SQL and Networking in Software

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SQL and Networking
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The dramatic growth of computer networking over the past decade has had a major impact on database management and given SQL a new prominence. As networks became more common, applications that traditionally ran on a central minicomputer or mainframe moved to local area networks of desktop workstations and servers. In these networks SQL plays a crucial role as the link between an application running on a desktop workstation with a graphical user interface and the DBMS that manages shared data on a cost-effective server. More recently, the exploding popularity of the Internet and the World Wide Web has reinforced the network role for SQL. In the emerging "three-tier" Internet architecture, SQL once again provides the link between the application logic (now running in the "middle tier," on an application server or web server) and the database residing in the "back-end" tier. The next few sections in this chapter discuss the evolution of database network architectures and the role of SQL in each one.
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Centralized Architecture
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The traditional database architecture used by DB2, SQL/DS, and the original minicomputer databases such as Oracle and Ingres is shown in Figure 3-2. In this architecture the DBMS and the physical data both reside on a central minicomputer or mainframe system, along with the application program that accepts input from the user's terminal and displays data on the user's screen. The application program communicates with the DBMS using SQL.
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Figure 3-2: Database management in a centralized architecture
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Suppose that the user types a query that requires a sequential search of a database, such as a request to find the average amount of merchandise of all orders. The DBMS receives the query, scans through the database fetching each record of data from the disk, calculates the average, and displays the result on the terminal screen. Both the application processing and the database processing occur on the central computer, so execution of this type of query (and in fact, all kinds of queries) is very efficient. The disadvantage of the centralized architecture is scalability. As more and more users are added, each of them adds application processing workload to the system. Because the system is shared, each user experiences degraded performance as the system becomes more heavily loaded.
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File Server Architecture
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The introduction of personal computers and local area networks led to the development of the file server architecture, shown in Figure 3-3. In this architecture, an application running on a personal computer can transparently access data located on a file server, which stores shared files. When a PC application requests data from a shared file, the networking software automatically retrieves the requested block of the file from the server. Early PC databases, such as dBASE and later Microsoft's Access, supported this file server approach, with each personal computer running its own copy of the DBMS software.
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Figure 3-3: Database management in a file server architecture
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For typical queries that retrieve only one row or a few rows from the database, this architecture provides excellent performance, because each user has the full power of a personal computer running its own copy of the DBMS. However, consider the query made in the previous example. Because the query requires a sequential scan of the database, the DBMS repeatedly requests blocks of data from the database, which is physically located across the network on the server. Eventually every block of the file will be requested and sent across the network. Obviously this architecture produces very heavy network traffic and slow performance for queries of this type.
Client/Server Architecture
Figure 3-4 shows the next stage of network database evolution the client/server database architecture. In this scheme, personal computers are combined in a local area network with a database server that stores shared databases. The functions of the DBMS are split into two parts. Database "front-ends," such as interactive query tools, report writers, and application programs, run on the personal computer. The back-end database engine that stores and manages the data runs on the server. As the client/server architecture grew in popularity during the 1990s, SQL became the standard database language for communication between the front-end tools and the back-end engine in this architecture.
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