This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs for the special pre-order price, click here for more information. Aimed at everyone from enterprise developers to self-taught scripters, Real-World AJAX: Secrets of the Masters is the perfect book for anyone who wants to start developing AJAX applications.

Browsing Applications and the Role of AJAX

AJAX - An Overview
AJAX isn't new. It's a combination of a number of existing technologies acting together. Namely:

• XHTML and CSS for standards-based presentation
• Document Object Model for dynamic display and interaction
• XML and XSLT for data interchange and manipulation
• XMLHttpRequest for asynchronous data retrieval
• JavaScript to tie everything together

Until AJAX came along, it wasn't easy to replicate the rich and responsive interaction design of native applications. AJAX is different from other previous attempts to address this problem since it's based on existing non-proprietary standards that are already familiar to developers.

In traditional Web applications, most user action triggers an HTTP request. The server does some processing and returns the result back to the user. While the server is processing, the user waits.

The "start-stop-start" nature of Web applications is good from a technical standpoint but not from a user standpoint since almost all user interaction results in trips to the server and the user waits while the server does the work.

AJAX solves this problem by using an AJAX engine. At the start of the session, the AJAX application loads the AJAX engine. The AJAX engine is written in JavaScript as a JavaScript library and sits in a hidden frame. The user interacts with the AJAX engine instead of the Web server. If the user interaction doesn't require a trip to the server, the AJAX engine handles the interaction on its own. When the user interaction needs some data from the server, the AJAX engine makes a call asynchronously on an XMLHttp request without interrupting the user's flow.

In this sense AJAX is asynchronous because the AJAX engine is communicating with the server asynchronously to the user interaction. So the user gets a seamless experience; in other words he's not waiting.

That's AJAX in a nutshell as we understand it.

AJAX on Mobile Devices
Now how does AJAX differ on mobile devices?

By definition, it doesn't.

In other words, as expected, a browser that supports AJAX applications on the mobile Internet will also support XMLHttp, JavaScript, CSS, and an AJAX engine.

As of May 2006, very few mobile browsers supported AJAX, but this is changing and we expect most vendors and device makers to support AJAX.

However, it's important to understand that mobile devices need more than the mere ability to support AJAX components. Support for AJAX components is necessary but isn't a sufficient condition to create new applications.

To appreciate this, we have to understand the technical limitations of the mobile browsing model. The browser model is document-centric; it's based on mark-up languages. In contrast, downloaded and native applications are application-centric since they're based on a programming language.

To be really useful, any mobile application development model must be able to access data elements that are tightly coupled to the device. These include the telephony API, phone book, text messages, messaging API, call records, SIM card, calendar, Bluetooth stack, media player, file system, and so on.

Applications running on the phone can access these services through APIs. For the most part, applications running on browsers can't access these functions except for a few proprietary solutions. Besides supporting AJAX components like JavaScript and XMLHttp, we also need an additional software component running on the device that abstracts the device APIs.

Currently only Opera browsers have announced support for AJAX but we expect others to follow suit (www.opera.com). The Opera platform provides an AJAX engine and access to device APIs (www.opera.com/products/mobile/platform/). The Opera platform is a browser-based programming environment that abstracts the native device APIs through a set of JavaScript APIs and so provides developers with access to the low-level functions on the device from the browser.

It also provides an application framework for developers to build their own mobile applications. We'll discuss the Opera platform and its APIs in greater detail when we discuss mobile applications development.

Widgets and the Disruptive Potential of AJAX
AJAX has some obvious advantages on the mobile Internet. AJAX is significant because it has momentum and the support of the developer community. It also provides a richer user interface. AJAX makes browsing applications richer and bridges the gap between the Web on the Internet and on the mobile Internet.

However, the disruptive potential of AJAX lies in its enabling widgets.

What's a widget?

Widgets aren't new. They have two meanings in computing. They can mean the components of the graphical user interfaces that the user interacts with (for example, radio buttons or combo boxes). Alternately, they can also refer to small "helper" applications.

We're concerned with the latter, the helper-type applications. These helper-type application widgets were originally seen on the Mac OS and they do small standalone tasks. Although widgets are very simple applications, the widget model is popular because it's easy to develop widgets using a few lines of JavaScript/VBScript.

Critically, from our perspective, the popularity of AJAX has led to a resurgence of interest in widgets. Of course, AJAX isn't the only path to creating widgets. Widgets can be implemented using mechanisms such as Yahoo widgets, formerly called Konfabulator (http://widgets.yahoo.com/), and other methods.

Increasingly widgets are also supported at the operating system and browser levels. Mac OS X 10.4 supports widgets via the Dashboard. Microsoft Windows Vista will provide widgets by Windows Sidebar and, in February 2006 Opera announced support for widgets in Opera 9. In Linux, SuperKaramba lets you put widgets on your desktop (www.superkaramba.com ).

Support for widgets at the mobile-browser level is very interesting for developers. The Opera platform is the first mobile AJAX framework. It also has a corresponding browser framework. However, unlike other frameworks, it's fully designed for mobile devices in the sense that it uses the same code base on the browser and the mobile device.

With minor configuration changes, the desktop/browser widget can also run on the mobile device. From a developer's perspective, there are more ways than one to monetize the widget (desktop, mobile, and browser).

Support for AJAX widgets at a browser level makes mobile widgets very disruptive.

AJAX is an open standard and conforms to the overall ethos of Web 2.0. Widgets can also call other widgets. Complex applications can be developed from simple widgets. Finally, AJAX widgets are quick to develop and easy to deploy and are capable of drawing revenue from the so-called "Long Tail."

In most situations, 80% of the revenue comes from 20% of the products/services. The remaining 80% of the products have low demand and low sales. These constitute the so-called "long tail". The principle of harnessing the long tail argues that collectively these low-volume/low-sales products can add up to market share that equals or exceeds the few bestsellers, provided the distribution channel is large enough and the per-unit production cost is low. The long tail is depicted in Figure 4.5 with the lighter shade

AJAX-based widgets can be used to develop quick and cheap applications for the long tail.

So to recap:

• Widgets built on AJAX are based on open standards and are part of Web 2.0.
• Because AJAX widgets can be deployed on the desktop, browser, and a mobile device with minor modifications, the developer can use the same code base to create applications that can earn revenue from three sources: desktop, browser, and mobile.
• Widgets can call other widgets, so complex applications can be developed from simple components.
• AJAX widgets are quick to develop and easy to deploy. They are capable of contributing revenue from the "long tail."

This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs, click here to order.