vcg19.htm

Visual C++与数据库的连接经典实例

<FONT SIZE=4 COLOR="#FF0000"><B>Workgroup Networks</B></FONT></CENTER></H5>

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<P>Workgroup networks connect a limited number of users (usually 25 or less) who share files, printers, and other computer resources. Microsoft's Windows 95 and Windows NT Workstation, Novell's NetWare Lite, and Artisoft's LANtastic networks are typical network operating systems (NOSs). Workgroup networks are usually self-administered; in other words, the members of the workgroup control <I>permissions</I> (also called <I>authority</I>) to share workgroup resources.

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<P>Workgroup computers usually are connected by peer-to-peer networks and use a single network protocol. Any computer in a peer-to-peer network may share its resources, such as files and printers, with other computers in the workgroup. Access is designed specifically for workgroup computing. Figure 19.1 shows a five-member workgroup network using Ethernet adapter cards and cabling. One of the workgroup computers shares a fax modem and a printer with other members of the workgroup.

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<P><B><A HREF="19vcg01.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/19vcg01.gif">Figure 19.1. A five-member workgroup network with a shared fax modem and laser printer.</A></B>

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<FONT SIZE=4 COLOR="#FF0000"><B>Departmental Networks</B></FONT></CENTER></H5>

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<P>Departmental networks use dedicated server computers that provide resources to client workstations, usually within a single facility. Novell NetWare 3.x and 4.x, Microsoft LAN Manager and Windows NT Server 3.51, IBM's LAN Server, and Banyan VINES are examples of client-server NOSs. Departmental networks often include remote access services (RAS) that let users, such as field salespeople, to connect to the server with a modem-equipped computer. Servers fall into the following three classes:

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<LI>File servers let many users share files that contain data required by a common application. Typically, these files contain word-processing documents, worksheets, or desktop database tables. The applications that access the files are responsible for page or record locking to prevent two users from modifying the same record in a database. Word-processing and spreadsheet applications usually lock the entire file while a user has the file open.

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<LI>Application servers let users run applications from the server instead of a local fixed disk drive. Running the .EXE files and opening the DLLs of today's Windows mega apps from the network can cause severe network congestion. The advantage of using an application server is that all users work with the same version of the application, and updates and upgrades to applications are much easier to administer. A single server can provide both file- and application-sharing services.

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<LI>Database servers are computers that usually are devoted to running the back end of a client-server RDBMS. The performance of the RDBMS is degraded if the server computer also is used for file or application sharing. The database administrator (DBA) usually administers the RDBMS from a workstation. Database servers often run as NOS applications. For example, the Sybase version of SQL Server and Raima's Database Server are available as NetWare-loadable modules (NLMs) that run under NetWare 3.11 or higher. Microsoft SQL Server for Windows NT 6.0 (NTSQLS) runs under Windows NT 3.51, which is considered a NOS (as well as a general-purpose operating system) in this book.

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<P>A variety of computer types (PCs, Macs, and UNIX workstations, for example), each of which uses a different network protocol (such as NetBEUI, IPX, and TCP/IP), may be connected as clients in a departmental network. Departmental networks use gateways to connect to mainframe computers. One or more full-time network administrators (NAs or NWAs) usually are assigned to manage departmental networks. Independent, self-administered workgroups may exist within the departmental network. Figure 19.2 shows a simple departmental network with a single file and/or application server. The server is equipped with a modem to provide remote access service.

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<P><B><A HREF="19vcg02.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/19vcg02.gif">Figure 19.2. A simple departmental network with a shared printer and a RAS modem.</A></B>

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<NOTE><B>NOTE</B>

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<BR>Technically, of the examples of NOSs just listed, only NetWare 3.11 and Banyan VINES are true network operating systems. NOSs use proprietary operating systems rather than DOS, UNIX, OS/2, or Windows NT. LAN Manager, Windows NT Server, and LAN server are network applications (layers) that run under (or on top of, depending on your point of view) an operating system. LAN Manager 2.2 runs under Microsoft OS/2 version 1.3, Windows NT Server runs under Windows NT 3.51, and LAN Server runs under IBM OS/2 2+. After Windows 95 has started, most of the DOS components are no longer accessed except under special&#151;exceptional&#151;circumstances. For the sake of simplicity, this book uses the term <I>NOS</I> to describe any software product that lets computers share files.</NOTE>

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<FONT SIZE=4 COLOR="#FF0000"><B>Enterprise-Wide Networks</B></FONT></CENTER></H5>

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<P>Enterprise-wide networks connect departmental LANs, often across large distances. Figure 19.3 shows one of the departmental or headquarters LANs that makes up an enterprise-wide network. Most enterprise-wide networks use a variety of communication methods to link LANs into a WAN; the type of interconnection depends on the distance between the individual LANs. Concentrators, bridges, and routers are hardware devices that transfer packets of data between the LANs.

