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Web application

From Academic Kids

In software engineering, a web application is an application delivered to users from a web server over a network such as the World Wide Web or an intranet. Web applications are popular due to the ubiquity of the web browser as a client, sometimes called a thin client. The ability to update and maintain web applications without distributing and installing software on potentially thousands of client computers is a key reason for their popularity. Web applications are used to implement webmail, online retail sales, online auctions, wikis, discussion boards, weblogs, and perform many other functions.

In earlier types of client-server computing, each application had its own client program which served as its user interface and had to be separately installed on each user's personal computer. An upgrade to the server part of the application would typically require an upgrade to the clients installed on each user workstation, adding to the support cost and decreasing staff efficiency.

In contrast, web applications dynamically generate a series of web pages in a standard format supported by common browsers such as HTML/XHTML. Client-side scripting in a standard language such as JavaScript is commonly included to add dynamic elements to the user interface. Generally, each individual web page is delivered to the client as a static document, but the sequence of pages can provide an interactive experience, as user input is returned through web form elements embedded in the page markup. During the session, the web browser interprets and displays the pages, and acts as the universal client for any web application.

The web interface places some limits on client functionality. Application-specific methods such as drawing on the screen, and more general-purpose techniques such as drag and drop are not supported by standard browser technology. Web developers often use client-side scripting to add functionality, especially to create an interactive experience that does not require page reloading (which many users find disruptive). Recently, technologies have been developed to coordinate client-side scripting with server-side technologies such as PHP; AJAX, a technology that works with JavaScript, is an example.

A signficant advantage of building web applications to support standard browser features is that they should perform as specified regardless of the operating system or OS version installed on a given client. Rather than creating clients for Windows, Mac OS X, Linux, and other operating systems, the application can be written once and deployed almost anywhere. However, inconsistent implementations of the HTML, CSS, DOM and other browser specifications can cause problems in web application developement and support. Additionally, the ability of users to customize many of the display settings of their browser (such as selecting different font sizes, colors, and typefaces, or disabling scripting support) can interfere with consistent implementation of a web application.

Another (less common) approach is to use Macromedia Flash or Java applets to provide some or all of the user interface. Since most web browsers include support for these technologies (usually through plug-ins), Flash- or Java-based applications can be implemented with much of the same ease of deployment. Because they allow the programmer greater control over the interface, this overcomes many browser-configuration issues, but incompabilities between Java or Flash implementations on the client can introduce different complications. Because of their architectural similarities to traditional client-server applications, with a somewhat "thick" client, there is some dispute over whether to call systems of this sort "web applications"; an alternative term is "rich internet application".

Though many variations are possible, a web application is commonly structured as a three-tiered application. In its most common form, a web browser is the first tier, an engine using some dynamic web content technology (e.g., CGI, PHP, Java Servlets or Active Server Pages) is the middle tier, and a database is the third tier. The web browser sends requests to the middle tier, which services them by making queries and updates against the database and generating a user interface.

Web interfaces have increasingly been used for applications that have previously been thought of as traditional, single-user applications. For example, Microsoft HTML Help replaced Windows Help as the primary help system in Microsoft Windows. Like their networked brethren, such applications generate web pages as their user interface and send them (sometimes via an embedded HTTP server) to a local web browser component, which then renders the pages for the user and returns user input to the application. Web applications powered by embedded web servers have also become commonplace as the user interfaces for configuring network components such as servers, routers, and gateways.

An emerging strategy for application software companies is to provide web access to software previously distributed as local applications. Depending on the type of application, it may require the development of an entirely different browser-based interface, or merely adapting an existing application to use different presentation technology. These programs allow the user to pay a monthly or yearly fee for use of a software application without having to install it on a local hard drive. A company which follows this strategy is known as an application service provider (ASP), and ASPs are currently receiving much attention in the software industry.

See also

External links

es:Aplicación web it:Applicazione web

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