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Summary In this chapter you learned that… . Information feeds use Internet protocols such as HTTP and TCP/IP. . MCF and CDF were precursors of RSS and Atom. . Several versions of RSS are still in widespread use. Exercises 1. Visit Syndic8.com and check out the usage of different flavors of language used in information feeds registered with Syndic8. Use the Search facilities on the Feeds page. This will allow you to see whether usage of different versions of RSS and Atom have changed much since this chapter was written. 2. In your favorite aggregator(s) look for the pieces of information that were displayed in Figures 2.1 and 2.2 to see how the tools you favor handle the information from the elements inside an item element. 21 Where Did Information Feeds Start?
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Use of RSS and Atom Versions The use of RSS 0.91 has persisted surprisingly well, considering that it was nominally replaced by version 0.92 in December 2000 and RSS 2.0 in September 2002. This chapter was written in October 2004, when RSS 0.91 remains in widespread use. Total information feeds at the time of writing are thought to be approaching 10 million, of which maybe half are active. In order to gain some impression of the relative usage of various versions of RSS and Atom, we used the feeds listed on the Syndic8.com Web site. Given that the numbers of registered feeds is small relative to the total estimated number of feeds, there has to be some uncertainty about exact proportions of usage of the different versions. RSS 0.91 remains a frequently used information feed technology among the feeds listed on Syndic8.com. RSS 0.91, 1.0, and 2.0 and Atom 0.3 all have widespread usage. RSS 0.92 has significant usage but perhaps an order of magnitude less than the other four technologies just mentioned. The following table summarizes one set of data, gleaned by exploring Syndic8.com, about the usage of different versions of Atom and RSS current at the time of writing. Technology Version Number of Feeds Atom Pre-IETF 20,470 RSS 0.91 41,444 RSS 0.91fn 2 RSS 0.92 1,489 RSS 0.92d2 0 RSS 1.0 15,801 RSS 2.0 37,771 Total Feeds 80,129 You may notice that the total number of feeds, 80,129, is less than the sum of the feeds for individual versions. We are not sure as to the cause of the discrepancy. In part it may arise because some feeds provide information in more than one format. However, the purpose of showing the table is simply to capture the relative market share of different flavors of RSS and Atom late in 2004. Despite the uncertainties about the figures, however, it seems clear that Atom 0.3 (the pre-IETF version), RSS 0.91, RSS 1.0, and RSS 2.0 are the dominant information feed technologies. RSS 0.92 is fifth in the pecking order, an order of magnitude lower than RSS 1.0. If you want to do a similar analysis on Synic8.com to estimate how market share has changed since this chapter was written, visit the Syndic8.com Web site, select the Feeds link, and use the search facilities to specify format and version to explore how numbers have changed. 20 Chapter 2
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RSS 0.9 RDF was pivotal to RSS 0.9, as the name RDF Site Summary indicates. After some resistance to the perceived complexity of RDF, RSS 0.91 made an appearance. RSS 0.91 The term RSS 0.91 is used for two distinctly different flavors of RSS. First, historically, was the Netscape version, which was RDF Site Summary. A little later, when UserLand became involved, RSS was then Really Simple Syndication. The UserLand version of RSS 0.91, which is described in more detail in Chapter 8, continues in widespread use. It was designed to be simple to use by the standards of June 2000. Its simplicity made it easy for automated tools to create RSS 0.91 feeds which, no doubt, contributed to its initial and continuing popularity. RSS 1.0 The way that RSS 0.9x progressed, or failed to progress according to your viewpoint, meant that many in the information feed community who had been attracted by the potential of the RDF-based Netscape flavor of RSS were looking for a flavor of RSS that had the metadata and semantic Web possibilities that RDF brought to the scene. The enthusiasm for the possibilities for more-sophisticated use of metadata than was possible with RSS 0.91 led to the development of the RDF-based RSS 1.0. RSS 1.0 is described in more detail in