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<P><B><A HREF="19vcg03.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/19vcg03.gif">Figure 19.3. A headquarters LAN that acts as the hub of a wide-area network.</A></B>

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<P>The LAN in Figure 19.3 uses Ethernet running the TCP/IP protocol and includes a connection to a mainframe computer through a gateway, as well as a bridge to a fiber-optic (FDDI) and copper token-ring network. Connections to North American LANs in the WAN are made through a T1 switch that provides access to high-speed telephone lines. Overseas subsidiaries communicate through a satellite link. Because of the complexity of WANs, most firms that operate enterprise-wide networks have a staff that manages the communications aspects of the WAN.

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<FONT SIZE=4 COLOR="#FF0000"><B>Domains, Workgroups, Servers, and Workstations</B></FONT></CENTER></H4>

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<P>Early in the history of PC-based networks, the most common configuration was the departmental LAN (shown in Figure 19.2), a single server sharing its resources with a group of workstations. As the number of users in a LAN grows, additional servers are added to accommodate more shared files and applications, as well as expanding database files. When users number in the thousands and WANs span continents, the simple client-server model no longer suffices for network administration. Therefore, an additional tier, the <I>domain,</I> was added to the client-server hierarchy.

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<P>Figure 19.4 shows the relationships between two domains (represented by the two domain controllers), servers, and workstations. The interconnection between the Ethernet backbone of each of the domains, represented by a lightning bolt in Figure 19.4, could be twisted-pair (10baseT) wire, coaxial cable (10base2), a fiber-optic link (FDDI), or a T1 data line. Only two workstations per domain are shown in this figure, but a single domain commonly supports 100 or more workstations. Domains are named, and users in one domain can share files that are stored on another domain's servers.

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<P><B><A HREF="19vcg04.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/19vcg04.gif">Figure 19.4. Two interconnected network domains.</A></B>

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<P>The advantage of assigning servers and workstations to domains is that a workstation user can gain access to any server in the domain with a single logon operation. Network administrators don't need to create new user accounts for each server in a domain, because each user account is validated by the domain controller, not by the server(s) in the domain. The domain controller maintains the user account records for each person who is authorized to use a workstation in the domain. Domain controllers also can act as conventional file, application, and/or database servers.

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<NOTE><B>NOTE</B>

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<BR>Windows NT Server carries the single-logon process one step further by authenticating user accounts across trusting domains. If the domain that is responsible for the user's account data is trusted by the other domains to which the domain with the user account is connected, the user automatically has an account in each of the other domains. A full discussion of trust relationships between domains is beyond the scope of this book. The <I>Concepts and Planning Guide</I> that accompanies Windows NT Server provides a complete explanation of domain topology.</NOTE>

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<P>If you have more than one server in a Windows NT Server domain, every five minutes or so the domain controller replicates the user-account data on each of the servers in the domain. To minimize replication overhead, only changes to the user-account records are reflected in the servers' user-account tables. Replicating user-account data provides a backup in case the domain controller fails. If a domain controller failed and then was restored online, it would be automatically demoted to a domain server. This is done to prevent the failed domain controller's possibly out-of-date user account data from being used.

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<P>The domain administrator can promote any server to domain controller status. Promoting a server to domain controller demotes the current domain controller to server status.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Topology and Protocols Used in This Chapter's Examples</B></FONT></CENTER></H4>

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<P>The examples in this chapter use Windows 95 as the peer-to-peer NOS, Windows NT Server 3.51 as the client-server NOS, and both Windows NT Workstation 3.51 and Windows 95 as client workstation environments. Windows NT Server 3.51 is a superset of Windows NT Workstation 3.51 that provides a number of additional features that aren't included in Windows NT's built-in peer-to-peer (workgroup) networking capability. One of the additional features offered by Windows NT Server is trust relationships between domains (as discussed earlier). Other added features, such as fixed-disk fault tolerance and directory replication, are discussed in a section later in this chapter. SQL Server for Windows NT 6.0, used in Chapter 20, &quot;Creating Front Ends for Client-Server Databases,&quot; runs as a service of Windows NT Server.

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<P>In order to understand many of the examples shown in this and the next chapter, you need to know the configuration of the computers and the topology of the network used to create the examples. Otherwise, you might not know why drives with letters such as G: and H: appear in the examples. Each of the computers uses generic NE-2000-compatible Ethernet cards connected by thin Ethernet (10base2) cabling. Following are the specifications and configurations of the disk drives of the server and workstation computers:

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<LI>The domain (Darkstar) controller and file server (GayDeceiver) is a Pentium-90 MHz PCI/ISA-bus PC with 32M of RAM and a 2G (2 gigabyte) PCI SCSI (Small Computer Systems Interface), 8-ms disk drive running Windows NT Server 3.51 and Windows 95 in a dual-boot configuration. The drive is configured as a 1.6G C: FAT drive, a Windows 95 DRVSPACE drive of 500M (I:), and a 300M NTFS partition (D:). A second 1G SCSI drive serves for temporary storage. The Windows NT file system (NTFS, which is accessible only by Windows NT Server), provides faster file access and additional security features.

