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Sams Teach Yourself Database Programming with Visual C++ 6 in 21 Days phần 4

Đa tầng ứng dụng thường bao gồm hơn hai tầng. Cấp khách hàng bao gồm máy (s) đang chạy một số loại của một chương trình khách hàng mỏng, chẳng hạn như một trình duyệt Web hoặc một ứng dụng đơn giản. Thường có một tầng lớp trung lưu, bao gồm máy (s) đang chạy một máy chủ Web, | Teach Yourself Database Programming with Visual C 6 in 21 days -- Day 9-Understanding COM happens to have the same name. Simpo PDF Merge and Split Unregistered Version - http www.simpopdf.com A true software component needs to be safely installable on any machine and accessible to all the appropriate applications on that machine. Hard-coded DLL names in the application EXE files are detrimental to this. Build-Time Dependencies Between the DLL and the EXEs That Use it The most common way for applications to use a DLL is to link with its import library implicit load-time linking . The other method-explicit runtime linking with the LoadLibrary and GetProcAddress functions-is not used nearly as often. Using LoadLibrary and GetProcAddress isn t as convenient for the application developer as simply linking to the DLL s import library and having the OS automatically load the DLL. However a problem occurs when it comes time to update the DLL. A true software component can be updated independently of the rest of the system. In other words you can install and use a new version of a component without breaking the existing system. With traditional Win32 DLLs when you modify the code and rebuild the DLL the information in the import library can change. You will recall that the import library contains the function names exported from the DLL. The import library also contains import records for those functions that are fixed up when the DLL is loaded and the addresses are known. This means that when a DLL is updated the applications that use the DLL by linking with the DLL s import library need to be rebuilt also to ensure that the system is stable. Therefore a build-time dependency exists between the application EXE and the DLLs that it uses. NOTE It might be possible to update a DLL without updating the EXEs that use it if you don t change any existing functions in the DLL. However there is no mechanism for the EXE to gracefully recover if the DLL does become out of sync. Also