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Here the aggregator is subscribed to three feeds: A, B, and C. The oldest item in each is number 1, the newest number 6. The aggregator is only looking at four items, which will have appeared in chronological order: 1, 2, 3, and 4. From the aggregator s point of view the oldest item of interest is Item 6 from Feed A and the newest is Item 6 from Feed B. The aggregator is in effect skimming off the most recent items from its subscribed feeds. Another benefit of the item-oriented model is that it makes it possible to create composite or so-called synthetic feeds. An online system can aggregate content from various different sources and then republished the combined material as a whole new feed. This makes it possible to create a multiauthor feed on a particular topic, or (as PubSub.com) build a feed dynamically from search results. In Figure 20-6, there is again an aggregator skimming off the most recent items from a number of feeds. The difference is that the aggregator here is building a composite feed from those items, which it publishes as an entirely separate feed, Feed D. Figure 20-6 Title Item 1 Date Content Title Item 2 Date Content Title Item 3 Date Content Title Item 4 Date Content Feed A Feed Metadata 1 2 3 4 5 6 Feed D Feed Metadata 1 2 3 4 5 6 Feed B Feed Metadata 1 2 3 4 5 6 Feed C Feed Metadata 1 2 3 4 5 6 Aggregator 249 Modeling Feed Data
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Figure 20-4 The most immediate advantage of the entry-oriented model is that it makes it possible for a user tool to group and sort items according to different criteria than the document order provided by a specific feed. The most likely criteria will be that of publication date and time, so that your aggregator displays the newest item first. The flow of items through your system might look something like what s shown in Figure 20-5. Figure 20-5 Title Item 1 Date Content Title Item 2 Date Content Title Item 3 Date Content Title Item 4 Date Content Feed A Feed Metadata 1 2 3 4 5 6 Feed B Feed Metadata 1 2 3 4 5 6 Feed C Feed Metadata 1 2 3 4 5 6 Aggregator Title Day 1 Feed Feed Metadata Item 1 Date Content Title Item 2 Date Content Title Item 3 Date Content Title Item 4 Date Content Day 2 248 Chapter 20
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From the document-oriented view the redundancy and opacity could be seen as bugs in the system. However, if we turn things around into a different model, magically the bug becomes a feature and what was opaque becomes transparent. Item-Oriented Models The document-oriented view is a fairly direct analogy of the traditional paper publication. But rather than consider the feed as a package, if you think of it more like a source of continual information, it takes on some different characteristics. Twenty-First Century Press Clippings One of the less glamorous jobs in the old media of newspapers is clipping. For a very long time, certain publishers and press clipping services have offered a selective view of relevant media to their customers. Say your company is in the environment-friendly lawn mower business. You ll probably want to know what s going on elsewhere in the world of environment-friendly lawn mowers. You could read every newspaper and periodical in your area or even internationally, but that wouldn t leave much time for lawn mowers. So you could get some poor souls to do the reading for you, extracting any articles or advertisements of interest with a pair of scissors. At the end of every week you d get a pile of individual clippings, all about lawn mowers. The material in such a system would be treated at the granularity level of the articles of course you d want to know the source of each clipping, but that information is auxiliary to the article itself. Afeed can be thought of as a kind of clipping service, except the clippings are available in real time, immediately after they appear. Your aggregator will visit each feed periodically to collect the clippings. Where this analogy is weak is in that current systems are rarely as specialized or personalized as a press clipping service. Such services do exist: PubSub.com will provide a feed based on the results of a custom search query. In fact, the following is the address of a feed of search results on the keywords environment-friendly lawn mowers : http://rss.pubsub.com/37/b5/3778cec38f25c9f8282681622a.xml PubSub is ahead of most of the field in this respect, but in general the idea of treating individual clippings (that is, syndicated items) as standalone, self-contained pieces of information is very powerful. Microcontent and Memes Information can appear in many formats, and traditional print media tends to favor large chunks: books, magazines, and academic papers tend to be multipage collections of text that cover wide areas. However the tendency on the Web has been toward smaller pieces, and the newsflash or blog post represents the ultimate in granularity. Essentially you will have a single idea or meme, with a few lines of human-readable content backed by several pieces of machine-readable metadata. It is believed that this approach to information will offer better personalization (you only get the information you want) supported by more efficient search and organization of these small chunks of information. The stereotype of this is of course the syndicated item. Being able to extract individual items from a feed enables the manipulation of information at this meme level. Which leads us to the entry or item-oriented model of feed data. Figure 20-4 shows the general idea. Items are detached from their originating feed, although they commonly will include a back reference to the source feed. The aggregator in its truest sense is a tool that aggregates items from a variety of sources. 247 Modeling Feed Data