Chapter 9. RSS 0.92, 0.93, and 0.94 Three further 0.9x versions of RSS were specified by UserLand. Of those three versions, version 0.92 was released only a couple of weeks after the RSS-Dev Working Group had released the RSS 1.0 specification. At that point UserLand was championing RSS 0.92, Really Simple Syndication, and the RSS-Dev Working Group was championing RSS 1.0, RDF Site Summary. By any realistic criteria the RSS world was a political minefield and technically, at least for the newcomer to the minefield, a confusing mess. Given the politics and differences in technical approach of the different versions of RSS it is amazing that there has been such rapid growth in adoption of information feeds. The relative simplicity for an aggregator author creating tools that can accept feeds in essentially any of the flavors of RSS (and later Atom) has likely been a significant factor ensuring the survival of the varied formats used for information feed documents. Versions 0.93 and 0.94 are essentially of historical interest only. RSS 2.0 RSS 2.0 was released in September 2002. Its lineage is the UserLand 0.9x family. A newcomer to the field might assume that RSS 2.0 was the replacement for RSS 1.0, which is not the case. The two specifications exist in parallel. RDF was part of RSS 0.9 and RSS 1.0. RSS 0.91, 0.92, 0.93, 0.94, and 2.0 did not use RDF. 19 Where Did Information Feeds Start?
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We will return to the various versions of RSS in a moment after a brief look at a similar Microsoft technology of that era, the Channel Definition Format (CDF). The Microsoft Channel Definition Format Like RDF Site Summary 0.90, CDF used the then-embryonic XML. A March 1997 Microsoft document, located at www.w3.org/TR/NOTE-CDFsubmit.html, contains a description of CDF. Like RSS, CDF had a channel element which, among many other child elements, included a number of item elements. CDF was intended to serve a similar purpose as Netscape s RSS. Microsoft envisaged users subscribing to channels that Microsoft would supply. My recollection of CDF was that it felt as if it were information being pushed at me by Microsoft. The use of the term webcasting by Microsoft contributed to that feeling. I didn t feel that it was information that I was choosing. It felt much more as if it were information that Microsoft was telling me that I should have. I was, at the time, very resistant to that notion. I suspect that others shared that sort of feeling, and uptake of CDF was poor. In the end Microsoft abandoned CDF. RSS: An Acronym with Multiple Meanings The following sections briefly describe some salient points about the various versions of RSS that were proposed or released. The history is a little confusing, not least because the versions were not released in the order that the version numbers might suggest. The following table summarizes the chronology. Technology Version Date Company RSS 0.9 March 1999 Netscape RSS 0.91 June 2000 UserLand RSS 1.0 December 2000 RSS-Dev Working Group RSS 0.92 December 2000 UserLand RSS 2.0 September 2002 UserLand Atom 0.3 December 2003 Mark Nottingham We think there is a very significant lesson to be learned there for creators of information feeds today. The feed creator needs to give the users the feeling that they are in control. Some creators of feeds are currently attempting to push advertisements into feeds in a way that disguises those advertisements. In time, users will rebel against that approach, just as they have done with pop-up advertisements in Web pages. With so many choices of information available to each user, any feed creator who attempts to push unwanted information at users will quickly lose his audience. 18 Chapter 2
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Precursors to RSS This section briefly describes technologies that led to RSS and Atom. Some of the technologies mentioned contained key concepts that have been carried forward into RSS and Atom. MCF and HotSauce One way of looking at information feeds is as metadata that informs users about the content available for viewing. Work on information feeds, in that sense, began to reach a wider audience in the mid-1990s. The Meta Content Framework (MCF) was intended to provide information about content available on Web sites, on FTP sites, and in relational and other databases. MCF was intended to document the relationships among objects. It was a research project, associated