Web Computing

8/2/2019 Web Computing

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Web ComputingWeb computing can be defined as a special kind of distributed computing that

involves internet-based collaboration of several remotely located applications.

Contrary to the "classical" distributed components that are static and rely upon a

fixed hardware configuration within a local area network, a new approachadvocates existence of dynamic functionalities that can be easily migrated and

executed on any machine connected to the Web. The new style of computing

requires a uniform run-time environment, broadband connections and flexible

software structures.

With the arrival of the Internet and the Web, we can now access information, even

do business from virtually anywhere. For this fulfillment, there are many

challenges ahead like designing and developing faster, lighter and more robust

applications that can be delivered across the Web using HTTP protocol. This sort

of applications are called Web applications. The tools and the technologies fordeveloping, delivering and deploying Web applications.

Web Architecture

The architecture is very simple and straightforward. To view a web page in

our browser, we have to type the URL for that page or click an existing hyper-link

to that URL. Once we submit this request and the web server receives this, the web

server locates the web page from its memory or from its hard disk and sends it

back to the browser. The browser then displays the page. Each image in the page isalso referenced by a URL and the browser requests each image URL from theserver in the same way it requested the main HTML page.

The Web Browser

The web browser can be thought of as a universal graphical user interface.

whether we are doing some simple web browsing or transacting online banking,

the web browser's responsibilities are that of presenting web content, issuing

requests to the web server and handling any results generated by the request. There

are two main web browsers in the market. They are Microsoft Internet Explorerand Netscape Communicator. Both brings some incredible power on the client side

and evolved into fully programmable document containers. Each has its own object

model allowing for scripts, or objects, to manipulate the elements of the document

itself. Scripting languages like VBScript or JavaScript can be used to perform

client-side data validation or provide some interactivity within the document.


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Dynamic HTML (DHTML) is a combination of HTML, Cascading Style

Sheets (CSS), the document object model, and scripting languages. CSS are a

better way of positioning and formatting HTML elements. And since each property

of the style sheet is made visible to the object model, we can use some script to

manipulate and reposition HTML elements. DHTML, as a whole, provides a

greater level of interactivity within our pages. It also adds much more control overthe presentation of them as well.

The latest must-have browser feature is Extensible Markup Language

(XML) support. XML allows us to define our own tag set to characterize our data,

and to construct documents and data structures using these tags. XML provides a

way for structured data to be self-describing, and that means the data can be

portable. Also there is XHTML, the combination of XML with HTML.

Furthermore, with Extensible Style-Sheets Language (XSL), we can select the data

we want to view, and even change tag names, allowing us to transform XML tagsto HTML. Depending upon the type of browsers, the XSL will change XML: toHTML for web browsers, to WML for WAP browsers etc.

The web browser is also capable of executing applications within the same

context as the document on view. The tow most popular choices for client-side

web applications are Microsoft's ActiveX technology and Java applets. ActiveX

components are downloaded from the web server, registered with the Windows

registry, and executed when called upon by other script elements. A Java applet is

a small Java program also downloaded from the web server, and executed within

the browser's own Java Virtual Machine (JVM). Both ActiveX objects and Javaapplets have full access to the browser's document object model and can exchange

data between the browser and themselves. Thus the web browser always serve the

purpose of being our 'window to the world'. It serves as our primary user interfaceas we browse the web, conduct online business or even play games.

The Web server

The web server is the heart of any web interaction. The web server is a

software program running on the server machine that listens for incoming requests

from the web browser and serves those requests at any time. Once the web server

receives a request, it then springs into action. Depending on the type of request, the

web server might look for a web page, or it might execute a program on the server.

Either way, it will always return some kind of results to the web browser, even if itmight be an error message.
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Currently there are two top web servers. They are the Apache web server

and Microsoft's Internet Information Server (IIS). The first one has been developed

as free software and has been contributed to by programmers around the world. Its

power, flexibility, ease of use and the availability for multiple platforms has

immensely contributed to its rise in popularity over the past few years. Microsoft's

IIS, on the other hand, runs on the Windows NT and Windows 2000 operating

systems. While it offers a wide range of features, its dependence on the Windows

platform is being a major drawback. After the advent of the Linux operating

system, the usage and popularity of the Apache web server gets multiplied. Thus

the web server is poised to play a very important role in our server-side

applications.

Web Application Architecture

A web application typically follows a three-tiered model. The first tierconsists of the presentation layer which, in the case of web application, includes

not only the web browser but also the web server, which is responsible for

assembling the data into a presentable format. The second tier is the application

layer. It usually consists of some sort of script or program. Finally the third tier

provides the second tier with the data that it needs. A typical web application will

collect data from the user (first tier), send a request to the web server, run the

requested server program (second and third tiers), package up the data to be

presented in the web browser, and send it back to the browser for display (first

tier).

Collecting the data - The first step of a web application usually involves collecting

some kind of data from the user using a simple HTML form. The user would type

some information into some form fields and the information will be taken to the

web server for being processed and finally the result will be sent back to the user.

