TABLE OF CONTENTS As explained in the introduction of this chapter, "push" content is sent from server to client spontaneously, without an explicit request. There are many types of push content. Radio and television broadcast, e-mail alerts, newsletters, SMS welcome messages, spam mail, and pop-up advertisements in Web browsers are all examples of push content.
In fact, it is not entirely correct to define push content as "content delivered without explicit request." Many benign forms of push content are provided after a user subscribes to it. It is more accurate to say that push content does not involve a strict request reply dialogue between client and server.
Payable push content is normally subject to prior subscription, for the simple reason that the content provider needs to establish a billing relationship with the subscriber. By analogy to radio and television broadcast, push content on the Internet is often organized in channels. The push client software tunes into these channels by subscribing to the corresponding server for particular information. The difference with radio and television is, of course, that a push client can receive information from as many channels at the time as the user wants.
Internet push content survived a short hype at the end of the 1990s, with companies like Microsoft, PointCast, and Marimba proposing new ways of sending content from servers to clients on the Internet. Many Web-based push technologies, like Microsoft's Active Channel technology, were in fact not strictly push but really intelligent and automated...
