3 Tier Architecture - In need of an example

Wikipedia have a nice explanation: Multitier architecture:

'Three-tier' is a client-server architecture in which the user interface, functional process logic ("business rules"), computer data storage and data access are developed and maintained as independent modules, most often on separate platforms.

Web development usage

In the web development field, three-tier is often used to refer to websites, commonly electronic commerce websites, which are built using three tiers:

• A front end web server serving static content, and potentially some cached dynamic content.
• A middle dynamic content processing and generation level application server, for example Java EE, ASP.net, PHP platform.
• A back-end database, comprising both data sets and the database management system or RDBMS software that manages and provides access to the data.

By "tier" do you mean a "layer" in your software stack? The word "tier" is better used to describe the physical components of your system. If you are using ASP.NET, you probably already have a "3 tiered" system -

1. Browser displaying web pages
2. IIS Server hosting your app
3. Database Server with your database

But you are possibly putting all of your code into a single software "layer" - specifically, the code behind file of your aspx pages. You want to move from a single layer to a 3 layer approach. The classic "3 layer" software architecture consists of the following -

1. Presentation Layer

2. Business Logic Layer (BLL)

3. Data Access Layer (DAL)

_{(source: asp.net)}

For a typical ASP.NET app, you might apply this as follows. First, you create a LINQ2SQL file (.dbml) containing the objects for your database access. This is your Data Access Layer (DAL).

Next you might create a DLL to contain your Business Logic Layer (BLL). This layer will access the database via the DAL, manipulate it as required, and then expose it via a simple interface. For example, if your application displays a client list, your BLL might have a public function called GetClientList() which returned a list of clients.

Finally you would set up your code behind files to instantiate the BLL and wire it up to the interface components. This is your Presentation Layer. For example, it might take the data returned from your GetClientList() function and bind it to a data grid on the web form. The idea is to have the presentation layer as thin as possible.

This seems a little long-winded to describe, but it's pretty straight-forward once you have done it a couple of times. You will find that separating out your application like this will make it much easier to maintain, as the separation of concerns leads to cleaner code. You will also find it much easier to upgrade or even replace your presentation layer, as it contains very little smarts. Finally, you will get to a point where you have a number of very useful BLL libraries that you can easily consume in new applications, greatly improving productivity.

This is what I have in my project. More than just a traditional 3-tier architecture.

1.) Application.Infrastructure

• Base classes for all businessobjects, busines object collection, data-access classes and my custom attributes and utilities as extension methods, Generic validation framework. This determines overall behavior organization of my final .net application.

2.) Application.DataModel

• Typed Dataset for the Database.
• TableAdapters extended to incorporate Transactions and other features I may need.

3.) Application.DataAccess

• Data access classes.
• Actual place where Database actions are queried using underlying Typed Dataset.

4.) Application.DomainObjects

• Business objects and Business object collections.
• Enums.

5.) Application.BusinessLayer

• Provides manager classes accessible from Presentation layer.
• HttpHandlers.
• My own Page base class.
• More things go here..

6.) Application.WebClient or Application.WindowsClient

• My presentation layer
• Takes references from Application.BusinessLayer and Application.BusinessObjects.

Application.BusinessObjects are used across the application and they travel across all layers whenever neeeded [except Application.DataModel and Application.Infrastructure]

All my queries are defined only Application.DataModel.

Application.DataAccess returns or takes Business objects as part of any data-access operation. Business objects are created with the help of reflection attributes. Each business object is marked with an attribute mapping to target table in database and properties within the business object are marked with attributes mapping to target coloumn in respective data-base table.

My validation framework lets me validate each field with the help of designated ValidationAttribute.

My framrwork heavily uses Attributes to automate most of the tedious tasks like mapping and validation. I can also new feature as new aspect in the framework.

A sample business object would look like this in my application.

User.cs

[TableMapping("Users")]
public class User : EntityBase
{
    #region Constructor(s)
    public AppUser()
    {
        BookCollection = new BookCollection();
    }
    #endregion

    #region Properties

    #region Default Properties - Direct Field Mapping using DataFieldMappingAttribute

    private System.Int32 _UserId;

    private System.String _FirstName;
    private System.String _LastName;
    private System.String _UserName;
    private System.Boolean _IsActive;

    [DataFieldMapping("UserID")]
    [DataObjectFieldAttribute(true, true, false)]
    [NotNullOrEmpty(Message = "UserID From Users Table Is Required.")]
    public override int Id
    {
        get
        {
            return _UserId;
        }
        set
        {
            _UserId = value;
        }
    }

    [DataFieldMapping("UserName")]
    [Searchable]
    [NotNullOrEmpty(Message = "Username Is Required.")]
    public string UserName
    {
        get
        {
            return _UserName;
        }
        set
        {
            _UserName = value;
        }
    }

    [DataFieldMapping("FirstName")]
    [Searchable]
    public string FirstName
    {
        get
        {
            return _FirstName;
        }
        set
        {
            _FirstName = value;
        }
    }

    [DataFieldMapping("LastName")]
    [Searchable]
    public string LastName
    {
        get
        {
            return _LastName;
        }
        set
        {
            _LastName = value;
        }
    }

    [DataFieldMapping("IsActive")]
    public bool IsActive
    {
        get
        {
            return _IsActive;
        }
        set
        {
            _IsActive = value;
        }
    }

    #region One-To-Many Mappings
    public BookCollection Books { get; set; }

    #endregion

    #region Derived Properties
    public string FullName { get { return this.FirstName + " " + this.LastName; } }

    #endregion

    #endregion

    public override bool Validate()
    {
        bool baseValid = base.Validate();
        bool localValid = Books.Validate();
        return baseValid && localValid;
    }
}

BookCollection.cs

/// <summary>
/// The BookCollection class is designed to work with lists of instances of Book.
/// </summary>
public class BookCollection : EntityCollectionBase<Book>
{
    /// <summary>
    /// Initializes a new instance of the BookCollection class.
    /// </summary>
    public BookCollection()
    {
    }

    /// <summary>
    /// Initializes a new instance of the BookCollection class.
    /// </summary>
    public BookCollection (IList<Book> initialList)
        : base(initialList)
    {
    }
}