Internet DRAFT - draft-sayre-atompub-protocol-basic
draft-sayre-atompub-protocol-basic
Atompub Working Group R. Sayre
Internet-Draft January 24, 2006
Expires: July 28, 2006
The Atom Publishing Protocol (Basic)
draft-sayre-atompub-protocol-basic-06
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo presents a protocol that uses XML and HTTP to publish and
edit Web resources.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. The Atom Publishing Protocol Model . . . . . . . . . . . . . . 3
4. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Listing . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Authoring . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7. Atom Protocol Feeds . . . . . . . . . . . . . . . . . . . . . 6
8. Media Feeds . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Service Description . . . . . . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
11. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. Informative References . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
The Atom Publishing Protocol uses HTTP [RFC2616] and XML [W3C.REC-
xml-20040204] to publish and edit Web resources.
2. Notational Conventions
The Atom Protocol 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 Atom Protocol 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.
4. Discovery
To discover the location of the feeds exposed by an Atom Protocol
service, the client must locate and request a Service Description
Document (Section 6).
Client Server
| |
| 1.) GET Service URI |
|------------------------------->|
| |
| 2.) Service Description Doc |
|<-------------------------------|
| |
1. The client sends a GET request to the Service Description URI.
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2. The server responds with a Service Description 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.
5. 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.
6. 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.
6.1. Create
Client Server
| |
| 1.) POST Entry to Feed URI |
|------------------------------->|
| |
| 2.) 201 Created @ Location |
|<-------------------------------|
| |
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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.
6.2. 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.
6.3. 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.
6.4. Delete
Client Server
| |
| 1.) DELETE to Entry URI |
|------------------------------->|
| |
| 2.) 204 No Content |
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|<-------------------------------|
| |
1. The client sends a DELETE request to the entry's URI.
2. The server responds with successful status code.
6.5. 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.
7. Atom Protocol Feeds
7.1. 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.
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An example Atom Protocol 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.
7.2. POST
An Atom Protocol feed also accepts POST requests. The client POSTs a
representation of the desired resource to the APP feed. Some feeds
only accept POST requests with certain media-types, so a POST request
could result in a response with a status code of 415 ("Unsupported
Media Type"). In the case of a successful creation, the status code
is 201 ("Created").
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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
8. Media Feeds
The entries within Media Feeds do not represent uniform types of
content. For example, they might contain JPEG images, text
documents, MPEG movies, or any other type of resource the server
allows.
8.1. GET
Media Feeds return an Atom feed much like the textual Atom Protcol
feeds described above, 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 edit links
used to edit the entry metadata. As with any Atom entry, related and
derived resources can be located by inspecting an entry's atom:link
elements.
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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/beach.tiff" />
</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).
8.2. 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.
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An example request:
POST /collection HTTP/1.1
Host: example.org
User-Agent: Cosimo/1.0
Content-Type: image/tiff
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.tiff
[@@ deal with response ambiguity noted in WG]
9. Service Description
Many Atom Protocol applications require a basic resource layout in
order to ease configuration requirements. Servers use Service
Description documents to convey information about related groups of
Atom Protocol feeds. On a blogging service, for example, each group
might represent a distinct blog and associated resources.
Example Service Description document:
<app xmlns="http://purl.org/atom/app#">
<service name="My Blog" class="feed"
href="http://example.com/entries">
<service name="Photos" class="media feed"
href="http://example.com/photos"/>
<service name="Drafts" class="feed"
href="http://example.com/drafts"/>
</service>
<service class="feed" name="Sidebar Blog"
href="http://example.org/details"/>
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</app>
Servers are not required to expose a Service Description document,
but experimental deployment experience has shown that a single
document which signals some basic information about the server's
configuration can greatly simplify client implementations. The
simplest useful Service Description document shows the location of a
single resource:
<app>
<service name="My Blog" class="feed"
href="http://blog.example.com/app.cgi"/>
</app>
If another service is added, the document can be upgraded to reflect
new resources.
<app>
<service name="My Blog" class="feed"
href="http://blog.example.com/app.cgi"/>
<service name="Another Blog" class="feed"
href="http://another.example.com/app.cgi"/>
</app>
Finally, more extensive services could require some amount of
hierarchical grouping.
