------------->|
| |
| 2.) 201 Created @ Location |
|<-------------------------------|
| |
1. The client sends a representation of a member to the server via
HTTP POST. The Request URI is that of the Collection.
2. The server responds with a response of "201 Created" and a
"Location" header containing the URI of the newly-created
resource.
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3.4.2 Read
Client Server
| |
| 1.) GET or HEAD to Member URI |
|------------------------------->|
| |
| 2.) 200 OK Atom Entry |
|<-------------------------------|
| |
1. The client sends a GET (or HEAD) request to the member's URI.
2. The server responds with an Atom Entry document.
3.4.3 Update
Client Server
| |
| 1.) PUT to Member URI |
|------------------------------->|
| |
| 2.) 200 OK |
|<-------------------------------|
1. The client PUTs an updated representation to the member's URI.
2. The server responds with a representation of the member's new
state.
3.4.4 Delete
Client Server
| |
| 1.) DELETE to Member URI |
|------------------------------->|
| |
| 2.) 204 No Content |
|<-------------------------------|
| |
1. The client sends a DELETE request to the member's URI.
2. The server responds with successful status code.
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3.5 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
failed. Consult the HTTP specification for more detailed definitions
of each status code.
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4. Collections
An Atom Collection is a set of related resources represented by one
or more Atom Feed documents [AtomFormat]. Atom Collections are
ordered the date their members were updated, with the most recently
updated member appearing first.
4.1 GET
Collections 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 reflected the full membership of the
collection, 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 collection's membership.
An example collection 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>
<id>urn:uuid:941e12b4-6eeb-4753-959d-0cbc51875387</id>
<pub:edit href="./entry7.atom"/>
<link href="/permalink7.html" />
</entry>
...
</feed>
Each member is represented by an Atom Entry, 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.
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4.2 POST
In addition to GET, a Collection Resource also accepts POST requests.
The client POSTs a representation of the desired resource to the
Collection Resource. Note that some collections only allow members
of a specific media-type and 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 MUST be 201
("Created").
Example request creating a resource in a collection.
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
4.3 Entry Collection
Entry Collections are Collections that restrict their membership to
Atom entries. The 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.
4.3.1 The 'pub:edit' Element
The pub:edit element has one attribute, 'href'. The value of this
attribute is an IRI reference interpreted relative to xml:base.
4.3.2 Role of Atom Entry Elements During Editing
The elements of an Atom Entry Document are either a 'Writable
Element' or a 'Round Trip Element'.
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Writable Element - An element of an Atom Entry whose value is
editable by the client and not enforced by the server.
Round Trip Element - An element of an Atom Entry whose value is
enforced by the server and not editable by the client.
That categorization determines the elements' disposition during
editing.
+--------------------+------------+
| Atom Entry Element | Property |
+--------------------+------------+
| atom:author | Writable |
| | |
| atom:category | Writable |
| | |
| atom:content | Writable |
| | |
| atom:contributor | Writable |
| | |
| atom:id | Round Trip |
| | |
| atom:link | Writable |
| | |
| atom:published | Writable |
| | |
| atom:source | Writable |
| | |
| atom:summary | Writable |
| | |
| atom:title | Writable |
| | |
| atom:updated | Round Trip |
+--------------------+------------+
Table 1
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5. Media Collections
Media Collections are Collections that do not have uniform
restrictions on the representations of the member resources. For
example, they might contain JPEG images, text documents, MPEG movies,
and any other type of resource the server allows.
5.1 GET
Media Collections return an Atom feed much like Entry Collections,
but with a few additions. The listing MUST 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 collection members, derived resources can be
located by inspecting an entry's atom:link elements.
An example Media Collection 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">title 25</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
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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 way for users to
associate their local resources with those they have uploaded to the
server (see POST below).
5.2 POST
To create media resources, clients POST the resource to the Media
Collection'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 when listed in a Media Collection. 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 request.
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
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6. Service Outlines
In order for authoring to commence, a client must first discover the
capabilities and locations of collections offered.
The Service Outline Document is a XOXO outline [XOXO]. The top level
list items describe distinct groups of resources offered by the
service. For example, a user with an account containing three blogs
would have 3 items at the top of the outline. There is no
requirement that servers support multiple top-level items, and a
collection may appear in more than one location in the document.
Clients can read entries contained in a collection by visiting an the
URI located in the 'href' attribute of a XOXO outline item. This URI
also serves as the location a client POSTs new entries to. The 'rel'
attribute of the XHTML anchor element conveys the nature of a
collection's member resources. This specification defines two
initial values for the 'rel' attribute:
o entry
o media
These values correspond to the two types of collection defined by
this specification. Extensibility for 'rel' values is specified in
XHTML Modularization [W3C.REC-xhtml-modularization-20010410].
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An example Service Outline:
<ul class="xoxo">
<li>
<a href="/app/collection.py?id=1&type=entries"
rel="entry" type="application/atom+xml">Main Blog</a>
<ul>
<li><a href="/app/collection.py?id=2&type=media"
rel="media" type="application/atom+xml">My Stuff2</a>
</li>
</ul>
</li>
<li>
<a href="/app/collection.py?id=3&type=entries"
rel="entry" type="application/atom+xml">Side Bar Blog</a>
<ul>
<li><a href="/app/collection.py?id=4&type=media"
rel="media" type="application/atom+xml">My Stuff4</a>
</li>
</ul>
</li>
<li>
<a href="/app/collection.py?id=5&type=entries"
rel="entry" type="application/atom+xml">A Third Blog</a>
<ul>
<li>
<a href="/app/collection.py?id=6&type=media"
rel="media" type="application/atom+xml">My Stuff6</a>
</li>
</ul>
</li>
</ul>
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7. Security Considerations
Because APP is a publishing protocol, it is important that only
authorized users can create and edit entries.
APP relies on HTTP Authentication. See [RFC2617] for more detailed
description of the security properties of HTTP Authentication.
8. Normative References
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.,
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-xhtml-modularization-20010410]
Altheim, M., Boumphrey, F., McCarron, S., Dooley, S.,
Schnitzenbaumer, S., and T. Wugofski, "Modularization of
XHTML", W3C REC REC-xhtml-modularization-20010410,
April 2001.
[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.
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[XOXO] Marks, K., Celik, T., Pilgrim, M., and M. Peterson, "XOXO
1.0: Extensible Open XHTML Outlines", October 2004.
Author's Address
Robert Sayre
Email: rfsayre@boswijck.com
URI: http://boswijck.com
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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.
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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