with Apple, but it was intended to have practical outcomes. The MCF research project led to a product, HotSauce, which allowed Apple users to browse Web sites that had MCF data. In 1996, HotSauce seemed set for growth, but a change of management at Apple led to a loss of momentum in the MCF project and the associated HotSauce metadata browser. Some key staff then left the project. While the MCF project at Apple languished, the concepts of MCF were developed further in an XML context to create the Resource Description Framework (RDF). Netscape Channels One of the individuals associated with Apple s MCF project was Ramanathan Guha, who moved to Netscape around the time that support for MCF from Apple management lessened. Around the same time interest in the embryonic XML specification was growing. The XML specification became a W3C Recommendation in February 1998. In addition, MCF was one of the technologies used to formulate the RDF specification at the W3C. RDF used XML as one of its serialization formats. RDF was used by Netscape in version 0.9 of RSS. At that time RSS stood for RDF Site Summary. For reasons that aren t entirely clear to the outside observer, Netscape lost interest in RSS, partly perhaps because of unease about how RDF did or didn t fit into an information feed document. At the time RDF was pretty new, and many who created software for the Web were only beginning to adjust to the existence of XML. Also, taking RDF on board seemed to some a step too far to expect software authors to accept. In the end My Netscape used RSS 0.91, which didn t use RDF. The My Netscape portal site closed in April 2001. At the time of writing, a copy of the My Netscape documentation relating to RSS 0.90, apparently copied from the Google cache of the original, was accessible at www.purplepages.ie/RSS/netscape/rss0.90.html. Information about MCF is located at www.xspace.net/hotsauce/mcf.html, and information about HotSauce is located at www.xspace.net/hotsauce/. 17 Where Did Information Feeds Start?
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The appearance of the same information feed item in RSS Bandit is shown in Figure 2-2. As you can see, the contents of the item element are processed in a noticeably different way. In the highlighted line in the upper pane, the contents of the title, dc:date, and dc:subject elements are displayed. The content of the description element is displayed in the lower pane. The content of the link element is displayed toward the top left of the RSS Bandit window. The content of the dc:creator element is displayed toward the upper part of the lower pane, together with the content of the dc:date and title elements, all laid out in a way that makes for ease of reading by a human reader. The information feed simply supplies a number of feed items. In RSS 1.0 they are contained in item elements and their associated child elements. How an aggregator or similar software module processes and displays those basic pieces of information is what serves to differentiate one aggregator from another. As we discuss elsewhere, ease of use of aggregators and ease of customization are likely to be significant determining factors differentiating successful from less successful aggregators. They all receive the same information from the RSS and Atom feeds to which a user subscribes. Figure 2-2 16 Chapter 2
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Figure 2-1 As you can see in the following code snippet, the information item contains information about a blog post. From that information the aggregator creates a display. The content of the title element is displayed in the highlighted line in the upper part of Thunderbird, together with the content of the dc:creator and dc:date elements. The content of the dc:date element is formatted in a way that makes it more acceptable to the human reader. Thunderbird doesn t display the content of the link element or the description element. The content of the link element is a URL used to retrieve the blog post and display it in the bottom pane of Thunderbird.
http://www.tfosorcim.org/archives/000277.html Naturally, during the writing of Beginning RSS and Atom, I am giving a lot of thought to information feeds, their characteristics and what they might (and should) become. Not ignoring the problems they can realistically solve. I was interested in… Information Feeds Andrew Watt 2004-11-25T12:11:09+00:00 15 Where Did Information Feeds Start?
