Computer language client program

Multi-tier Architecture. An expansion of a client-server architecture to include a middle layer of software. The middle tier may run on a computer different from either the client or server computers. The middle tier isolates the client and server programs, so that changes in either of them do not require corresponding changes in the other. The middle tier acts as to receive, authenticate, and transform data as it passes between client and server computers. To make middle tier software easy to change and maintain, it is often written in Java, a modem object-oriented computer language that is available free on most of today s computer platforms. 

The RDBMS is installed and runs on a computer that functions as a database server. Any SQL commands are executed on the server by the RDBMS. Functions written in SQL or in any of the procedural languages mentioned above are also executed by the RDBMS. This has the advantage that the data tables used by these SQL commands or procedural functions are under the control of the server. This is the most efficient way to access the data. The disadvantage is that the server may have many requests to handle from many users. Another way to operate on data tables is indirectly, using a client program typically (although not necessarily) run from another computer. 

There is a smaller set of tools that are typically run on the server. Any SQL commands and any procedural language functions are run on the server. In principle, there is complete flexibility of the server side tools, since in principle any computer program can be written in any computer language. Later chapters of this book show how the RDBMS server itself can be extended using server side programming to handle chemical information. These extensions may directly solve the needs of a particular project, but more importantly they increase the flexibility of the RDBMS to handle chemical information. Client programs can use the results of chemical searches and other computations as well. 

Selecting rows from a table is only one common operation needed for a client program to work with an RDBMS. In addition to a simple SQL select statement, other more complex select statements will become necessary as more complex client programs are developed. It is also important to see how to properly use the SQL Insert statement. Rather than showing examples in several computer languages, the use of more complex SQL statements is discussed in Chapter 12. 

This chapter discusses ways in which more complex client applications can be written. These programs use SQL to select, insert, delete, or update tables in the database. Depending on the computer language used for the client program, a variety of interface libraries is available. 

Applet A unit of a program written in Java programming language. An applet can be downloaded onto a client platform along with a web page over a computer network and executed within its virtual environment called the Java Virtual Machine. 

Object-oriented, platform-independent programming language it allows complete applications to be transported over the WWW and executed on. the client s computer. 

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