Web Client Programming with Perl: Chapter 1: Introduction
A relationship in which two computers, usually a server and a client, communicate across a network. Usually one computer manages or supplies services to the. A web client is an application that communicates with a web server, using Hypertext The student may flunk biology, but at least he'll be the first to know who won the Figure demonstrates the relationship between web clients and web servers. However, Chapter 4 does show some examples of writing simple clients. Client–server model is a distributed application structure that partitions tasks or workloads The client-server characteristic describes the relationship of cooperating programs in an application. The server component provides a function or.
The Web seems to have permanently captured the imagination of the world. It brings up visions of vast archives that can now be made globally available from every desktop, images and multimedia that can be distributed to every home, and But the soul of the Web is pure and unchanged.
BBC Bitesize - GCSE Computer Science - Network hardware - Revision 7
When you get down to it, it's just about sending data from one machine to another--and that's what HTTP is for. With a browser, you can download web documents and view them formatted on your screen.
When you request a document with your browser, you supply a web address, known as a Universal Resource Locator or URL. The URL identifies the machine that contains the document you want, and the pathname to that document on the server. The browser contacts the remote machine and requests the document you specified.
After receiving the document, it formats it as needed and displays it on your browser.
- Client-Server Model
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- Client–server model
For example, you might click on a hyperlink corresponding to the URL http: Your browser contacts the machine called www. When the document arrives, the browser formats it and displays it on the screen. If the document requires other documents to be retrieved for example, if it includes a graphic image on the pagethe browser downloads them as well.
But as far as you're concerned, you just clicked on a word and a new page appeared. Clients and Servers Your web browser is an example of a web client.
The remote machine containing the document you requested is called a web server. The client and server communicate using a special language a "protocol" called HTTP. Figure demonstrates the relationship between web clients and web servers. Client and server relationship To keep ourselves honest, we should get a little more specific now. Although we commonly refer to the machine that contains the documents as the "server," the server isn't the hardware itself, but just a program that runs on that machine.
The web server listens on a port on the network, and waits for client requests using the HTTP protocol. After the server responds to the request using HTTPthe network connection is dropped and the browser processes the relevant data that it received, then displays it on your screen.Servers: What are they? Client - Server Relationship explained - Blonde Dictionary
In practice, many clients can be using the same server at the same time, and one client can also use many servers at the same time see Figure If you master HTTP, you can request documents from a server without needing to go through your browser. Similarly, you can return documents to web browsers without being limited to the functionality of an existing web server. HTTP programming takes you out of the realm of the everyday web user and into the world of the web power user.
But before your client can actually communicate with a server, it needs to establish a connection. Now, this is when we separate our audience into the lucky and the. One of the great virtues for which Perl programmers are extolled is laziness.
The Perl community encourages programmers to develop modules and libraries that perform common tasks, and then to share these developments with the world at large. Only for those of us on UNIX, though.
For now, however, NT users have to cope with what's out there. But on the brighter side, NT's Perl environment is getting better over time. Also, some readers may be stuck with Perl 4, in which case LWP is off limits. Many Internet Service Providers do not support software "extras" like Perl, and thus will not upgrade the version of Perl 4 that was distributed with their operating system. Perl 4 is considered unsupported and buggy by most Perl experts, but for many readers, it's all they have.
However, Chapter 4 does show some examples of writing simple clients using Sockets, for those readers who cannot use LWP or choose not to. A Word of Caution There are some dangers in developing and configuring Web client programs.
A buggy client program may overload a web server. It could cause massive amounts of network traffic.
Or you might receive flame mail or lawsuits from web maintainers. Worst of all, web clients could cause data integrity problems on servers by feeding bad data to Common Gateway Interface CGI programs that don't bother to check for proper input.
To avoid these disasters, there are a few things you can do: Test your code locally. The ideal environment for web development is a machine running both the web client and the web server. When you use this type of setup, communication between the client and server doesn't actually go though a network connection. Instead, communication is done locally by the operating system. If the computer dramatically slows down shortly after running your newly written client, you know there's a problem.
Such a program would be even slower over a network. Run your own server. Many excellent servers are freely available on the Internet, and it is far better to accidentally overload your own server than the one used by your Internet Service Provider ISP or company.
When you finally decide to run your client program with someone else's server, leave your "verbose" options on and watch what your program is doing. Make sure you designed your program so you can stop it if it is getting out of hand.
The sharing of resources of a server constitutes a service. Whether a computer is a client, a server, or both, is determined by the nature of the application that requires the service functions. For example, a single computer can run web server and file server software at the same time to serve different data to clients making different kinds of requests.
Client software can also communicate with server software within the same computer. Client and server communication[ edit ] In general, a service is an abstraction of computer resources and a client does not have to be concerned with how the server performs while fulfilling the request and delivering the response. The client only has to understand the response based on the well-known application protocol, i.
Clients and servers exchange messages in a request—response messaging pattern. The client sends a request, and the server returns a response.
This exchange of messages is an example of inter-process communication. To communicate, the computers must have a common language, and they must follow rules so that both the client and the server know what to expect. The language and rules of communication are defined in a communications protocol. All client-server protocols operate in the application layer. The application layer protocol defines the basic patterns of the dialogue.
To formalize the data exchange even further, the server may implement an application programming interface API. By restricting communication to a specific content formatit facilitates parsing.
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By abstracting access, it facilitates cross-platform data exchange. A computer can only perform a limited number of tasks at any moment, and relies on a scheduling system to prioritize incoming requests from clients to accommodate them.
To prevent abuse and maximize availabilityserver software may limit the availability to clients. Denial of service attacks are designed to exploit a server's obligation to process requests by overloading it with excessive request rates. Example[ edit ] When a bank customer accesses online banking services with a web browser the clientthe client initiates a request to the bank's web server. The customer's login credentials may be stored in a databaseand the web server accesses the database server as a client.
An application server interprets the returned data by applying the bank's business logicand provides the output to the web server. Finally, the web server returns the result to the client web browser for display. In each step of this sequence of client—server message exchanges, a computer processes a request and returns data. This is the request-response messaging pattern. When all the requests are met, the sequence is complete and the web browser presents the data to the customer. One context in which researchers used these terms was in the design of a computer network programming language called Decode-Encode Language DEL.