Creating ASP.NET Applications with N-Tier Architecture

This article describes how to build ASP.NET applications using n-tier architecture. The benefits of having n-tier architecture is that all the modules having dedicated functionality will be independent of each other. Changing one tier will not effect other tiers and there is no single point of failure even if some tier is not working. 


In a typical n-tier application there will be 4 Layers. The bottom most layer is the Data layer which contains the tables and stored procedures, scaler function, table values function. This Data layer is typically the database engine itself. We will be using SqlServeras the data layer in our example.

On top of Data Layer, we have a Data Access Layer (DAL). This layer is responsible for handling Database related tasks i.e. only data access. This Data access layer is created as a separate solution so that the changes in DALonly need the recompilation of DAL and not the complete website. The benefit of having this layer as a separate solution is that in case the database engine is changes we only need to change the DALand the other areas of the website need not be changed and recompiled. Also the changes in other areas outside this solution will not demand for DAL recompilation.

On top of DAL, we have our Business Logic Layer(BLL). BLLcontains all the calculations and Business Rule validations that are required in the application. It is also in a separate solution for the reason that if the Business rules change or the calculations change we only need to recompile the BLL and the other layers of the application will remain unaffected.

Finally on top of BLLwe have our Presentation Layer. The Presentation layer for an ASP.NET web forms application is all the Forms ( apsxpages and their code behinds) and the classes contained in the App_Code folder. The Presentation layer is responsible for taking the user input, showing the data to the user and mainly performing input data validation.

Note: Input data filtration and validation is typically done at the Presentation Layer(Both client side and server side). The business Rule validation will be done at the BLL.

So to visualize the above mentioned architecture:


Note: The Data Access Layer in this article was written using classic ADO.NET, due to which the amount of code in DALis little too much. Nowadays using ORMs like Entity framework to generate the DALis recommended. The DALcode will be generated by ORM itself.

Using the code

Let us develop a small Toy ASP.NET application that will use n-tier architecture. We will develop a small Employee Management application for the NorthWindDatabase. (For simplicity, I have removed all other tables from the DB and some columns from the Employee table). This application should be able to perform the basic CRUD operations on the DB.

The solution for this application will contain separate projects for DALand BLL. The Data Layer will be SqlServer. The Presentation Layer is an ASP.NET website running on top of these projects.


The Data Layer

The data layer in this example contain only one table called Employee. The data layer also contains the stored procedures for all the basic operations on the Employee table. So let us look at the table and all the stored Procedures we have in our Data Layer.


Now we will create a set of stored procedures to perform the operations on the Employees Table.

Now we have our Data Layer ready.

The Data Access Layer

Now we will go ahead and create a Data Access Layer for our application. The data access layer will contain 2 main type of classes. A set of classes that will represent the Table entities. And classes to perform the CRUDoperations on the database.


The Employeeclass in the above diagram is the Entity that will represent the Employeetable. This class has been created so that the Layers above the DALwill use this class to perform operations in Employee table and they need not worry about the table schema related details.

The EmployeeDBAccessclass expose the methods to perform the CRUDoperations on the Employee table.

The class SqlDbHelperis a wrapper class for ADO.NETfunctions providing a more simpler interface to use by the rest of DAL.

Note: If we use any ORM(Object Relation Mapper) then DAL need not be written. The ORMwill generate all the DAL code. Entity frameworkis one of the best ORMsavailable. This DAL can simply be replaced with a class library containing the Entity Framework generated Entities and Contexts.

The Business Logic Layer

The business logic layer will have a reference to the DAL and will mainly perform Business rule validation and business logic specific calculations. In out example, I will write a simple BLLthat will govern the IO between the DALand Presentation layer. In real applications the BLLwill contain more logic and code.


The Presentation Layer

The presentation layer now contains only a set of pages and code behinds and it will use the BLLand the the Employee class to perform all the operations. The add Operation can be seen as an example how the BLLis being used to perform an operation.


Note: All the CRUD operations have been implemented. Please refer tio the sample code for all the details. When we run the application we can see all the EDIT/UPDATE, DELETE and ADD operations in action.


Point of Interest

I created this small application to demonstrate application development using n-tier architecture. The demo application has been created to show the basic idea behind the 3-tier architecture. There are many things that are still missing from this sample from the completion perspective. Client side validation and server side validation in presentation layer, Business rule validation and calculations in BLL are some missing things.

Since the idea here was to talk about how to put n-tier architecture in actual code, I think this article might have provided some useful information on that. I hope this has been informative.

[UPDATE] Note: In this article I am reusing the Employeemodel in the presentation layer. This model is defined in Data Access Layer. Due to this the presentation layer has to refer to the data access layer. This is not ideal in the real world scenarios(as pointed out in many of the comments below). Ideal solution for this would be to have two different models for Employee. the current model which is defined in the data access layer can be called as the data model and the business logic layer can create a model for employee which will be called as domain model. The business logic layer will then have to contain the code for mapping the data model to the domain model and vice versa. This mapping can be done either manually or a tool like AutoMappercan also be used to perform such mapping. With this change the presentation layer need not refer to the data access layer but it can refer to the business logic layer and use the Employee domain model from that.

In this article the n-tier architecture is specifically a data centric n-tier and not a domain centric one. If we need to design the application in a domain centric n-tier architecture then we need to follow a different way of organizing our layers. But perhaps that is a topic which deserves a separate discussion altogether but I wanted to point out the possibility of a domain centric n-tier architecture in this article.

Download sample code for this article: nTierDemo