Analysis and Testing of Ajax-based Single-page Web Applications

Figure 1.7 Screen shot of Google Documents, a web-based word processor,
spreadsheet, presentation, and form application.
connection is established between client/server and the server is not required
to maintain any state information from the clients.
This pull-based (polling) style of interaction, although scalable, has limitations
for applications that require fast data delivery to the clients. Examples
of such applications include auction web sites where the users need to be
informed about higher bids, web-based stock tickers where stock prices are
frequently updated, multi-user collaboration applications, web-based chat applications,
or news portals.
An alternative to the traditional pull-based approach is the push-based
style, where the clients subscribe to their topic of interest, and the server publishes
the changes to the clients asynchronously every time its state changes.
In 1995, Netscape introduced a method (Netscape, 1995) for pushing data
on the web through HTTP Streaming, by using a special MIME type called
multipart/x-mixed-replace. This method simply consists of streaming server
data in the response of a long-lived HTTP connection that is kept open by
server side programming.
The push-based approach has recently gained much attention, thanks to
many advancements in client and server web technologies that make pushing
data from the server, in a seamless manner, possible.
Comet (Russell, 2006) is a neologism to describe this new model of web
data delivery. Although Comet provides multiple techniques for achieving
high data delivery on the web, the common ground for all of them is relying
on standard technologies supported natively by browsers, rather than on
proprietary plugins. Generally, Comet (also known as Reverse Ajax) applications
rely on Ajax with long polling (see Chapter 4) to deliver state changes
to the clients, as fast and reliable as possible. Well-known examples include
Chapter 1. Introduction 11