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Figure 20-3 If you wanted your system to have a memory and keep track of the feed as it changed over time, the document- oriented model would suggest taking a snapshot of the document between every change. However, if you save what s in the feed on Day 1 and Day 2 as in Figure 20-3, then you ll have duplicate data corresponding to Item 2 and Item 3. In this little example that would mean a third of the item data (2/6 items) would be redundant. The data in feeds is usually text, and in these days of ever-cheaper storage this redundancy probably isn t a big deal. The Opaque Feed But there s a more significant problem. The information of interest on a given day is contained in the items that appear on that day. By wrapping up collections of items into documents, it makes it harder to see what s inside those items what you see are daily feeds, not items. In effect the document-oriented approach makes the individual pieces of content and metadata opaque to systems that might want to index or search the items. Title Day 1 Feed Feed Metadata Item 3 Date Content Title Item 2 Date Content Title Item 1 Date Content Day 2 Title Feed Feed Metadata Item 4 Date Content Title Item 3 Date Content Title Item 2 Date Content 246 Chapter 20
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The styling code here follows the CSS 2 specification. Not all browsers offer support for all CSS 2 features. In particular Internet Explorer will ignore the :before pseudo-element, with the result that the explanatory line will be missed out of the display. Another part of the CSS code that may not be obvious is this part: language, managingEditor, generator, image, guid, ttl, skipHours, skipDays, webMaster, lastBuildDate, updateBase, updateFrequency, updatePeriod, docs { display: none; } This again overrides the default display formatting (inline) for the listed elements, although this time rather than treating each as a block, the rendering style is none, and the instruction is not to render them at all. This is a handy way of dumping the more obscure elements that aren t likely to be of interest to the reader. Note that the
value is displayed verbatim in the browser you get the text of the URI but it isn t clickable. This specific fault is something that can be fixed by using XSLT style sheets, but it points toward a deeper problem. Drawbacks of a Feed-Oriented Model Treating syndication feeds as documents does suggest nifty tricks like CSS rendering, but this view overlooks some of the important characteristics of real feeds. As mentioned earlier, they don t necessarily have to include content. There may still be benefit in displaying a list of titles, links, even dates and the rest of the metadata, but this is stretching the idea of a document somewhat. A more significant drawback is that most syndication feeds are dynamic, changing every few days or perhaps even every hour. The Dynamic Feed The fact that a feed generally changes over time a lot more rapidly than a regular Web page is really only one consideration. In the example listing of Tech Idol it wasn t clear what the order of the items related to. If they were a kind of popularity chart, with the number one ranked Idol listed first, then the document-oriented approach is probably quite a good fit. Where the document model really goes awry is when the order of items in the feed is governed directly by time, such as in the reverse-chronological order of Weblogs and other news-like sources. Only a certain number of items can be realistically included in a feed, and the reverse-chrono order suggests new items got to the top of the feed. So what happens to the old items? Generally they drop off the bottom of the feed. This can be seen in Figure 20-3, which shows a feed that holds three items on two consecutive days. Here the numbering of the items corresponds to their creation order, so Item 1 was created first, 2 second, and so on. On Day 1 the newest item is Item 3, this is shown at the top of the feed. By Day 2 another item, Item 4 has been created. Being the newest, this now takes the position at the top of the feed. As this particular feed is designed to carry only three items at a time, the oldest item, Item 1, no longer appears on Day 2 (see Figure 20-3). 245 Modeling Feed Data
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The result of this particular piece of content with styling looks like what you see in Figure 20-2. Figure 20-2 The content of the