A web application is an application that is accessed over a network such as the Internet or an intranet. The term may also mean a computer software application


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NameA web application is an application that is accessed over a network such as the Internet or an intranet. The term may also mean a computer software application
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Introduction


A web application is an application that is accessed over a network such as the Internet or an intranet . The term may also mean a computer software application that is coded in a browser-supported language (such as JavaScript, combined with a browser-rendered markup language like HTML) and reliant on a common web browser to render the application executable.

Web applications are popular due to the ubiquity of web browsers, and the convenience of using a 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, as is the inherent support for cross-platform compatibility. Common web applications include webmail, online retail sales, online auctions, wikis and many other functions.

A web application is a collection of static and dynamic web pages. Static web page as the name signifies is the one that does not change when a site visitor requests it. The web server sends the page to the web browser without any modifications. In contrast, a Dynamic web page is modified by the server before it is sent to the browser. The changing nature of the page is why it’s called dynamic.

History

In earlier computing models, e.g. in client-server, the load for the application was shared between code on the server and code installed on each client locally. In other words, an 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-side code of the application would typically also require an upgrade to the client-side code installed on each user workstation, adding to the support cost and decreasing productivity.

In contrast, web applications use web documents written in a standard format such as HTML and JavaScript, which are supported by a variety of web browsers. Web applications can be considered as a specific variant of client-server software where the client software is downloaded to the client machine when visiting the relevant web page, using standard procedures such as Http. Client web software update may happen each time the web page is visited. During the session, the web browser interprets and displays the pages, and acts as the universal client [2] for any web application.

In the early days of the Web each individual web page was delivered to the client as a static document, but the sequence of pages could provide an interactive experience, as user input is returned through web form elements embedded in the page markup.

In 1995 Netscape introduced a client-side scripting language called JavaScript allowing programmers to add some dynamic elements to the user interface that ran on the client side. So instead of sending data to the server in order to generate an entire web page, the embedded scripts of the downloaded page can perform various tasks such as input validation or showing/hiding parts of the page.

In 1996, Macromedia introduced Flash, a vector animation player that could be added to browsers as a plug-in to embed animations on the web pages. It allowed the use of a scripting language to program interactions on the client side with no need to communicate with the server.

In 1999, the "web application" concept was introduced in the Java language in the Servlet Specification version 2.2. [2.1?].[3][4] At that time both JavaScript and XML had already been developed, but Ajax had still not yet been coined and the XMLHttpRequest object had only been recently introduced on Internet Explorer 5 as an ActiveX object.[5]

In 2005, the term Ajax was coined, and applications like Gmail started to make their client sides more and more interactive. A web page script is able to contact the server for storing/retrieving data without downloading an entire web page.

In 2011 HTML5 was created, which provides graphic and multimedia capabilities without the need of client side plugins. HTML5 also enriched the semantic content of documents. The APIs and document object model (DOM) are no longer afterthoughts, but are fundamental parts of the HTML5 specification. WebGL API paved the way for advanced 3D graphics based on HTML5 canvas and JavaScript language. These have significant importance in creating truly platform and browser independent rich web applications.

[edit] Interface

Through Java, JavaScript, DHTML, Flash, Silverlight and other technologies, application-specific methods such as drawing on the screen, playing audio, and access to the keyboard and mouse are all possible. Many services have worked to combine all of these into a more familiar interface that adopts the appearance of an operating system. General purpose techniques such as drag and drop are also supported by these technologies. Web developers often use client-side scripting to add functionality, especially to create an interactive experience that does not require page reloading. Recently, technologies have been developed to coordinate client-side scripting with server-side technologies such as PHP. Ajax, a web development technique using a combination of various technologies, is an example of technology which creates a more interactive experience.

[edit] Structure

Applications are usually broken into logical chunks called "tiers", where every tier is assigned a role.[6] Traditional applications consist only of 1 tier, which resides on the client machine, but web applications lend themselves to a n-tiered approach by nature.[6] Though many variations are possible, the most common structure is the three-tiered application.[6] In its most common form, the three tiers are called presentation, application and storage, in this order. A web browser is the first tier (presentation), an engine using some dynamic Web content technology (such as ASP, ASP.NET, CGI, ColdFusion, JSP/Java, PHP, Perl, Python, Ruby on Rails or Struts2) is the middle tier (application logic), and a database is the third tier (storage).[6] The web browser sends requests to the middle tier, which services them by making queries and updates against the database and generates a user interface.

For more complex applications, a 3-tier solution may fall short, and it may be beneficial to use an n-tiered approach, where the greatest benefit is breaking the business logic, which resides on the application tier, into a more fine-grained model.[6] Another benefit may be adding an integration tier that separates the data tier from the rest of tiers by providing an easy-to-use interface to access the data.[6] For example, the client data would be accessed by calling a "list_clients()" function instead of making an SQL query directly against the client table on the database. This allows the underlying database to be replaced without making any change to the other tiers.[6]

There are some who view a web application as a two-tier architecture. This can be a "smart" client that performs all the work and queries a "dumb" server, or a "dumb" client that relies on a "smart" server.[6] The client would handle the presentation tier, the server would have the database (storage tier), and the business logic (application tier) would be on one of them or on both.[6] While this increases the scalability of the applications and separates the display and the database, it still doesn't allow for true specialization of layers, so most applications will outgrow this model.[6]

[edit] Business use

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.

