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Comparison between AJAX and Flex

Web Design & Development Guide

Comparison between AJAX and Flex

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Ajax and Adobe Flex are popular Web development technologies to create Rich Internet Applications. They can be used separately or in combination. Ajax developers often use an Ajax framework to speed up development. Flex developers use Adobe's tools for development. This article compares Ajax frameworks with Adobe Flex.

About this Comparison

Adobe Flex is a single-vendor product (proprietary to Adobe) while Ajax is a collection of techniques to create web applications that is not specific to a vendor. With Ajax the following options can be considered:

  • Custom Ajax (develop from scratch)
  • Open Source Frameworks
  • Commercial Frameworks

The comparison below will take these options into account when comparing Ajax with Flex.

Other RIA Technologies

There are more technologies that can be used for building RIAs, other than Ajax and Flex. A full list can be found in the ‘Methods and Techniques’ section in the Rich Internet Application article.


Ajax and Flex can also be used together. There are two tools available to facilitate this integration: the Flex Ajax Bridge and Ajax Data Services.

Comparison of Product Capabilities


Adobe Flex relies on the Flash 9 browser plug-in, which needs to be present in the browser of the website visitor. Ajax uses the various Internet browsers as its runtime. Some Ajax Frameworks have an additional JavaScript engine that abstracts away from differences in browser implementations: developers use the engine which in turn communicates with the browser.

The benefit of using a proprietary plug-in as Flash 9 is the controlled runtime environment, which is identical across all web browsers. This make development easier and it allows Adobe to add additional features and to improve performance. The downside is that the plug-in needs to be installed, which can pose a problem in environments with locked-down operating systems. Adobe periodically publishes data on the market penetration of the Flash Player. Some critics of flash, such as Jakob Nielsen[1], state that reliance on a plug-in as a break with web standards, as the web browser is only used to launch the player which does not use web standards such as HTML, CSS and JavaScript. The Flash plug-in offers some support for HTML, CSS and an extended version of JavaScript (ECMAScript).

The benefit of Ajax is that it runs in various web browsers, without the need for a plug-in. The downside is that there are differences between browser implementations of DOM, JavaScript and CSS, which can result in Ajax applications that work in one browser, but not in another. Ajax Frameworks mitigate this issue by offering a browser compatibility layer. Another downside of Ajax is that it requires JavaScript to be enabled in the web browser. It is possible to make Ajax applications degrade gracefully when JavaScript is disabled, but this requires additional application design.


Flex has a declarative development model, in which the application is defined with XML tags. Additional programming is done in ECMAScript. Styling can be added using CSS. Ajax can be programmed in JavaScript or with Ajax frameworks that offer a declarative programming model (e.g. Backbase). Ajax styling is also done with CSS. There are also server-side Ajax frameworks that allow Ajax programming with JavaServer Faces (e.g. Icesoft and Backbase) or .NET (e.g. Telerik and Infragistics).

Adobe Flex has its own IDE: Adobe Builder, which is based on Eclipse. There are many IDEs that can be used for Ajax development. They include:

  • Generic IDEs such as Eclipse and Visual Studio.NET
  • Dedicated Ajax IDEs such as Aptana
  • IDEs bundled with Ajax Frameworks

Some Ajax Frameworks develop their IDE on top of an existing IDE: for example, Backbase uses Eclipse. Other Ajax Frameworks develop their IDE in the web browser, often using their own framework, such as [[Tibco_Software|Tibco] GI.

Adobe bundles a framework for automated testing with Flex, while for Ajax most existing web page testing tools can be used.


RIA performance consists of various aspects, such as the size for the initial download, the speed of the runtime, and speed of data manipulation.

Flex applications have a minimum initial download size of approximately 125 kilobyte, which users may consider slow on dialup connections. Ajax programs can be very small, although most frameworks introduce a certain initial download size, ranging from several kilobytes to a megabyte. Flex applications are already compressed, while for Ajax, GZip compression on the web server is typically used to compress the files.

