COIN78 - XML Lesson 11: RSS, RDF, and XML Metadata
RSS
RSS stands for Really Simple Syndication. It is also known as Rich Site Summary. RSS provides a simple way to share and view headlines and web content while also allowing the user to manage site content. It was created to automatically inform users regarding updates to their websites or other files without requiring them to have to go to the actual site. It is extremely useful for sites that constantly change or update their information. Examples of this would be myyahoo.com and myspace.com which allow the user to determine the site’s content.
Generally, for those who regularly use the web, RSS solves a problem by allowing you to see the latest content from the sites you are interested in without needing to visit each site individually. Also, with RSS feeds, you do not need to join each site’s e-mail newsletter, ensuring your privacy. The number of site offering RSS feeds is continually growing including CNN News and the Washington Post along with Yahoo News.
How it works
RSS is used to share content between sites. It allows computers to retrieve and understand information from other computers’ feeds. This information is then presented through a feed aggregator, which is a program or website that gathers information and sorts the RSS feeds. In order to use it, your RSS feed should describe your site content. Then, you must register your RSS feed with an aggregator. The aggregator is customized, based on the user’s interests. RSS uses XML to create a text-based file with information regarding a blog, website or podcast. A new RSS document is saved as an .xml and uploaded to a website. Once this is done, it allows the aggregator to retrieve the file and read the file’s new information. Now, the aggregator can display the content.
History
The name “RSS” encompasses a format that spans a few different versions of at least two (but parallel) formats. Netscape designed version 0.90, which is considered to be the original RSS. It was created as a format to build portals of headlines to mainstream news sites, but it was found to be too complex for what it wanted to do. Therefore, a simpler version, dubbed 0.91, was proposed but then dropped by Netscape when it lost interest in this area of business.
Another vendor, however, named UserLand Software, picked up 0.91 hoping to use it for the company’s weblogging products and other web-based software. Meanwhile, another group was designing a new format based on RSS 0.90. This format was based on RDF and is called RSS 1.0. UserLand did not accept this new format and instead decided to continue to evolve their 0.9x branch producing versions 0.92, 0.93, 0.94 and 2.0. Therefore, there are now seven different formats called “RSS”. If you want to make your web content available by syndication, which format should you choose? You will need to be flexible enough to handle all seven variations.
Here is a table from xml.com that contains the seven RSS versions and recommendations.
RSS versions and recommendations
| Version | Owner | Pros | Status | Recommendation |
|---|---|---|---|---|
| 0.90 | Netscape | Obsoleted by 1.0 | Don't use | |
| 0.91 | UserLand | Drop dead simple | Officially obsoleted by 2.0, but still quite popular | Use for basic syndication. Easy migration path to 2.0 if you need more flexibility |
| 0.92, 0.93, 0.94 | UserLand | Allows richer metadata than 0.91 | Obsoleted by 2.0 | Use 2.0 instead |
| 1.0 | RSS-DEV Working Group | RDF-based, extensibility via modules, not controlled by a single vendor | Stable core, active module development | Use for RDF-based applications or if you need advanced RDF-specific modules |
| 2.0 | UserLand | Extensibility via modules, easy migration path from 0.9x branch | Stable core, active module development | Use for general-purpose, metadata-rich syndication |
For a quick tutorial on preparing a podcast (for which knowledge of RSS is virtually essential) please see this link. The sample file for this tutorial is itunes_demo.xml or itunes_demo.rss. Note that depending on which Web browser you are using, and how your MIME types are set up, you may or may not be able to see this file. If not try the text version itunes_demo.txt.
RDF
Resource Description Framework (RDF) is a family of World Wide Web Consortium (W3C) specifications originally designed as a metadata model but which has come to be used as a general method of modeling information, through a variety of syntax formats. Wikipedia has a very good overview of the history of RDF, and links to other articles on semantic web and machine knowledge. An excellent tutorial on RDF at W3 schools can be accessed from this link.
The RDF metadata model is based upon the idea of making statements about resources in the form of subject-predicate-object expressions, called triples in RDF terminology. The subject denotes the resource, and the predicate denotes traits or aspects of the resource and expresses a relationship between the subject and the object. For example, one way to represent the notion "The sky has the color blue" in RDF is as a triple of specially formatted strings: a subject denoting "the sky", a predicate denoting "has the color", and an object denoting "blue".
This mechanism for describing resources is a major component in what is proposed by the W3C's Semantic Web activity: an evolutionary stage of the World Wide Web in which automated software can store, exchange, and use machine-readable information distributed throughout the web, in turn enabling users to deal with the information with greater efficiency and certainty. RDF's simple data model and ability to model disparate, abstract concepts has also led to its increasing use in knowledge management applications unrelated to Semantic Web activity.
For further reading, read the 2001 article in Scientific American titled The Semantic Web, and the more recent article The Semantic Web in Action.
Additional W3C entries on SWRL, which combines OWL and the RuleML markup language will give you additional background in these areas.
An application of XML and RDF that facilitates machine learning and description of social networks is FOAF, which you can also generate from the FOAF-a-matic page.
Learning to use RSS, RDF, and XHTML metadata
The sample files below will introduce you to the grammar and vocabulary of RSS, RDF, and more formal expressions of XHTML metadata.
Section 1:
- RSS - Podcast climate example .
- Basic structure - (put XML code here for the channel and item recordsets).
- Example files - basic RSS tags RSS_1.xml , a channel definition RSS_2.xml , an RSS file with five recordsets RSS_3.xml , an RSS file with RDF declaration RSS_4.xml , and the same file with five RDF declarations RSS_5.xml.
Section 2:
- RDF - rdf from Informaticus site.
- Basic structure (put XML code here)
- Example files - RDC_RDF.rdf and RDC_RDF.txt and Informaticus_RDF.rdf and RDC_RDF.txt (view the text files if your Web browser can't view RDF). Right click to download if needed, and open in Notepad if your web browser doesn't support these formats (it likely will not).
Section 3:
- XHTML has the capacity to add metadata.
- Syntax of metadata is complex, with very few standards. There is a syntax for FOAF which is a form of RDF metadata.
- Example files of XHTML metadata are complex. I have included an XHTML-RDF file as well as an assertion language example. Here is my FOAF file foaf.rdf and in a second language foaf.xrdf . You can build your own FOAF file at the FOAF Project wiki.