In cloud computing model web applications are Software as a Service (SaaS). There are business applications provided as SaaS for enterprises for fixed or usage dependent fee. Other web applications are offered free of charge, often generating income from advertisements shown in web application interface.

[edit] Writing web applications

There are many web application frameworks which facilitate rapid application development by allowing the programmer to define a high-level description of the program.[7] In addition, there is potential for the development of applications on Internet operating systems, although currently there are not many viable platforms that fit this model.

The use of web application frameworks can often reduce the number of errors in a program, both by making the code simpler, and by allowing one team to concentrate just on the framework. In applications which are exposed to constant hacking attempts on the Internet, security-related problems can be caused by errors in the program. Frameworks can also promote the use of best practices[8] such as GET after POST.

[edit] Applications

Examples of browser applications are simple office software (word processors, online spreadsheets, and presentation tools), but can also include more advanced applications such as project management, computer-aided design, video editing and point-of-sale.


Web application terminology


Before knowing the concepts of Web application completely, it is essential to know all the terminologies employed in a web application. This section defines frequently used terms relating to web applications.

An application server is software that helps web server process web pages containing server-side scripts or tags. When such a page is requested from the server, the web server hands the page off to the application server for processing before sending the page to the browser. Common application servers include Macromedia Coldfusion, Macromedia JRun Server, the Microsoft .NET Framework, IBM Web Sphere, and Apache Tomcat.

A database is a collection of data stored in tables. Each row of a table constitutes one record and each column constitutes a field in the record, as shown in the following example.


database

FIG 1: Database model

A database driver is software that acts as an interpreter between a web application and a database. Data in a database is stored in a proprietary format. A database driver lets the web application read and manipulate data that would otherwise be undecipherable.

A database management system (DBMS or database system) is software used to create and manipulate databases. Common database systems include Microsoft Access, Oracle 9i, and MySQL.

A database query is an operation that extracts a record set from a database. A query consists of search criteria expressed in a database language called SQL. For example, the query can specify that only certain columns or only certain records be included in the record set. Using this database query we can effectively communicate with the database. Insert, Delete, Select and Update are the most commonly used commands using which we can insert a record, delete the records, select the records and update the records respectively.

A dynamic page is a web page customized by an application server before the page is sent to a browser.

A record set is a set of data extracted from one or more tables in a database, as in the following example:


recordsa

FIG 2: Record Set

In short, a record set is a set of data extracted from one or more tables in a database. The record set is returned to the application server, which uses the data to process the request..

A relational database is a database containing more than one table, with the tables sharing data. The following database is relational because two tables share the DepartmentID column.


join

FIG 3: Relational Database Illustration

A server technology is the technology that an application server uses to modify dynamic pages at runtime.

A static page is a web page that is not modified by an application server before the page is sent to a browser.

A web application is a website that contains pages with partly or entirely undetermined content. The final content of these pages is determined only when a visitor requests a page from the web server. Because the final content of the page varies from request to request based on the visitor's actions, this kind of page is called a dynamic page.

A web server is software that sends out web pages in response to requests from web browsers. A page request is generated when a visitor clicks a link on a web page in the browser, selects a bookmark in the browser, or enters a URL in the browser's address text box. Popular web servers include Microsoft Internet Information Server, Microsoft Personal Web Server, Apache HTTP Server, Netscape Enterprise Server, and Sun ONE Web Server.

Static and Dynamic Web Pages- In Brief


The majority of the web pages on the internet today are static HTML pages. HTML pages are simple text files that are displayed by the browser upon request. Essentially, static pages are those on the site that send exactly the same response to every request.


Dynamic web pages are more complex and usually give the website visitors easier access to the information that they need. Setting cookies, creating member login areas, pulling product data from a database that is easily updated, maintaining a web site visitors session data to remember products that they have selected.

Visitors find dynamically generated web pages to be useful because they are provided instant access to highly relevant information. Dynamic sites are also much easier to update and maintain. Adding new products and editing products in your online searchable catalog is as easy as filling out a form. This saves countless hours (and dollars) to update multiple static pages. Dynamic web pages are created using technologies like CGI, ASP, Cold Fusion, JSP, PHP etc.

URL's of dynamically generated pages often contain question marks (?) (Used in Coldfusion to pass the variables), percentage signs (%) and other symbols (&, + and $) or text (cgi-bin) making up the "query string".









Processing a Static Web Page


A static website comprises a set of related HTML pages and files hosted on a computer running a web server.

A web server is software that serves web pages in response to requests from web browsers. A page request is generated when a visitor clicks a link on a web page, selects a bookmark in a browser, or enters a URL in a browser's address text box.

The final content of a static web page is determined by the page designer and doesn't change when the page is requested. Here's an example:





Static Page- An example





About Static Pages




Static page output will never change.

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