vcg19.htm

Visual C++与数据库的连接经典实例

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<NOTE><B>NOTE</B>

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<BR>RAID level 0 (disk striping only) provides improved fixed-disk read performance by sequentially placing blocks of data on multiple disks. RAID level 0 isn't included in the preceding list because RAID level 0 disk striping by itself doesn't provide fault tolerance.</NOTE>

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<P>Windows NT Server lets you choose either RAID level 1 or RAID level 5 redundancy to keep the network operating despite the failure of a single disk drive. Unless you have a particular reason for choosing RAID level 1, RAID level 5 is currently the favored method of providing disk drive fault tolerance. If more than one disk drive at a time fails, you're out of luck, because you can't reconstruct the missing data with either RAID level 1 or level 2 strategies. Low-cost fixed-disk drives ($200 per gigabyte and still falling) with an advertised MTBF (mean time between failures) of 800,000 to 1,000,000 hours were available at the time this book was written, so multiple-drive failure is unlikely unless the server experiences a power surge that your power-line conditioning system can't take in stride.

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<P><B>WARNING</B>

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<BR>No form of RAID is effective when drives are killed. Natural disasters, especially lightning, can easily destroy all the drives in a RAID system at one time.

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<NOTE><B>CAUTION</B>

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<BR>If you believe advertised MTBF figures, the probability of two disk drives failing simultaneously because of mechanical or on-board electronic component malfunction is extremely small. On the other hand, the first disk drive installed in the Darkstar domain controller experienced an unexplained catastrophic failure within the first few days of operation. The replacement drive then failed less than 60 days after it was installed. MTTF (mean time to failure) is a more useful measure of the reliability of fixed disk drives, but manufacturers rarely report MTTF values. Fault-tolerance strategies are never a substitute for regular backups of server data.</NOTE>

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<P>Windows NT Server doesn't expand on Windows NT 3.51's tape backup system. Windows NT supports a variety of SCSI and QIC tape drives, including drives that use the 4-mm DAT (digital audio tape) format. Many suppliers of backup tape drives that aren't supported by the drivers included with Windows NT Server provide their own drivers for Windows NT 3.51. Windows 95 supports QIC drives connected to floppy controllers and some limited QIC drives connected either to parallel or dedicated controllers. At the time this book was written, Windows 95 didn't support SCSI or DAT tape drives. Aftermarket backup programs for SCSI and DAT tape drives are available to Windows 95 users.

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<NOTE><B>NOTE</B>

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<BR>Hardware products that have been tested and found to perform satisfactorily with Windows NT 3.51 and the hardware drivers that are included with the retail versions of Windows NT Server 3.51 and Windows NT Workstation 3.51 are listed in the <I>Hardware Compatibility </I><I>List</I> that accompanies Windows NT. Periodic additions to the list of tested hardware products appear in updated versions of the list and are available for downloading from Library 1 of the WINNT forum on CompuServe. Microsoft also maintains a WWW server at www.microsoft.com, where there are links to a vast array of information about Microsoft, including the extensive Knowledge Base. Virtually any files that are available from Microsoft may be found on ftp.microsoft.com.</NOTE>

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<FONT SIZE=4 COLOR="#FF0000"><B>Network Adapter Cards and Operating Protocols</B></FONT></CENTER></H4>

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<P>Windows 95, Windows NT Workstation, and Windows NT Server support a variety of network adapter cards (also called <I>NICs,</I> Network Interface Cards) and operating protocols. Both Windows NT 3.51 and Windows 95 use the Open Systems Interconnection (OSI) Reference Model, which divides the flow of data in a connection between an application running under a computer operating system and the network hardware into the seven layers shown in Figure 19.13. This layered configuration results in the various protocols used to communicate between networked computers as a stack. Each of the layers in the workstation's stack communicates with the same layer in the server's stack.