The performance of Ajax applications is based on the performance of the browser, which can vary from one browser to another. Frameworks such as Backbase and Jackbe have optimized the performance of their framework across browsers. Adobe optimizes the Flash Runtime for performance, and therefore has more control over performance. One of the benefits is that Flex can handle large data sets on the client-side, while Ajax requires more optimization to get good performance, especially in Internet Explorer[2]. The best Ajax frameworks can handle more client-side data than other frameworks, due to careful optimization strategies.


Adobe Flex comes with many user interface widgets in different skins. Custom Ajax does not come with pre-built widgets, but many Ajax frameworks do, ranging from a couple of widgets to over 100. Some Ajax widgets are stand-alone, while others are tightly integrated into an Ajax framework, using the framework’s underlying functionality for event handling, drag-and-drop, resizing, sorting, and so on. An underlying framework makes it easier to customize or extend widgets, and to create completely new widgets. Some Ajax frameworks come with multiple skins for the widgets.


Now, Flex SDK has been Open Source so it also can be extended by external code contributions. Create custom component in Flex is very easy. Ajax is typically more extensible, because Ajax runs in browsers instead of a proprietary runtime. Custom widgets can also be created with Ajax and Ajax frameworks simplify this task by providing reusable functionality.


Flex uses multimedia capabilities from Flash, such as vector graphics, animation, image manipulation, audio and video. Ajax does not have multimedia functionality built-in, but can use other browser functionalities and plug-ins such as SVG and Quicktime, or even the Flash plugin.

Server Communication

Ajax uses the web browser’s underlying communication mechanism, which is HTTP. This supports pulling data from the server, as well as pushing data to the client using Server Push. Remoting is supported by various Ajax frameworks such as DWR. Adobe Flex also supports binary communication.

SEO and Web Analytics

Ajax and Flex are often used to build web applications that are not very content-rich. However, if there is a fair amount of content that needs to be indexed by search engines, search engine optimization (SEO) techniques become relevant. Adobe Flex has limited support for SEO because content cannot be read directly by search engines, which instead use meta data and HTML shadow pages to index Flex applications. HTML within Ajax applications can be indexed by search engines, although search bots cannot follow links that are generated by JavaScript logic. Proper application design can make Ajax applications fully accessible for search engines. Web analytics typically works well with Ajax applications, as all HTML files that are loaded can be tracked, even if they are loaded into an existing page. What happens within a Flex application is hidden from web analytics tools.


Adobe Flex is Section 508 compliant. Some Ajax frameworks such as Bindows and Backbase are also Section 508 compliant, but most other Ajax applications are not.


Both Flex and Ajax run in the browser’s sandbox, and are generally seen as equally secure, though the Flash plug-in can introduce additional security risks. Most Ajax code is plain text and thus easy to read: this could make it easier for attackers to find vulnerabilities.

Non-Functional Comparison

User Experience

Adobe has attempted to make Flex as user friendly as possible. However, critics of Flash such as Jakob Nielsen[1] claim that Flash-based applications have inherent usability issues because visitors are used to using web pages as interfaces for web applications. Critics of Flash usability limitations such as the following:

  • Flash apps have a different feel: the cursor looks different, fonts look different, links and right-click does not work as expected
  • Copy and paste of text does not work
  • Back-button does not always work as expected
  • Developers are more likely to introduce unintuitive interface concepts

Flex proponents state that many of these issues can be addressed with proper application design and thorough usability tests.

Standards Support

Both Flex and Ajax support many web standards. Ajax uses a standards-based runtime (the web browser) that supports many standards out-of-the-box. Flex uses a proprietary runtime that has implemented support or partial support for many standards. Any standard that is available in web browsers is immediately available for Ajax applications, while Adobe has to implement these standards in its Flash runtime.

Vendor Support

Flex is supported by a single large vendor: Adobe. Ajax in general is not supported by a vendor, though some Ajax frameworks provide support and several open source Ajax projects are supported by their developer community on a voluntary basis. Most commercial Ajax vendors provide support plans, ranging from basic email support to 24x7 phone support with a SLA.


  1. ^ a b Flash: 99% Bad (2000-10-29).
  2. ^ http://dojotoolkit.org/pipermail/dojo-contributors/2005-July/000881.html

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This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.


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