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So we can see current push technologies of extensions of earlier accomplishments. Like Email, they are both digital and network-based. Unlike Email, the modern delivery mechanism is modeled after the metaphor of telecasting than those of digital information access and delivery. Our technical lexicon for push-phase information access has grown from radiobroadcasting, telecasting, re-broadcasting, simulcasting, broadcasting and narrow casting to include "netcasting", "web casting", "group casting", "personal casting." One wonders how long politicians will be able to resist the temptation of "spincasting," he said, wryly.
Push technology is so current and so widely discussed that it behooves us to take this first pass at investigating this new multi-casting phenomena.
Dynamic updating, at least in Netscape's sense, took one of two forms: server push and client pull. Server push refreshed information displayed on the client through pre-determined, timed, server-initiated transmissions of HTML documents. However, this approach is server-invasive, requiring special server-side executables to create and deliver the refresh stream, and has, not surprisingly, fallen into disuse.
Client pull, on the other hand remains in use within the Netscape community for the display of constantly-updated HTML pages. Unlike server push, client pull requires no special programs to operate. The Web browser client initiates an HTTP connection and request for information from a server when it sees a particular token of the <META> tag in an HTML document. To illustrate, the tag
<META http-equiv="refresh" content="5; url=http://www.widget.com">
would cause a pull-compliant browser to refresh the current browser window with the document at http://www.widget.com 5 seconds after loading the current page. Without a URL specified, the browser will refresh itself with a re-load of the current page. The "pull" is shut off as soon as a document is reached which does not have a refresh <META> tag. (Interactive demonstration server-push and client-pull, along with a slew of other browser features and enhancements, may be found in the World Wide Web Test Pattern).
In either case, the idea is a simple one: provide data downloads without requiring user intervention. However, server-push and client-pull are deficient in one major respect: they are context and content insensitive. That is, all accesses to a URL - whether pushed or pulled - produce the same results for all users at any given moment in time. This context/content insensitivity became the bete noir of Netscape's dynamic updating technology because it produced an information access and delivery system that wasn't scalable. The delivery of numerous, complex, timely and personalized documents require as many URL's as there are documents. In order to minimize information overload, some mechanism needed to be created to build the content and context sensitivity into the push technology, itself.
In current push environments, the content is handled via "content identification" while the context is recognized via "channels." In this way, downloads are pre-filtered on servers and organized, consolidated and distributed as coherent streams. Also, push-phase access is desktop-compatible and stand-alone, rather than browser-centric through either a Web client launchpad or plug-in. This liberates both end-user and information provider from dependence on Web-based clients. Today's info-pushers are really in the business of content, developing the technology they need to deliver it.
Currently, the push paradigm is "targeted and solicited" (with a caveat, below). Individuals voluntarily "subscribe" to network information providers, perhaps through intermediate push delivery systems. Pointcast, Wayfarer, and BackWeb illustrate this paradigm, although their business models are different (Pointcast's business model is advertising-based, while Wayfarer's and BackWeb's is based on subscriptions). However, in each case, users connect to a central server which connects them with a variety of different Internet and intranet information feeds which are filtered and screened. These techniques rely on desktop perusers which function as autonomous windows. In fact, Pointcast was the first to deploy the peruser as a screen saver toward the end of encouraging information uptake even during workstation idle time. BackWeb expands on this theme by adding wallpaper and flash to the screen saver option. Both BackWeb and Wayfarer invest heavily in information filtering though information re-packaging and bridging technologies. However, at its core, variations of this type of push technology use a model not unlike that used by the cable television industry offering pay-per-view. The caveat is that most push-phase technologies are actually "automated client-poll," which means that the information is still pulled by the client software. Since the end-user isn't directly involved, push still seems an appropriate term to use.
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Marimba Corporation's (www.marimba.com) approach to solicited push is different. Here an analogy with direct broadcast television satellite systems is closer. Marimba's proprietary client-server software, Castanet, allows the end-user to connect to an arbitrary number of third-party server "transmitters" from the client side. The connection between a Castanet client "tuner" and each of the Castanet server "transmitters" is called a "channel". In basic terms the channel amounts to network access to some server's file structure. Anyone can create their own channel which will bring them updates of changed information. Where Pointcast is a 1:many network transmitter, Castanet is many:many. (see Figure 3) Marimba also adds UpdateNow SDK to their offering. An UpdateNow-enabled transmitter will automatically download new releases of software as they become available. If a way could be enhance this technology to remove obsolete applications as well, problematic legacy apps might become a thing of the past.
At this writing push technology developers are complementing their peruser technology with advanced proxy server and firewall software to minimize network traffic and information duplication on the intranet's server while increasing its throughput. Security and encryption is also being built in as are expanded intelligent agency and brokerage capabilities to increase the content coherence on the channels.
Push-phase information technology offers several advantages over manual, pull-phase information distribution technologies:
Currently, media-rich content channels are widely available for push technology. However, they tend to cluster into a few, non-compatible formats. Consider that Netscape's Netcaster is based upon Marimba's Castanet technology are Java-based, while Microsoft's Webcaster uses a proprietary Channel Definition Format. These are fundamentally different approaches to the mediation of content. In the former case, content is considered to be "executable," while the latter uses a more traditional "filter-based" delivery system by using the CDF descriptors to do the mediation via meta-information files. The difference cannot be overstated, for it involves two radically different approaches to network information delivery.
This compatibility-issue is but one of many which will determine the effectiveness of push technology for today's and tomorrow's commercial needs.
While it remains to be seen whether push-phase information access and delivery is the latest "silver bullet," one thing is clear: the concept of filtered and channeled multimedia is here to stay. In the absence of extensive information-theoretic studies of the recall, precision, fallout and generality of the output, there's not much that can be said at this point because the network brand of push technology remains in its infancy. Empirical studies of this sort are of immense practical importance and will no doubt follow in due course.
Further, there are both social and privacy issues which are just now being discussed. We do not know how to measure the long term efficacy and behavioral effects of "passive viewing" (where end-users might stare at a push channel window - perhaps a stock ticker - while on the telephonel, or working with another workstation application). Speculation ranges from a "numbing" effect to a highly-useful form of rapid information uptake. In addition, society needs to deal with the issue of the delivery of "sensitive" (perhaps pornographic, inflammatory, etc.) material. It is worth remembering that everything that can be pulled can also be pushed. Finally, commercial interests will inevitably drive this technology toward "unsolicited" push so that vendors may advertise through existing distribution channels. To illustrate the point, Pointcast advertisements are not filterable - they are an integral part of the feed. Perhaps we should concern ourself with the question "Will push lead to shove?"
In the end, our skepticism about push technology is based on a simple axiom which for want of another term we'll call Boyle's Law for Cyberspace: data will always fill whatever void it can find. The primary lemma is that even if network overload can be avoided, information overload of the end-user will always remain. Such being the case, client-side solutions such as information customization (cf. http://www.acm.org/~hlb/publications/cb5/cb5.html) must be sought. We will follow up with a technical comparison of push and pull alternatives in Digital Village at a later date. Until then, some of the major vendors of push technology are accessible through the URLs in the sidebar.