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HTTP The existence of a physical network infrastructure was, of course, essential to being able to move information feeds around once they were created. In addition a transport protocol, or pair of transport protocols (in this context TCP/IP), needed to be in place. All information on the Web is sent in TCP/IP packets. On a physical system that s not totally reliable, the use of TCP/IP produces a remarkable approximation to reliability. Over those protocols HTTP, the Hypertext Transport Protocol, is in place. When information feeds are retrieved by a browser or by an aggregator, retrieval is done using HTTP. For example, the URL for the information feed for my blog is located at http://www.tfosorcim.org/blog/index.rdf. HTTP is used in retrieving the RSS 1.0 document, which constitutes the information feed document for the blog. If the physical and software infrastructure had not been in place, it would not have been possible to implement generally available information feeds, at least not in the way that they were implemented. HTML HTML, the HyperText Markup Language, was also a key part in the use and acceptance of information feeds. HTML, and more recently XHTML, has been the display format chosen for the Web pages represented as items in an information feed. Converting RSS (and later Atom) information feeds to HTML was a relatively straightforward task. Scripting languages, such as Perl, were familiar to many programmers. And software modules were created which allowed the creation of HTML (or XHTML) from RSS and, later, Atom. Some software modules use XML parsing (discussed in more detail in the next section), whereas others treated an information feed as text. When faced with a malformed feed that didn t meet the well-formedness constraints of XML (discussed in Chapter 6), the use of regular expressions can support appropriate handling of the character sequences in the information feed and an HTML page can be created that the aggregator or a Web browser can display to the user. XML If an RSS or Atom feed has been created correctly, then the information feed document should be wellformed XML. In that case the feed document can be parsed using widely available XML parsers and each component part of the feed document can be displayed according to choices specified by the author of the aggregator or by the user. XML software tools are available for many platforms. For example, XSLT transformations can be used to transform all or part of the feed document in ways to make it suitable for display. Polling the Web An aggregator polls a specified URL, using HTTP, to find an information feed. Once the feed document has been retrieved, it is parsed and, depending on the approach taken by the aggregator writer, all or parts of the retrieved information is displayed to the users. Figure 2-1 shows the appearance of the information feed from www.tfosorcim.org/blog/index.rdf in Mozilla Thunderbird 0.9. 14 Chapter 2
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2 Where Did Information Feeds Start? The precise date when information feeds started is debatable depending on which aspects of information feeds or similar technologies are viewed as seminal. This chapter takes the view that information feeds started with the My Netscape portal, which aggregated content from many sources in a portal that could be customized by the user. The My Netscape portal provided a way for users to have a customized Web page where the information that interested them was what they saw. On the vast majority of Web sites at that time the information presented to a user was standardized for all users and what the user saw was the information that the Web site author chose. For users to be able to define the information that was presented to them was a fairly radical departure from the previous situation on the Web, whereby users were largely passive recipients of information. Even when, the user saw only Web pages with non-customized content there was limited interactivity, such as clicking on hyperlinks and, of course, a user could choose which sites to visit. However, when users were able to influence or determine page content, things began to change. In this chapter you learn about: . The history of information feeds . The history of the various versions of RSS The Nature of the Web My Netscape didn t occur in a vacuum. Several other technologies and factors were required to be in place for the customization of content to be possible. Many factors are essential to the creation and delivery of information feeds. Some of those factors are related to the invisible infrastructure of the Web; others to its more visible parts.
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146 Chapter 6 Similarly, the following Hourly class extends the Employee class using the extends keyword. public class Hourly extends Employee { public float hourlyRate; //Pay rate public float hoursWorked; //Weekly hours worked public float computePay() { System.out.println( Computing hourly pay for + name); float pay = 0.0F; if(hoursWorked <= 40) { pay = hourlyRate * hoursWorked; } else //Need to compute overtime { pay = (hourlyRate * 40) + (hourlyRate * (hoursWorked - 40) * 1.5F); } return pay; } } A child class has access to the fields and methods in the parent class, depending on the access specifier, which is discussed in Chapter 7, Advanced Java Language Concepts. The computePay() method of the Salary class displays the name of the employee being paid, but there is no name field in the salary class. The name field is in Employee, the parent of Salary. In this example, because name is public, Salary has acces to it and can use it at any point in the Salary class. Notice that the Hourly class prints out the employee s name within its computePay() method, using the name field inherited from the Employee class. Instantiating Child Objects Now that we have defined the Employee, Salary, and Hourly classes, let s look at a program that instantiates and uses these classes. The following Inherit- Demo program creates an Employee, Salary, and Hourly object. Study the program carefully and try to determine its output. public class InheritDemo { public static void main(String [] args) { System.out.println( Instantiating an Employee );
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