DataSnap Distributed Databases (Multitier)
Distributed applications are also referred to as multitier applications. This means that the program is split in two or more pieces that might reside on separate servers. These pieces communicate using CORBA, DCOM, or HTTP. CORBA is open , multiplatform , and considered easy to use, but it must be purchased separately. DCOM is from Microsoft and is for Windows only. HTTP is open and multiplatform, but it is less powerful and, therefore, has more limited use. DataSnap, formerly called MIDAS, is covered in Chapter 20, "Distributed Databases with DataSnap." DataSnap provides the mechanism for spreading your database applications across multiple servers.
The standard multitier model allows an application with a specific task to use other program parts residing on other servers. This is so that the business logic can be encapsulated and shared between any number of applications. As you can imagine, there are many benefits in terms of data integrity, logistics, and maintenance (to name a few).
This is also the architecture of choice if your project requires the thin-client model. This is when only the interface part of the program runs on the client computer. All the application logic and business rules are on another server. This is ideal for low-bandwidth networks (such as the Internet), for network computers, when centralized processing is required, or when the interface must run on many different operating systems.
Another model is also supported by the multitier components ”it's called briefcase . It enables the client part of the application to work even when it is not connected to the network. The trick is that the components can use a cached copy of the data in a local flat file. As a result, users can get the data they need from the server and take it on the road or home with them. In this model, the client-side must also contain the application logic and some business rules. When the mobile user returns, changes can be automatically reconciled between the two separate databases, but such reconciliation sometimes requires someone to identify the appropriate result of contradictory changes.
The Remote Data Modules that Borland provides as part of DataSnap can use BDE components, or any of the other component sets listed here.
NOTE
Sources for More Information on Borland's Database Architectures are
Borland C++Builder 5 Developer's Guide (the manual provided with C++Builder), Inprise Corporation: Chapters 13 “15, Chapter 23, and Chapter 25
Calvert, Charlie, Accessing Databases Using ADO and Delphi , Borland Community Web site, http://community.borland.com/soapbox/techvoyage/article/1,1795,10270,00.html
Cant ¹, Marco, Data Access Dilemma , Borland Community Web site, http://community.borland.com/article/0,1410,20191,00.html
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