What's next after AJAX?

The rapid spread of the term AJAX (asynchronous JavaScript and XML) -- from Weblog to Wall Street Journal within weeks! -- might lead developers to assume it’s a breakthrough that heralds the death of desktop applications. There’s certainly a kernel of truth in that: The recent spate of new Web applications under the AJAX banner have redefined end-users’ expectation of what’s even possible within a Web browser by offering smooth scrolling, incremental updates, and more responsive input forms.

Nevertheless, so-called fat-client UIs still retain one fundamental advantage over Web UIs: real-time event notification. AJAX alone does not address IM, stock tickers, and other collaborative applications that require “push” data streaming.

The key goal of AJAX-style applications is to decompose jarring transitions that download an entire new Web page into a series of smaller, more frequent transactions. Developers consider AJAX to be “asynchronous” because data can be updated without interrupting the user. For example, Google Maps dramatically reduces the perceived latency of scrolling a map by only downloading the newly visible tiles and moving the rest.

In the middleware community, however, the formal definition of asynchrony refers to the capability of sending a message at any time, in either direction. AJAX provides the upstream direction, but HTTP would appear to make server-initiated transmission impossible.

Fortunately, clever developers have exploited a loophole in HTML to address this. Browsers are designed to display pages incrementally while downloading pages from slow Web sites. Using hidden frames and JavaScript tags, HTTP can be used to hold open a long-lived response connection, allowing an application to stream data into the browser.

The simplest way to exploit this is to turn the browser into a 21st-century “green screen” dumb terminal. Manuel Kiessling’s open-source ARSC (A Really Simple Chat) uses AJAX techniques to send input lines upstream, whereas a modified HTTP server that holds open thousands of simultaneous connections rebroadcasts the chat stream to other users. Another example is KnowNow’s SpeedReader product, which is useful for alerting employees to relevant RSS news items.

The subtler and broader implication of combining AJAX with asynchronous event notification is to extend publish-and-subscribe application integration across the Internet.

Several open-source platforms provide powerful abstractions for connecting fully interactive Web UIs to enterprise applications and Web services. Nevow (née Woven LivePage) and Pushlets extend the event loop familiar from model-view-controller GUIs for Python and Java, respectively. Mod_PubSub is designed as an event bus that uses URL path names as topics to implicitly invoke programs written in a wide range of languages. Commercially, KnowNow’s LiveServer provides enterprise-class scalability (and even connects to and from Excel spreadsheets).

The clear benefits of migrating desktop applications to the Web in terms of maintenance, security, and scalability must be weighed against the costs of slower response times, limited interactivity, and less-than-beautiful graphical interfaces. With AJAX, push technology, and the ubiquitous plug-ins for PDF and Flash, the Web is closer than ever to becoming a viable default platform for application development.