3 Tier Architecture - In need of an example

Question

Presently I am working using single tier architecture. Now I am wanting to learn how to write code using 3 tier architecture. Please can you provide me with a simple example?

Answer 1

A good tutorial, with complete source control download of a well written tiered application would be here:

http://nerddinnerbook.s3.amazonaws.com/Intro.htm

This isn't a tutorial about tiered architecture, but it's a well written app and gives some insight into why you might consider this architecture.

Additionally, as has only been briefly touched on above, this is about keeping your logic/storage/presentation code separate, so if you have to change one of them (e.g change from asp.net front end to a desktop application), it's not so hard to do.

Answer 2

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)

alt text http://weblogs.asp.net/blogs/fredriknormen/WindowsLiveWriter/UsingWebServicesina3tierarchitecture_134F6/3tier_2.jpg

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.

Answer 3

Presentation layer: put everything that is related to user interface. (What the user sees)

Business layer: everything that is related to the logic of the application (How is the information coming from presentation layer treated)

Data layer: provide an abstraction of the underlying data source(s) (Where and how the information coming from/going to business layer is stored)

Each layer should know as less as possible about the other and it should be a top down approach:

• the data layer should know nothing about business and presentation
• business layer should know about data but not about presentation
• presentation should know about business but not about data

Simple example:

Website:

• Presentation: all the graphical things, fields where user inserts data, menus, pictures, etc.
• Business: all constraints about the data (unique name, name without symbols, valid date, etc), methods for manipulating business objects (create new user, add new order, etc)
• Data: Methods that access the underlying database.

Answer 4

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

Basically, there are 3 layers:

• tier 1 (presentation tier, GUI tier)
• tier 2 (business objects, business logic tier)
• tier 3 (data access tier). These tiers can be developed and tested separately.

What is the need for dividing the code in 3-tiers? Separation of the user interface from business logic and database access has many advantages. Some of the advantages are as follows:

Reusability of the business logic component results in quick development. Let's say we have a module that handles adding, updating, deleting and finding customers in the system. As this component is developed and tested, we can use it in any other project that might involve maintaining customers.

Transformation of the system is easy. Since the business logic is separate from the data access layer, changing the data access layer won’t affect the business logic module much. Let's say if we are moving from SQL Server data storage to Oracle there shouldn’t be any changes required in the business layer component and in the GUI component.

Change management of the system is easy. Let's say if there is a minor change in the business logic, we don’t have to install the entire system in individual user’s PCs. E.g. if GST (TAX) is changed from 10% to 15% we only need to update the business logic component without affecting the users and without any downtime.

Having separate functionality servers allows for parallel development of individual tiers by application specialists.

Provides more flexible resource allocation. Can reduce the network traffic by having the functionality servers strip data to the precise structure needed before sending it to the clients.

Answer 5

A 3-tier architecture usually has the following components:

1. Client Browser
2. Web server hosting the ASP.NET application
3. Some backend storage such as database that is being accessed by the ASP.NET application

So to answer your question on how to write code for a 3-tier architecture, you develop an ASP.NET application that communicates with a data storage.

Answer 6

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)
    {
    }
}