Client-server

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Client server is network architecture which separates a client (often an application that uses a graphical user interface) from a server. Each instance of the client software can send requests to a server. Specific Types of servers include: application servers, file servers, terminal servers, and mail servers. While their purpose varies somewhat, the basic architecture remains the same.

Although this idea is applied in a variety of ways, on many different kinds of applications, the easiest example to visualize is the current use of web pages on the internet. For example, if you are reading this article on Wikipedia, your computer and web browser would be considered a client, and the computers, databases, and applications that make up Wikipedia would be considered the server. When your web browser requests a particular article from Wikipedia, the Wikipedia server finds all of the information required to display the article in the Wikipedia database, assembles it into a web page, and sends it back to your web browser for you to look at.


Characteristics of a server:

  • Passive (slave)
  • Waits for requests
  • Upon receipt of requests, processes them and then serves replies

Characteristics of a client:

  • Active (master)
  • Sends requests
  • Waits for and receives server replies

Servers can be stateless or stateful. A stateless server does not keep any information between requests. A stateful server can remember information between requests. The scope of this information can be global or session. A HTTP server for static HTML pages is an example of a stateless server while Apache Tomcat is an example of a stateful server.

The interaction between client and server is often described using sequence diagrams. Sequence diagrams are standardized in the UML.

Another type of network architecture is known as a peer-to-peer architecture because each node or instance of the program is both a "client" and a "server" and each has equivalent responsibilities. Both architectures are in wide use.

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[edit] Tiered architecture

A generic client/server architecture has two types of nodes on the network: clients and servers. As a result, these generic architectures are sometimes referred to as "two-tier" architectures.

Some networks will consist of three different kinds of nodes: server, application servers which process data for the clients, and database servers which store data for the application servers. This is called a three-tier architecture.

The advantage of an n-tier architecture compared with a two-tier architecture (or a three-tier with a two-tier) is that it separates out the processing that occurs to better balance the load on the different servers; it is more scalable. The disadvantages of n-tier architectures are:

  1. It puts a more load on the network.
  2. It is much more difficult to program and test software than in two-tier architecture because more devices have to communicate to complete a users transaction.

[edit] Addressing

Methods of addressing in client server environments can be described as follows

  • Machine process addressing; where the address is divided up as follows process@machine. Therefore 56@453 would indicate the process 56 on computer 453
  • Name Server; Name servers have an index of all names and addresses of servers in the relevant domain.
  • Localization packets; Broadcast messages are sent out to all computers in the distributed system to determine the address of the destination computer
  • Trader; A trader is a system that indexes all the services available in a distributed system. A computer requiring a particular service will check with the trading service for the address of a computer providing such a service.

[edit] Examples

Visiting a web site is a good example of Client-Server architecture. The web server serves the webpages to the web broser. Most Internet services are types of servers. Some examples are file servers, DNSs, printer servers, etc.

[edit] See also

[edit] Other network architectures

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