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 and Atom 1.0. 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.
2006/01/22: Atom 1.0 in basic form is widely supported. Many aggregators fail on feeds that do not resemble the structure of an RSS 2.0 feed closely. However, several have accepted bug reports, and a few have even released fixes. See ConformanceTests for current status.
2006/05/21: Atom 1.0 is widely supported in major consumer feed-reading applications, the notable exception being Bloglines.
2006/06/20: Bloglines rolled out its new Atom 1.0 parser, so now Atom 1.0 is widely supported in all major consumer feed-reading applications.
The RSS 2.0 specification is copyrighted by Harvard University and is frozen. No significant changes can be made (although the specification is under a Creative Commons licence) and it is intended that future work be done under a different name; Atom is one example of such work.
Atom 1.0 is specified in RFC 4287 (HTML Version); it represents the consensus of the Atompub Working Group within the IETF, as reviewed and approved by the IETF community and the Internet Engineering Steering Group. 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 underlying specifications.
There are two popular protocols widely used with RSS, MetaWeblog and Blogger. There are frequent reports of problems with interoperability and feature shortcomings.
The Atompub working group is in the late stages of developing the Atom Publishing Protocol, which is closely integrated with the Atom feed format and is based on the experience with the existing protocols.
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.
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&amp;T) and has been a source of difficulty for implementors. The content model for titles is underspecified; titles containing angle brackets or ampersands will be misinterpreted by a significant fraction of readers regardless of how they are represented. The RSS 2.0 content model does not permit actual well-formed XML markup, which reduces the re-usability of content.
Atom has a carefully-designed payload container. Content must be explicitly labeled as one of:
plain text, with no markup (the default)
escaped HTML, as commonly used with RSS 2.0
well-formed XHTML markup
some other XML vocabulary
base64-encoded binary content
a pointer to Web content not included in the feed
There is no guarantee that the recipient will be able to do anything useful with arbitrary XML or binary content. However, it is guaranteed that recipients will not inappropriately attempt to process content they cannot.
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).
RSS autodiscovery has been implemented several times in different ways, often relies on unregistered (thus invalid) application/rss+xml MIME type, and has never been standardized. This is a common source of difficulty for non-technical users.
Atom standardizes autodiscovery. The application/atom+xml MIME Type is registered with IANA. 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.
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. Atom also has support for aggregated feeds, allowing entries to point back to the feed they came from when they are included into other feeds.
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 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 of <link rel=""> values. Finally, Atom 1.0 provides recommended extension points and guidance on how to interpret simple extensions.
RSS 2.0 does not specify the handling of relative URI references. Different feed readers implement differing heuristics for their interpretation. There is no interoperability. In practice, relative URI references cannot be used in RSS feeds.
Atom 1.0 specifies use of the XML's built-in xml:base attribute for allowing the use of relative references.
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:
Libraries for processing Atom 1.0:
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 attribute.
RSS 2.0 can be encrypted or signed like any other web content, if treated as a bag of bits.
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.
RSS 2.0 categories have two parts: label and domain.
Atom 1.0 categories have three, with the addition of optional human-readable title.
The RSS 2.0 specification includes no schema.
Atom 1.0 includes a (non-normative) ISO-Standard RelaxNG schema, to support those who want to check the validity of data advertised as Atom 1.0. Other schema formats can be generated from the RelaxNG schema.
<?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>firstname.lastname@example.org (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>
<?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>email@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>
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|
|link||link||Atom defines an extensible family of rel values|
|language||-||Atom uses standard xml:lang attribute|
|managingEditor||author or contributor|
|pubDate||published (in entry)||Atom has no feed-level equivalent|
|lastBuildDate (in channel)||updated||RSS has no item-level equivalent|
|ttl||-||<ttl> is problematic, prefer HTTP 1.1 cache control|
|image||logo||Atom recommends 2:1 aspect ratio|
|-||icon||As in favicon.ico|
|description||summary and/or content||depending on whether full content is provided|
|enclosure||-||rel="enclosure" on <link> in Atom|
|source||-||rel="via" on <link> in Atom|
|-||source||Container for feed-level metadata to support aggregation|