4.5 Distributed Computing

4.5.2   Client/Server Architecture

The client/server architecture is a way to dispense a service from a central source. A server provides a service and multiple clients communicate with the server to consume that service. In this architecture, clients and servers have different roles. The server’s role is to respond to service requests from clients, while a client’s role is to issue requests and make use of the server’s response in order to perform some task. The diagram below illustrates the architecture.

The most influential use of the model is the modern World Wide Web. When a web browser displays the contents of a web page, several programs running on independent computers interact using the client/server architecture. This section describes the process of requesting a web page in order to illustrate central ideas in client/server distributed systems.

Roles. The web browser application on a Web user’s computer has the role of the client when requesting a web page. When requesting the content from a domain name on the Internet, such as www.nytimes.com, it must communicate with at least two different servers.

The client first requests the Internet Protocol (IP) address of the computer located at that name from a Domain Name Server (DNS). A DNS provides the service of mapping domain names to IP addresses, which are numerical identifiers of machines on the Internet. Python can make such a request directly using the module.

The client then requests the contents of the web page from the web server located at that IP address. The response in this case is an HTML document that contains headlines and article excerpts of the day’s news, as well as expressions that indicate how the web browser client should lay out that contents on the user’s screen. Python can make the two requests required to retrieve this content using the module.

Upon receiving this response, the browser issues additional requests for images, videos, and other auxiliary components of the page. These requests are initiated because the original HTML document contains addresses of additional content and a description of how they embed into the page.

An HTTP Request. The Hypertext Transfer Protocol (HTTP) is a protocol implemented using TCP that governs communication for the World Wide Web (WWW). It assumes a client/server architecture between a web browser and a web server. HTTP specifies the format of messages exchanged between browsers and servers. All web browsers use the HTTP format to request pages from a web server, and all web servers use the HTTP format to send back their responses.

HTTP requests have several types, the most common of which is a request for a specific web page. A request specifies a location. For instance, typing the address into a web browser issues an HTTP request to port 80 of the web server at for the contents at location .

The server sends back an HTTP response:

On the first line, the text indicates that there were no errors in responding to the request. The subsequent lines of the header give information about the server, the date, and the type of content being sent back.

If you have typed in a wrong web address, or clicked on a broken link, you may have seen a message such as this error:

It means that the server sent back an HTTP header that started:

The numbers 200 and 404 are HTTP response codes. A fixed set of response codes is a common feature of a message protocol. Designers of protocols attempt to anticipate common messages that will be sent via the protocol and assign fixed codes to reduce transmission size and establish a common message semantics. In the HTTP protocol, the 200 response code indicates success, while 404 indicates an error that a resource was not found. A variety of other response codes exist in the HTTP 1.1 standard as well.

Modularity. The concepts of client and server are powerful abstractions. A server provides a service, possibly to multiple clients simultaneously, and a client consumes that service. The clients do not need to know the details of how the service is provided, or how the data they are receiving is stored or calculated, and the server does not need to know how its responses are going to be used.

On the web, we think of clients and servers as being on different machines, but even systems on a single machine can have client/server architectures. For example, signals from input devices on a computer need to be generally available to programs running on the computer. The programs are clients, consuming mouse and keyboard input data. The operating system’s device drivers are the servers, taking in physical signals and serving them up as usable input. In addition, the central processing unit (CPU) and the specialized graphical processing unit (GPU) often participate in a client/server architecture with the CPU as the client and the GPU as a server of images.

A drawback of client/server systems is that the server is a single point of failure. It is the only component with the ability to dispense the service. There can be any number of clients, which are interchangeable and can come and go as necessary.

Another drawback of client-server systems is that computing resources become scarce if there are too many clients. Clients increase the demand on the system without contributing any computing resources.

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