1. RSS and Atom ¶

People who generate syndication feeds have a choice of
feed formats. As of mid-2005, the two
most likely candidates will be RSS 2.0(http://blogs.law.harvard.edu/tech/rss) and Atom 1.0(http://ietfreport.isoc.org/idref/draft-ietf-atompub-format/).
The purpose of this page is to summarize, as clearly and simply as possible, the differences between the RSS 2.0 and Atom 1.0 syndication languages.

Toru Marumoto has produced a Japanese translation(http://www.witha.jp/Atom/RSS-and-Atom.html).

1.1. Major/Qualitative Differences ¶

1.1.1. Deployment ¶

2005/07/21: RSS 2.0 is widely deployed and Atom 1.0 only by a few early adopters, see KnownAtomFeeds and KnownAtomConsumers.

1.1.2. Specifications ¶

The RSS 2.0 specification is copyrighted by Harvard University and is frozen. No significant changes can be made and it is intended that future work be done under a different name; Atom is one example of such work.

The Atom 1.0 specification (in the course of becoming an
IETF standards track RFC) represents the consensus of the
Atompub Working Group(http://www.ietf.org/html.charters/atompub-charter.html)
within the IETF(http://www.ietf.org/), as reviewed and approved by the IETF community and the
Internet Engineering Steering Group(http://www.ietf.org/iesg.html). The specification is structured in such a way that the IETF could conceivably issue further versions or revisions of this specification without breaking existing deployments, although there is no commitment, nor currently expressed interest, in doing so.

See the Extensibility section below for how each can be extended without changing the specifications themselves.

1.1.3. Publishing Protocols ¶

There are two popular protocols widely used with RSS, MetaWeblog(http://www.xmlrpc.com/metaWeblogApi)
and Blogger(http://www.blogger.com/developers/api/). There are frequent
reports(http://www.bblfish.net/blog/page7.html#2005/06/20/22-28-18-208) of problems with interoperability and feature shortcomings.

The Atompub working group is in the late stages of developing the
Atom Publishing Protocol(http://ietfreport.isoc.org/idref/draft-ietf-atompub-protocol/), which is closely integrated with the Atom feed format and is based on the experience with the existing protocols.

1.1.4. Required Content ¶

RSS 2.0 requires feed-level title, link, and description. RSS 2.0 does not require that any of the fields of individual items in a feed be present.

Atom 1.0 requires that both feeds and entries include a title (which may be empty), a unique identifier, and a last-updated timestamp.

1.1.5. Payload ¶

RSS 2.0 may contain either plain text or escaped HTML, with no way to indicate which of the two is provided. Escaped HTML is ugly (for example, the string AT&T would be expressed as “AT&T”) and has been a source of difficulty for implementors. RSS 2.0 cannot contain actual well-formed XML markup, which reduces the re-usability of content.

Atom has a carefully-designed payload container. Content may be explicitly labeled as any one of:

plain text, with no markup (the default)
escaped HTML, like is commonly used with RSS 2.0
well-formed, displayable XHTML markup
some other XML vocabulary (There is no guarantee that the recipient will be able to do anything useful with such content)
base64-encoded binary content (again, no guarantee)
a pointer to Web content not included in the feed

1.1.6. Full or Partial Content ¶

RSS 2.0 has a “description” element which is commonly used to contain either the full text of an entry or just a synopsis (sometimes in the same feed), and which sometimes is absent. There is no built-in way to signal whether the contents are complete.

Atom has separate “summary” and “content” elements. The summary is encouraged for accessibility reasons if the content is non-textual (e.g. audio) or non-local (i.e. identified by pointer).

1.1.7. Autodiscovery ¶

RSS
autodiscovery(http://diveintomark.org/archives/2002/06/02/important_change_to_the_link_tag) has been implemented several times in different ways and has never been standardized. This is a common source of difficulty for non-technical users.

Atom standardizes autodiscovery(http://www.ietf.org/internet-drafts/draft-ietf-atompub-autodiscovery-01.txt). Additionally, Atom feeds contain a “self” pointer, so a newsreader can auto-subscribe given only the contents of the feed, based on Web-standard dispatching techniques.

1.1.8. Extraction and Aggregation ¶

The only recognized form of RSS 2.0 is an <rss> document.

Atom 1.0 allows standalone Atom Entry documents; these could be transferred
using any network protocol, for example XMPP(http://ietfreport.isoc.org/idref/draft-saintandre-atompub-notify/). Atom also has support for aggregated
feeds, where entries from multiple different feeds are combined, with pointers
back to the feed they came from.

1.2. Differences of Degree ¶

1.2.1. Extensibility ¶

RSS 2.0 is not in an XML namespace but may contain elements from other XML namespaces. There is no central place where one can find out about many popular extensions, such as dc:creator and content:encoded.

Atom 1.0 is in an XML namespace(http://www.w3.org/2005/Atom) and may contain elements or attributes from other XML namespaces. There are specific guidelines on how to interpret extension elements. Additionally, there will be an IANA managed directory rel= values for <link>. Finally, Atom 1.0 provides recommended extension points and guidance on how to interpret simple extensions.

