An In-Depth Guide to Web Applications
A web application is a computer programme that uses web browsers and web technology to perform internet-related tasks.
This programmes contains pages of indeterminate content, in part or in whole.
The final content of a page is not determined until the user requests a page from the web server.
As the final content of the page varies from request to request depending on the visitor's actions, this type of page is called a dynamic page.
Web applications are created in response to various needs or problems.
Here are some of the most web applications, with examples:
Common uses of web applications
· Enable users to quickly and easily find information on a website where a large amount of content is stored
This type of web application gives visitors the ability to search for content, organize it, and navigate through it in any way they deem most appropriate. Examples include corporate intranets, Microsoft MSDN and Amazon.com.
Collect, store and analyze data provided by site visitors
In the past, data entered into HTML forms was emailed to employees or CGI applications for processing.
A web application allows you to save data from forms directly to a database, as well as extract data and create reports online for analysis.
These include, for example, online banking pages, online store pages, surveys and forms containing data provided by users.
Update websites with constantly changing content
A web application saves the site designer from having to continually update the site's HTML code.
Content providers, such as news publishers, deliver the content to the web application, and the web application updates the site automatically.
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How a web application works
A web application is a collection of static and dynamic web pages.
A static web page is a page that does not change when a user requests it: the web server sends the page to the requesting web browser without modification.
Rather, the server modifies dynamic web pages before sending them to the requesting browser.
The changing nature of this type of page is what gives it the name dynamic.
For example, you can design a page to display health program results and leave out certain information (like employee name and results) to calculate when the page is requested by a particular employee.
The following sections describe in more detail how web applications work.
Processing of static web pages
A static website is made up of a collection of pages and related HTML files hosted on a computer equipped with a web server.
A web server is software that provides web pages in response to requests from web browsers.
A page request is generated when a user clicks a link on a web page, chooses a bookmark in a browser, or types a URL in the Address text box of the browser.
The designer writes each line of HTML code on the page before placing it on the server.
The HTML code does not change once it is placed on the server, so this type of page is called a static page.
Strictly speaking, a static page may not be static at all.
For example, a replacement image or Flash content (a SWF file) can bring a static page to life.
However, this guide refers to a static page when it is sent to the browser without modification.
When the web server receives a static page request, it reads it, locates it, and sends it to the requesting browser, as the following example shows:
Processing of static web pages
A. The web browser requests the static page.
B. The server locates the page.
C. The web server sends the page to the requesting browser.
D. In the case of web applications, some lines of code are not determined when the user requests the page. These lines must be determined by some mechanism before the page is sent to the browser.
Processing dynamic pages
When a web server receives a request to display a static web page, the server sends it directly to the requesting browser.
However, when the web server receives a request to display a dynamic page, it reacts in a different way: it transfers the page to special software which takes care of finishing it.
This special software is called an application server.
The application server reads the code from the page, completes the page as instructed in the code, and removes the code from the page.
The result is a static page that the application server sends back to the web server, which in turn sends it to the requesting browser.
The only thing the browser receives when the page arrives is pure HTML code.
A view of this process is included below:
Processing dynamic pages: A Step by Step
A. The web browser requests the dynamic page.
B. The web server locates the page and sends it to the application server.
C. The application server looks for instructions on the page and completes it.
D. The application server transmits the completed page to the web server.
E. The web server sends the completed page to the requesting browser.
Access to a database
An application server allows you to work with server-side resources, such as databases.
For example, a dynamic page can ask the application server to extract data from a database and insert it into the HTML code for the page.
Using a database to store content helps separate the design of the website from the content you want to show to users of the site.
Instead of writing individual HTML files for each page, just write one page - or template - for the different types of information you want to present.
You can then upload the content to a database and have the website retrieve the content in response to a user's request.
You can also update the information in a single source and then apply that change to the entire website without having to manually edit each page.
You can use Adobe Dreamweaver to design web forms that insert, update, or delete data from the database.
The statement to extract data from a database is called a database query. A query consists of search criteria expressed in a database language called SQL (Structured Query Language). The SQL query is written in scripts or tags on the server side of the page.
An application server cannot communicate directly with a database because the format of the database prevents data decryption, just as a Microsoft Word document cannot be decrypted when opened with Notepad or BBEdit.
The application server can only communicate with the database via a driver which acts as an intermediary with the database: the software then acts as an interpreter between the application server and the database.
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