<app>
<service name="My Blog" class="feed"
href="http://blog.example.com/app.cgi">
<service name="Photos" class="media feed"
href="http://example.com/photos"/>
</service>
<service name="Other Things">
<service name="Another Blog" class="feed"
href="http://another.example.com/app.cgi"/>
<service name="A Third Blog" class="feed"
href="http://third.example.com/app.cgi"/>
</service>
</app>
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This example shows that links to APP feeds can appear in <service>
elements used to group other resources. The <service> element named
"Other Things" does not contain an 'href' attribute, so it functions
as a simple named group of the services it contains.
9.1. Categories
[@@ tbd]
9.2. Document Format
Service Description documents MUST be well-formed XML [W3C.REC-xml-
20040204].
The root element of an APP Service Description Document is "<app>".
This specification does not define any attributes of the <app>
element, but the element can have any number of attributes.
Zero or more <service> elements appear as child elements of <app>.
Also, <service> elements may contain zero or more <service> elements.
This specification defines three attributes of the <service> element.
<service> elements contain at least a 'name' or 'href' attribute.
Additional service properties too large or structured to include in
attribute values could appear as child elements of the service
element.
<app> elements can contain any number of elements that are not
<service> elements, and <service> elements can contain any number of
elements that are not <service> elements.
9.2.1. The 'name' Attribute
The 'name' attribute contains a short string describing the service
element. Entities such as "&" and "<" represent their
corresponding characters ("&" and "<" respectively), not markup.
9.2.2. The 'href' Attribute
The 'href' attribute contains an IRI reference interpreted relative
to the in-scope base IRI [RFC3987]. Most protocols require URIs
[RFC3986], so IRIs usually need to be converted to URIs before being
dereferenced.
9.2.3. The 'class' Attribute
The 'class' attribute contains a space-separated list of strings used
to classify the service element. This specification defines two
values for the 'class' attribute:
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o feed
o media feed
These values correspond to standard feeds and media feeds,
respectively. If the 'class' attribute is not present, the <service>
element can be processed as if the attribute were present with a
value of 'feed'.
9.2.4. Relax NG Schema
Service Description documents conform to the schema below.
default namespace = "http://purl.org/atom/app#"
start = app
app = element app {
anyAttribute*,
(service* & anyElement*)
}
service = element service {
(nameAtt | hrefAtt), anyAttribute*,
(service* & anyElement*)
}
nameAtt = attribute name { text }
hrefAtt = attribute href { text }
classAtt = attribute class { text }
anyElement = element * { (anyAttribute | text | anyElement)* }
anyAttribute = attribute * { text }
9.2.5. Extending Service Description
The Service Description document format can be freely extended by
adding attributes and elements not defined by this specification.
Valid Service Description document with extensions:
<app xmlns="http://purl.org/atom/app#" foo="bar">
<blog-userid>42</blog-userid>
<service name="Baz" qux="hmmm" href="http://example.com">
<some-other-extension>hmm</some-other-extension>
</service>
</app>
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Additional service properties too large or structured to include in
attribute values could appear as child elements of the <service> or
<app> elements. <app> elements may contain any number of elements
that are not <service> elements, and <service> elements may contain
any number of elements that are not <service> elements.
9.2.6. User Agent Conformance
Foreign markup is markup not defined by this specification.
Software consuming Service Description documents must not halt
processing when any foreign markup is encountered. Software may
ignore the markup and process any content of foreign elements as
though the surrounding markup were not present. For example,
software may process
<app>
<workspace>
<service name="My Blog"
href="http://example.com/entries">
<service name="Photos" class="media feed"
href="http://example.com/jpgs"/>
<view title="Archives" seek="...">
<view title="2005" href="..." />
...
</view>
</service>
</workspace>
</app>
as though the <workspace> and <view> elements were not present.
Software conforming to this specification may halt processing when
documents that do not conform to the schema are encountered.
10. IANA Considerations
[@@ fill out in for application/sd+xml (service description)]
11. Security Considerations
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12. Informative References
Appendix A. Acknowledgements
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
-06: Change service description format.
Change IPR terms to full3978
-interlude: More unproductive WG thrashing.
-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.
-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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Author's Address
Robert Sayre
Email: rfsayre@boswijck.com
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Internet Society.
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