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<P><B><A HREF="19vcg13.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/19vcg13.gif">Figure 19.13. OSI Reference Model protocol stacks for a workstation and a server.</A></B>

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<P>The OSI Reference Model has been adopted by the United Nations' International Standards Organization (ISO) and is accepted on a worldwide basis as the standard methodology for network software implementation. The following sections provide the details of Microsoft's implementation of the OSI Reference Model.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Network Driver Interface Specification and Network Adapter Card Drivers</B></FONT></CENTER></H5>

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<P>The protocol stack (also called protocol, transport protocol, or protocol driver) for WfWg 3.11, Windows 95, Windows NT Workstation 3.51, and Windows NT Server 3.51 includes the transport and network layers. The application, presentation, and session layers are attached to the operating system kernel (Windows 95, Windows NT Workstation, and Windows NT Server) or the environment (WfWg 3.11). The data-link layer of each of the three products is based on Microsoft's Network Driver Interface Specification (NDIS) standard for Windows. When you install WfWg 3.11, Windows 95, Windows NT Workstation 3.51, or Windows NT Server 3.51, you choose the NDIS driver supplied by Microsoft for the adapter card in your computer. The process of connecting the driver to the data-link layer and the adapter card is called <I>binding</I>.

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<NOTE><B>NOTE</B>

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<BR>Some network protocols include the transport, network, data link, and physical layer in a single monolithic protocol. The advantage of Microsoft's NDIS approach is that you can use more than one protocol with a single adapter card. In this case, the multiple protocols share the transport and network layers of the protocol stack. The computer first transmits data in the primary protocol and then in the other protocols (if multiple protocols are used). Windows automatically assigns a new number (LANA number) to the adapter card for each protocol in use. Novell's Open Datalink Interface (ODI) is similar in concept to NDIS and also allows multiple protocol stacks.</NOTE>

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<FONT SIZE=4 COLOR="#FF0000"><B>Network Protocol Stacks Included with Windows NT Server</B></FONT></CENTER></H5>

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<P>Windows NT 3.51 and Windows 95 include a number of protocol stacks: NetBEUI, TCP/IP, IPX/SPX, Novell IPX/ODI, DEC Pathworks, Banyan Vines, SunSelect PC-NFS protocol, and DLC. Windows NT Server also includes a protocol stack for the AppleTalk networking system built into all Macintosh computers. (Macintosh connectivity was an extra-cost option for LAN Manager 2.2.) The following list briefly describes the purpose and capabilities of each of the most commonly used protocols supplied with Windows NT Server:

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<LI>NetBEUI (NetBIOS Extended User Interface) is a protocol developed by IBM that has been used in both IBM and Microsoft products since the mid-1980s. (IBM LAN Server and Microsoft LAN Manager both derive from the original 3-Com NOS that Microsoft licensed for incorporation in OS/2.) NetBEUI is a very fast protocol within a single LAN, but it's not a routable protocol. WANs require routable protocols, which include a hierarchical addressing method to specify the destination of data packets. Therefore, NetBEUI is best suited for use on a single LAN.

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<LI>TCP/IP (Transmission Control Protocol/Internet Protocol) is a routable protocol that was developed by the ARPA (later DARPA, the U.S. Defense Advanced Research Projects Agency) for communication among government agencies and contractors (especially university research institutes) participating in defense-related research activities. The wide-area network created by ARPA was called the Internet. Admission to the Internet later was granted to corporations, to individuals able to afford a direct connection, and to Internet service providers who provided dial-up access to the Internet. At the time this book was written, about 50 percent of the Internet host sites were commercial. The Internet is now the ultimate in wide-area networks and links several million computers around the world. TCP/IP is discussed in two later sections.

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<LI>IPX/SPX (Internetwork Packet Exchange) is the Novell protocol for NetWare networks. Windows NT and Windows NT Server implement IPX through an NDIS-compliant data-link layer called NWLink, which supports the STREAMS interface. NWLink currently isn't a complete solution to accessing files on NetWare servers. You need to install and run NetWare workstation software on PCs that share files on NetWare servers. This two-vendor approach to a protocol stack has been the subject of much consternation among NetWare users. (Roughly 60 percent of the networked PCs in North America used NetWare in 1993, but the percentage of non-NetWare networks is increasing.)

