three-tier

I don't understand clearly what is meant by three-tier architecture. Front end is programming language, backend is database and the middle one is business unit i suppose.what is that middle layer the so called business unit

re: three-tier

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re: three-tier

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re: three-tier

Generally speaking, 3 tier apps have a display layer, business logic layer (called, creatively, a BLL), and a Data access layer (also imaginative, the DAL). I believe this architecture is basically worthless on real projects, but for the record, the display layer asks for information to display from the BLL. The BLL decides what information to access, and asks the DAL for it. the DAL decides how to get that actual information, and returns it to the BLL in the format the BLL asked for. The BLL then does some acrobatics to format the data for viewing, then the display layer displays it.