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<BR>The V: drive is a simple Panasonic CD drive connected to a SoundBlaster sound card. The U: drive is a Sony CDU-33A double-speed CD-ROM drive connected to an IBM Audiovation mWave Sound System. A Panasonic IDE/ATPI quad-speed CD drive provides fast access to CDs when needed. There is also a Toshiba CD drive connected to the SCSI bus. The SCSI adapter is an Adaptec 2940 PCI interface adapter that uses Windows 95's protected-mode drivers when running under Windows 95. The file server also acts as a database server for Microsoft SQL Server for Windows NT (SQLNT), which is located on the NTFS D: drive. The remaining 250M partition was doubled using DRVSPACE to allow for efficient storage of many thousands of sample files. Darkstar supports NetBIOS Extended User Interface (NetBEUI), IPX/SPX, and TCP/IP protocols, both for Ethernet and for dialup connections. Connections to the Internet are made through a 28K BPS dialup PPP connection. The Internet node name is darkstar.mv.com.

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<LI>One client workstation (Pixel) is an 80486DX2/50 ISA-bus PC with 16M of RAM running Windows 95. A 2G IDE drive is divided into two partitions on this system. The first partition, C:, is about 1G and contains the operating system and system files. The remaining space (1G) then becomes the D: partition, which is used for temporary storage. Drive S: is a Mitsumi CD-ROM drive connected to a Mitsumi card, and drive Z: is a Panasonic IDE/ATPI quad-speed CD drive that provides an additional CD drive. Server shares (shared server directories) appear as drives H: and higher on Pixel. Pixel uses both NetBEUI and IPX/SPX protocols. Pixel accesses the Internet using Windows 95's TCP/IP protocol and dialup networking.

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<BR>Pixel also hosts a digital voice mail and fax system that is connected to a special telephone line. You can call in to this system and have it read any pending e-mail. In the future, maybe you'll be able to call in from a telephone and create outgoing e-mail!

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<LI>Another client workstation (Long) is an 80386DX-33 ISA-bus PC with 8M of RAM and an 850M IDE fixed disk drive (C:) that runs Windows 95. The Long workstation is used primarily for testing and developing Windows applications and as a FAX/e-mail server. If you send me a fax, Long gets it. The Server shares appear as drive D: and higher on Long. This workstation uses only the NetBEUI protocol.

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<P>Windows NT Server 3.51 is used as the client-server NOS in this chapter because Windows NT Server 3.51 and SQL Server for Windows NT (NTSQLS) replace LAN Manager 2.2 and Microsoft SQL Server 4.2 for OS/2 in Microsoft's new enterprise networking strategy. With the release of Windows NT 3.51, the success of Windows NT is expected to hinge on the acceptance of Windows NT Server as an enterprise-wide NOS for both large and small organizations. Acceptance of NTSQLS also will influence the adoption of Windows NT Server, because the two products are designed to work best when they are combined. Oracle Corporation has announced the availability of a Windows NT implementation of its ORACLE 7 RDBMS. Oracle also has been experimenting with distributing software using the Internet.

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<NOTE><B>NOTE</B>

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<BR>Oracle Corporation has been experimenting with several methods of software distribution, including low-cost CDs using shareware techniques and online distribution using the Internet. In late 1995 you could download a fully functional version of Oracle's product from the Internet and try it for free for 30 days. Whether Oracle continues to support these new policies is yet to be seen, but the techniques have garnered Oracle substantial press coverage. Check the www.oracle.com World Wide Web page for more information and pricing.</NOTE>

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<P>Although the GayDeceiver workstation is configured to boot either Windows NT 3.51 or Windows 95, this chapter doesn't include examples of using Windows NT 3.51 as a network client, except for remote domain administration. Virtually all Windows 95 applications run quite effectively under Windows NT 3.51. The resources required to run Windows NT effectively (an 80486DX PC with at least 16M of RAM) and the lack of mainstream client-side applications that take advantage of Windows NT's 32-bit multithreaded, multitasking capabilities preclude Windows NT from replacing a substantial portion of the 30 million or so registered copies of Windows 3.1 and Windows 95 running on today's PCs. In late 1995, Microsoft released 32-bit versions of Excel and Word for Windows, which will help make Windows NT Workstation 3.51 more accepted than its predecessor. This book was written using the Windows 95 version of Word for Windows, which runs under both Windows NT and Windows 95.

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