There is an alternative mechanism for this using Java applets, which can be used as

a client to a server-side program by simply opening up a socket connection to the

web server. This approach can help to move the majority of data formatting and

validation off the server and onto the client. This approach reduces the volume of

network traffic to a major extent. Sending the Web Server a Request - In order forthe web server to spring into action and execute a server program, the web browser

needs to package up the user data and issue an HTTP request to the web

server HTTP/HTTPS is a simple, stateless, lightweight application-level protocol

generally implemented over TCP/IP connections. The HTTP is a stateless protocol

based on requests and responses. It was originally meant for serving static

information. In this paradigm, client applications, such as a Web browser, send


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requests to the servers, such as a Web server, to receive information or to initiate a

specific processing on the server. As an application level protocol, HTTP defines

certain types of requests that clients can send to servers. The protocol also specifies

how the requests and responses be structured. The latest version HTTP/1.1, in

addition to GET, POST and HEAD, has five additional request methods:

OPTIONS, PUT, TRACE, DELETE, and CONNECT. Of these, the GET and

POST requests meet most of the common application development needs.

The GET request method is the simplest and most frequently used request

method for accessing static resources such as HTML documents, images etc. GET

requests can also be used to retrieve dynamic information by using additional

query parameters in the request URL. For example, I can send a parameter

name=peter appended to a URL as http://www.peterindia.com?name=peter. The

Web server can use this parameter name=peter to send the content specific to

"peter".

The POST request method is normally being used to access dynamic

resources. POST requests are meant to transmit information that is request

dependent, and are used when we need to send large amounts of complex

information to the Web server. The POST request helps to encapsulate multi-part

messages into the request body. For example, we can use POST requests to upload

text or binary files. Also POST requests can be used to serializable Java objects to

the Web server through our applets. Thus POST requests offer a wider choice in

terms of the contents of a request.

There are certain differences between GET and POST requests. With GET

requests, the request parameters are transmitted as a query string appended to the

request URL. In the case of POST requests, the request parameters are transmitted

within the body of the request. Since a GET requests contains the complete request

information appended to the URL itself, it allows browsers to bookmark the page

and revisit later. If the parameters to be passed happen to be sensitive, then the

GET method may be not suitable. Also some Web servers, not complying with

HTTP/1.1, may put some restrictions on the length of request URL.

These two method requests are set to be idempotent, that is, they are not

programmed to modify the information on the Web server. An idempotent request

can be reapplied without changing any data on the server side. These are used for

pure information retrieval. Along with the type of the request, the client applicationalso specifies the resource that it needs as part of the request header.


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HTTP Response - For a HTTP request from a Web browser, the Web

server responds with the status of the response, and some meta-information

describing the response. All these are part of the response header. Except for the

case of the HEAD method request, the server also sends the body content that

corresponds to the resource specified in the request. The body content is actually

what we want to get from the Web server. But the content header fields in the

response contain useful information, such as "Date", "Content-Type", "Expires",

etc.

In HTTP, servers and clients use MIME (Multi-Purpose Internet Mail

Extensions) to indicate the type of content in requests and responses. Examples of

MIME types are text/html, image/gif, etc. Here the first part of the header indicates

the type of data, such as text and image, while the second part indicates the

standard extension, such as html for text and gif for image. MIME is an extension

of the email protocol to allow exchange of different kinds of data files over theInternet. HTTP servers use MIME headers at the beginning of each transmission.

Web browsers use this information to decide how to parse or render the response.

Web browsers also use MIME headers while transmitting data in the body of

requests, to describe the type of data being sent. That is all about the HTTP

request-response process.

Executing the Server Script or Program - An important function of the web

server is that of passing a request to a specific script, or program to be processed.

The web server first determines which type of operating environment it needs to

load by looking at the extension of the requested file or the directory the file islocated in. This is being done through mapping. When a web server is configured,

it is told how to handle specific file types. For example, typically anything in the

cgi-bin directory will be treated as a CGI script or anything with a .asp or .jsp

extension will be treated as a Active Server Page or a Java Server Page

respectively.

Once the web server determines the type of the requested file, it then loads

any required runtime environment for the file to be executed. For example, if a

CGI program were written in Perl, the web server would create a new process andload the Perl interpreter into it. For some types of programs depending on the web

server, there is no necessity for loading a separate runtime environment.

Returning the Results to the Browser - The final step in a web application

is to make some kind of response to the operation and return that to the browser.

Generally speaking, the server script specifies the content type and then writes the


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response to an output stream. When the web browser receives the response, it will

first look at the response header and determine the MIME type so that it knowshow to render the data.

Server-Side Web Programs

A server-side web program has some important exceptions from a regular

program. To make a program accessible to a web server, it must possess thefollowing characteristics:

i. When a user issues a request from a web browser, the web server has to

be able to locate and execute the requested program

ii. There must be a way in which the web server passes any form data to theprogram.

iii. Once the program is invoked, there has to be a standard entry point

iv. After the program has processed the input data, it has to package up the

results and send them back to the web server which will, in turn, send them back to

the web browser. The exact division of responsibility may vary for different web

servers. But all web servers just talk HTTP.


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Web Computing