Distributed system architectures:

There are a number of different system architectures, or ways of structuring the network of computers in a distributed system. For example, each computer in the system may be defined as either a client or a server. A server is normally a powerful computer that manages data, printers or network traffic. A client is normally a PC or workstation on which users run application programs or user interface code. Clients request services from servers, as shown in Figure, where a web client sends requests to a web server using a special language defined by the HyperText Transfer Protocol (HTTP).

Figure: Examples of 2-tier and 3-tier system architectures

Client-server architectures are sometimes called 2-tier architectures. Architectures may have three or more tiers, leading to the so-called n-tier architecture, where n is 2, 3, 4 or more. Each of the tiers (or levels) of the system has its own responsibilities.

A particular computer may not always be fixed in the role of client or server. That is, a computer may act as a server to a number of clients and yet act as a client to another server. For example, a web server maintaining a number of web pages may act as a server to clients running browsers. However, if the web server wants data from a relational database in order to satisfy some client request (for example, a list of products that are out of stock) then it acts as a client to a database server. The request to the database server is usually expressed in the standard database query language SQL.

Another type of network architecture is known as a peer-to-peer (P2P) architecture because each computer has similar capabilities and responsibilities - each can act as a server as well as a client. Some P2P systems are hybrid in that they also have central computers that act only as servers, as shown in Figure.

Figure: An example of a peer-to-peer architecture - hybrid P2P systems also use central servers for some functions

The P2P approach is used in many of the popular but controversial music-sharing systems. It is a robust approach in that it does not rely on one central server. It also makes it harder for hostile organizations to close down the system than it would be with a single-server system.

Both client-server and peer-to-peer architectures are widely used, and each has unique advantages and disadvantages. A full discussion of distributed system architecture is outside the scope of this course, so we will concentrate on client-server architectures.

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