Client/Server Databases

Date: 2015-10-07; view: 550.

Local Databases

The simplest type of database is the local database. A local database is a database that resides on a single machine. Imagine that you have a program that needs to store a list of names and addresses. You could create a local database to store the data. This database would probably consist of a single table. The table is accessed only by your program; no one else has access to it. Any edits made to the database are written directly to the database. Paradox, dBASE, and Access databases are usually local databases.

Another way a database can be implemented is as a client/server database. The database itself is stored and maintained on a file server (the server part of the equation). One or more users (the clients) have access to the database. The users of this type of database are likely to be spread across a network. Because the users are oblivious to one another, more than one might attempt to access the database at the same time. This isn't a problem with client/server databases because the server knows how to handle all the problems of simultaneous database access.

The users of a client/server database almost never work with the database directly. Instead, they access the database through applications on their local computer. These applications, called client applications, ensure that the users are following the rules and not doing things to the database that they shouldn't be. It's up to the client application to prevent the user from doing something that would damage the database.

1. DATABASE SERVERS

As long as I am talking about client/server databases, let's take a moment to talk about database servers. Database servers come in several flavors. Some of the most popular include offerings from InterBase (a Borland-owned company), Oracle, Sybase, Informix , and Microsoft. When a company purchases one of these database servers, it also purchases a license that enables a maximum number of users to access the database server. These licensed users are often referred to as seats. Let's say a company buys InterBase and purchases licenses for 50 seats. If that company grows to the point that 75 users require access to the database, that company will have to buy an additional 25 seats to be in compliance with the license. Another way that client/server databases are sold is on a per connection basis. A company can buy a license for 50 simultaneous connections. That company can have 1,000 users of the database, but only 50 can be connected to the database at any one time. The database server market is big business, no question about it.