>I've been following this thread, and I really don't know a lot aout most of this stuff, so a simple question would be: What's the difference between middle teir front end ect.

Mike,

Here's a brief description on N-Tier architecture (which BTW has nothing to do with Client/Server).

In a conventional single tier architecture there is one body of code that handles all aspects of an application's functionality, the user interface, the enforcement of business logic restrictions, and the storage and retrieval of data.

In mutlitier architectures these responsibilities are divided amongst multiple layers, in some cases simply various objects within a single application, in other cases by ActiveX servers or database servers that exist outside the main application. 3-Tier is one of the architectures that fall into the N-tier model.

In the 3-Tier model there is a user interface tier that only deals with presenting and getting data to and from the user. It applies no rules to that data and is only marginally aware of the nature of the data it presenting.

The second tier is often called the business logic tier. It is responsible for applying business rules against the data and fetching and sending data from and to the databse layer.

The third tier is the databse engine itself which is responsible for storing and retrieving the data and enforcing the domain constraints on that data. It is also responsible for enforcing the referential integrity constraints.

The advantage of the 3-tier architecture is seen during the evolution of the application. When a business rule changes (a very likely scenario) the only tier requiring adjustment is the business logic layer.

The backend data engine can be changed only requiring the alteration of the business logic layer and the database layer, the user interface doesn't even know where the data comes from so it is unaffected by the change.

Also, you can create alternate interface layers that use the same business and database layers therefore insuring that all interfaces are forced to comply with the established rules on the data. One example would be a VFP system using a VFP activeX server as the business layer and SQL server as the database. One day the accounting folks ask you to give them an Excel sheet to access and update the data. Excel can use the same VFP ActiveX server for the middle tier. This emans that if later the databse changes to Oracle, you don't need to change the VFP interface or the Excel interface as they both access the data through the ActiveX server. Once you change the ActiveX server the two user interface layers both now get the data from somewhere else.