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<LI>Services for Macintosh includes the ability to share files and printers between PCs and Macs. Windows NT Server can act as the sole file server for all Macs connected to the network. Windows NT Server's Services for Macintosh also lets you create an AppleTalk &quot;internet&quot; that connects two or more separate AppleTalk networks. Services for Macintosh supports AppleTalk Phase 2, which supports routing and network protocols other than the AppleTalk protocol. (The current version of Services for Macintosh doesn't support AppleTalk Phase 1.)

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<LI>DLC (IBM data-link control) protocol can be (but seldom is) used to connect Windows NT clients to IBM mainframe computers. DLC also lets you use printers that are connected directly to the network rather than connected to a network server or WfWg workstation. (The Hewlett-Packard LaserJet IIIsi printer, for example, supports DLC using the JetDirect interface card.) Microsoft's SNA Server for Windows NT, which costs about $400, is a more satisfactory approach to IBM mainframe connectivity. Windows 95 supports the Microsoft DLC protocol.

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<P>Windows 95 includes five categories of network protocols arranged by manufacturers. Figure 19.14 shows the Select Network Protocol dialog box for Windows 95.

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<P><B><A HREF="19vcg14.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/19vcg14.gif">Figure 19.14. Windows 95 network protocols.</A></B>

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<P>The five manufacturer protocols include Banyan, Digital Equipment (DEC), Microsoft, Novell, and SunSelect. The following list briefly describes the purpose and capabilities of each of the protocols supplied with Windows 95:

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<LI>Banyan Vines Protocol was developed by Banyan for their networks. It's not as common as the Microsoft and Novell protocols.

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<LI>DEC protocols are included so that Windows 95 can communicate directly with DEC VAX servers. There are four supported protocols under DEC: PATHWORKS V4.1 Ethernet, PATHWORKS 4.1 Token Ring, Pathworks V5.0A Ethernet, and Pathworks V5.0A Token Ring.

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<LI>Microsoft NetBEUI is a protocol developed by IBM that has been used in both IBM and Microsoft products since the mid-1980s. (IBM LAN Server and Microsoft LAN Manager both derive from the original 3-Com NOS that Microsoft licensed for incorporation in OS/2.) NetBEUI is a very fast protocol within a single LAN, but it's not a routable protocol. WANs require routable protocols, which include a hierarchical addressing method to specify the destination of data packets. Therefore, NetBEUI is best suited for use on a single LAN.

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<LI>Microsoft TCP/IP is a routable protocol that was developed by ARPA for communication among government agencies and contractors (especially university research institutes) participating in defense-related research activities. The wide-area network created by ARPA was called the Internet. Admission to the Internet later was granted to corporations, to individuals able to afford a direct connection, and to Internet service providers who provided dial-up access to the Internet. At the time this book was written, about 50 percent of the Internet host sites were commercial. The Internet is now the ultimate in wide-area networks and links several million computers around the world. TCP/IP is discussed in two later sections.

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<LI>Microsoft IPX/SPX is the Novell protocol for NetWare networks. Windows NT and Windows NT Server implement IPX through an NDIS-compliant data-link layer called NWLink, which supports the STREAMS interface. NWLink currently isn't a complete solution to accessing files on NetWare servers. You need to install and run NetWare workstation software on PCs that share files on NetWare servers. This two-vendor approach to a protocol stack has been the subject of much consternation among NetWare users. (Roughly 60 percent of the networked PCs in North America used NetWare in 1993; however, the percentage of non-NetWare networks is increasing.)

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<LI>Microsoft Network Monitor is a protocol used for network monitoring and is useful for debugging network problems.

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<LI>Microsoft DLC (IBM data-link control) protocol can be (but seldom is) used to connect Windows NT clients to IBM mainframe computers. DLC also lets you use printers that are connected directly to the network rather than connected to a network server or to a WfWg workstation. (The Hewlett-Packard LaserJet IIIsi printer, for example, supports DLC using the JetDirect interface card.) Microsoft's SNA Server for Windows NT, which costs about $400, is a more satisfactory approach to IBM mainframe connectivity. Windows 95 supports the Microsoft DLC protocol.

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