Dave Winer recently wrote that at least one person has asked if it is safe to ignore Atom in his weblog. If you are a cautious person like Tim Bray's Mr. Safe or you fit more on the right than the left side of the Technology Adoption Life Cycle then you are probably wondering why you should want to support the Atom syndication format over one of the many flavors of RSS. There are two parts to this question, if you are a consumer of syndication feeds or if you are a consumer of syndication feeds.
The Safe Syndication Producer's Perspective
An RSS feed is a regularly updated XML document that contains metadata about a news source and the content in it. Minimally an RSS feed consists of a channel that represents the news source, which has a title, link, and description that describe the news source. Additionally, an RSS feed typically contains one or more item elements that represent individual news items, each of which should have a title, link, or description
There are two primary flavors of RSS; Dave Winer's family of specifications (the most popular being RSS 0.91 & RSS 2.0) and the RDF-based RSS 1.0. The most popular are Dave Winer's family of specifications which have been adopted by a number of well-known organizations such as Yahoo! News, the BBC, Rolling Stone magazine, the Microsoft Developer Network (MSDN) , the Oracle Technology Network (OTN), the Sun Developer Network and Apple's iTunes Music Store. According to Syndic8 which tracks over 50,000 RSS feeds RSS 0.91, RSS 1.0 & RSS 2.0 all have about 30% of the RSS marketshare.
Most news aggregators support all 3 major versions of RSS although few actually take advantage of the fact that RSS 1.0 is an RDF vocabulary. If all one want is simple syndication of news items the RSS 0.91 should be satisfactory. If one plans to use extensions to the core RSS specification that expose application or domain specific functionality such as the ability to post comments one can use one of the many RSS modules in combination with RSS 2.0. The only advantage that RSS 1.0 gives over RSS 0.91/RSS 2.0 is that it is an RDF vocabulary and thus fits nicely into the dream of the Semantic Web.
The Atom syndication format can be considered to be a more sophisticated implementation of the ideas in RSS 2.0. It adds richer syndication capabilities such as the ability to put binary formats such as Word documents and Powerpoint documents in feeds and formalizes some of the best practices in the RSS world around putting [X]HTML in feeds.
The average user of a news aggregator will not be able to tell the difference between an Atom or RSS feed from their aggregator if it supports both. However users of aggregators that don't support Atom will not be able to subscribe to feeds in that format. In a few years, the differences between RSS and Atom will most likely be the same as those that are different between RSS 1.0 and RSS 0.91/RSS 2.0; only of interest to a handful of XML syndication geeks. Even then the simplest and safest bet would still be to use RSS as a syndication format. This is the same as the fact that even though the W3C has published XHTML 1.0 & XHTML 1.1 and is working on XHTML 2.0, the safest bet to get the widest reach with the least problems is to publish a website in HTML 3.2 or HTML 4.01.
The Safe Syndication Consumer's Perspective
If you plan to consume feeds from a wide variety of sources then one should endeavor to support as many syndication formats as possible. The more formats a feed consumer supports the more content is available for its users.
Based on their current popularity, degree of support and ease of implementation one should consider supporting the major syndication formats in the following order of priority
- RSS 0.91/RSS 2.0
- RSS 1.0
- Atom
RSS 0.91 support is the simplest to implement and most widely supported by websites while Atom is the most difficult to implement being the most complex and will be least supported by websites in the coming years.