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Many RDBMS vendors implement their database products using client/server architecture. Today's popular client/server RDBMS vendors include Oracle, Informix, and Sybase.
Some RDBMS systems also support server-to-server communication in order to provide distributed access to data maintained across a network of systems. In this kind of environment, a client still issues a SQL statement to a server to perform some operation. That server returns results to the client, but may communicate with a second server to perform part of the requested operation.
Figure 2. Server-to-Server Communication
One major drawback of today's RDBMS systems is that the SQL used for communication between the client and server usually contains proprietary, vendor-specific extensions to the standard ANSI version of the SQL language. This fact makes it difficult for a company to develop a client that communicates with databases from more than one RDBMS vendor at a time. The standards bodies that define the SQL language are currently developing a new standard to which all RDBMS servers should conform. In the meantime, many of the RDBMS vendors provide products called gateways which allow clients to communicate with more than one RDBMS product at a time.
Figure 3. RDBMS Gateway
Another potential drawback of RDBMS servers is that each RDBMS vendor provides its own communication protocol for sending SQL requests to the server and returning results to the client. This makes it difficult for a client application to communicate with database servers from more than one vendor. In recent years, however, RDBMS have vendors moved to support a standard communication protocol called ODBC. Client applications that use ODBC can operate more easily in an environment that contains databases from multiple vendors.
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