3-Tier Architecture MVC 4 Project

Question

I am working on a Web Application using ASP.Net MVC 4 3-Tier Architecture and I am stuck at certain points. I know here exist similar threads but none were clear enough. I have already created the needed layers which are UI(MVC4 project), BLL- Business Logic Layer(library class), BOL- Business Object Layer(library class that contains ADO.net) and DAL- Data Access Layer (library class). Layer dependencies are as follows:

• UI depends on BOL and BLL
• BLL depends on BOL and DAL
• DAL depends on BOL

I want you to correct me if I am wrong in the following. The BOL is the master reference layer which exchanges raw dB records with DAL then sends them to BLL which is responsible for any logical computations then gets the updated records and sends them to the controller in the UI.

Knowing the above,

1. Where should we place the CRUD functions?
2. Where and why should we create a class for declaring (plus set and get) the useful database fields?
3. What exactly should we put in the ViewModel folder; in other words since we have already defined the variables in the previous step and in the Entity then does it add any value to keep the Model folder?

Thank you in advance.

Answer 1

Can't be unambiguously correct answers on these issues, so any answer should be evaluated simply as an opinion. Here are my answers:

1. Where should we place the CRUD functions? CRUD is a frequent pattern at different levels. Hi-level Repository provides similar methods, as a low-level Table Gateway.

2. Where and why should we create a class for declaring (plus set and get) the useful database fields? These classes are usually simple Data Transfer Objects (DTO) or slightly more complicated Active Records. In accordance with the Dependency Inversion Principle the interface for these classes should provide the Business Logic Layer, and implementation should provide the Data Access Layer. Since DTO is very simple classes, they can be provided "as is" without interface/implementation separation.

3. What exactly should we put in the ViewModel folder; in other words since we have already defined the variables in the previous step and in the Entity then does it add any value to keep the Model folder? In theory entities should be our models; but in practice it's often not the case. F.e. in ASP.NET MVC models should provide not only the value of drop-down field, but also all possible values; so it requires the separate model class.

The result may be that you have three or four very similar classes at different levels of the application. Of course, it's not very good, so Aspect Programming may be applied in this case.