1.2.2. URIs ¶

RSS 2.0 does not specify the handling of relative URI references, and in practice they cannot be used in RSS feeds.

Atom 1.0 specifies use of the XML's built-in xml:base(http://www.w3.org/TR/xmlbase/) attribute for allowing the use of relative references.

1.2.3. Software Libraries (Parsing, Generating) ¶

Both RSS 2.0 and Atom 1.0 feeds can be accessed via standard HTTP client libraries. Standard caching techniques work well and are encouraged. Template-driven creation of both formats is quite practical.

Libraries for processing RSS 2.0:

FeedParser(http://feedparser.org/)
Rome(http://rome.dev.java.net/)

Libraries for processing Atom 1.0:

XML::Atom(http://bulknews.typepad.com/blog/2005/07/searchcpanorg_t.html)

1.2.4. Language Tagging ¶

For identification of the language used in feeds, RSS 2.0 has its own <language> element, while Atom uses XML's built-in
xml:lang(http://www.w3.org/TR/REC-xml/#sec-lang-tag) attribute.

1.2.5. Digital Signature/Encryption ¶

RSS 2.0 can be encrypted or signed like any other web content, if treated as a
bag of bits.

Rules for applying standard
XML Encryption(http://www.w3.org/TR/xmlenc-core/)
and XML Digital Signature(http://www.w3.org/TR/xmldsig-core/) on entries are included in Atom 1.0.
Alternatively, the feed can be encrypted or signed, like RSS 2.0, as a bag of bits.

1.2.6. Authors ¶

RSS 2.0 provides the ability to specify email addresses for a feed’s “managingEditor” and “webMaster”, and for an item’s “author”. Some publishers prefer not to share email addresses, and use “dc:creator” from the dublin core extension instead.

Atom 1.0 provides “author” and “contributor” elements at both the feed and entry level. These elements must contain a name, and also have optional email and URI sub-elements.

1.2.7. Categories ¶

RSS 2.0 categories have two parts: label and domain.

Atom 1.0 categories have three, with the addition of optional human-readable title.

1.2.8. Schema ¶

The RSS 2.0 specification includes no schema.

Atom 1.0 includes a (non-normative) ISO-Standard RelaxNG(http://relaxng.org/) schema, to support those who want to check the validity of data advertised as Atom 1.0. Other schema formats can be generated(http://www.thaiopensource.com/relaxng/trang.html) from the RelaxNG schema.

1.3. Sample RSS and Atom Feeds ¶

1.3.1. RSS 2.0 (687 bytes) ¶

~cpp 
<?xml version="1.0" encoding="utf-8"?>
<rss version="2.0">
  <channel>

   <title>Example Feed</title>
   <description>Insert witty or insightful remark here</description>
   <link>http://example.org/</link>
   <lastBuildDate>Sat, 13 Dec 2003 18:30:02 GMT</lastBuildDate>
   <managingEditor>johndoe@example.com (John Doe)</managingEditor>

   <item>
     <title>Atom-Powered Robots Run Amok</title>
     <link>http://example.org/2003/12/13/atom03</link>
     <guid isPermaLink="false">urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</guid>
     <pubDate>Sat, 13 Dec 2003 18:30:02 GMT</pubDate>
     <description>Some text.</description>
   </item>

  </channel>
</rss>

1.3.2. Atom 1.0 (677 bytes) ¶

~cpp 
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">

 <title>Example Feed</title>
 <subtitle>Insert witty or insightful remark here</subtitle>
 <link href="http://example.org/"/>
 <updated>2003-12-13T18:30:02Z</updated>
 <author>
   <name>John Doe</name>
   <email>johndoe@example.com</email>
 </author>
 <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

 <entry>
   <title>Atom-Powered Robots Run Amok</title>
   <link href="http://example.org/2003/12/13/atom03"/>
   <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
   <updated>2003-12-13T18:30:02Z</updated>
   <summary>Some text.</summary>
 </entry>

</feed>

1.4. Element Comparison Table ¶

Atom 1.0 defines 21 elements; RSS 2.0, 30. Most of the RSS 2.0 elements missing in Atom 1.0 are either those that were never widely implemented in practice, or the capabilities are provided in other ways.

RSS 2.0	Atom 1.0	Comments
rss	-	Vestigial in RSS
channel	feed
title	title
link	link	Atom defines an extensible family of "rel" values
description	subtitle
language	-	Atom uses standard "xml:lang" attribute
copyright	rights
webMaster	-
managingEditor	author or contributor
pubDate	published (in entry)	Atom has no feed-level equivalent
lastBuildDate (in channel)	updated	RSS has no item-level equivalent
category	category
generator	generator
docs	-
cloud	-
ttl	-	Use the Expires and max-age cache hints in HTTP 1.1
image	logo	Atom recommends 2:1 aspect ratio
-	icon	As in "favicon.ico"
rating	-
textInput	-
skipHours	-
skipDays	-
item	entry
author	author
-	contributor
description	summary and/or content	depending on whether full content is provided
comments	-
enclosure	-	rel="enclosure" on <link> in Atom
guid	id
source	-	rel="via" on <link> in Atom
-	source	Container for feed-level metadata to support aggregation

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