Client/Server must Know

Posted by arlene

One serious problem that people have with modern computing is that terms are not always defined and used in the same manner. Not only are there multiple meanings in use, but many people are not aware of this inexactness, resulting in confusion for the uninitiated. The use of ambiguous terms becomes even worse with client/server technology. Even the designation “client/server” has several meanings. The terms “distributed computing”, “cooperative computing” and “distributed transaction processing” are used with even less accuracy. This section defines these and other terms exactly. See the glossary for additional definitions.

Distributed Database

A database car be defined as a collection of information permanently stored in a computer and accessible by a computer application to support application processing. This general definition has been made more specific by the use of specialized software, called database management systems (DBMS), and the more modern relational database management system (RDBMS).

In a distributed database, the data is divided among more than one database instance but may be treated as a single logical database by applications. A distributed database can exist on the same platform or, more usually, on multiple platforms. The concept of a distributed database is essential to an understanding of the implications of transaction management.

Most DBMSs provide for defining multiple instances of the database management software. In this case, each instance is called a database. These systems often also allow the definition of multiple spaces, which define the physical location of the data. When multiple spaces are defined for the same database, the DBMS manages the spaces as if they were logically one space (spanning the spaces), and the database is not considered distributed.

The concepts of distributed database and distributed processing are very different. A distributed database spreads the data into various locations. Distributed processing spreads the processing of the business application into various locations.

Client/Server General Definition

The term client server refers to a relationship between two systems or processes. The client is the system that requests work to be done on its behalf by the server system. In most situations, which is client and which is server is determined by the relationship of requester (client) to server.

Servers provide services to requesting clients. It is important to distinguish between servers and the services they provide. Since a single server can provide several services, the designation “server” should be carefully separated from the designation “service.”

It is convenient to refer to a system or process that usually receives requests as the server and to a system or process that usually sends requests as the client. This can create some confusion, as when a server sends a request to another server. In later discussions, the context serves to distinguish clearly which type of object is being discussed.

Hardware Servers

It is common today to provide a variety of services from one or more networked computer systems. In this case, the machine providing the services is referred to as a server. Normally there is no end user interaction directly with the server machine. User interaction is performed with some other hardware—the hardware client—which may be a completely different type of computer and is sometimes referred to as a work station.

The operating system and the network software make the hardware server function. In other words, although the machine is referred to as the server, it needs specialized software to be a server.

Commonly a hardware server functions:

• As a print server, with printers attached to the servers and used by everyone on the network.
• As a file server, with common files stored on the server.
• As a storage mechanism supporting work stations that have little or no storage capacity.
• As a central storage for enterprise information.

When a hardware server is provided, it is transparent to the user and usually has little or no effect on application design.

Software servers are provided by special software designed specifically to handle requests. This special software is often provided on server hardware. It is not uncommon to find that the hardware provides the software server also contains the software clients.

The services provided by software servers usually fall into one of the following types:

• Data file services

• Remote procedure call services

• Database services

• Enhanced C/S capabilities, which provide a true distributed processing capability

Running programs are called processes in the open systems venue. The client process sends requests to the server process. The request for service names the service and includes any information required to perform it. The server performs the requested service and returns the results in the form of a reply. The client and server process may be on the same hardware system or on different hardware systems.

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