Designing N-Tier client/server architecture is no less complex than developing two-tier architecture, however the N-Tier architecture, produces a far more flexible and scalable client/server environment. In two-tier architecture, the client and the server are the only layers. In this model, both the presentation layer and the middle layer are handled by the client. N-Tier architecture has a presentation layer and three separate layers - a business logic layer and a data access logic layer and a database layer. The next section discusses each of these layers in detail.
In a typical N-Tier environment, the client implements the presentation logic (thin client). The business logic and data access logic are implemented on an application server(s) and the data resides on database server(s). N-tier architecture is typically thus defined by the following layers:
Database Layer: Made up of a RDBMS database component such as SQL Server that provides the mechanism to store and retrieve data.
Now that you have a general understanding of the different layers in a N-Tier application, let us move onto discuss the implementation of a N-Tier Web application.
In this article, I will consider an example web site (that displays authors and author titles information) constructed using N-Tier principles and use the example Web site to demonstrate the new features of ASP.NET 2.0 and SQL Server 2005. The sample Web site shown in this example is very simple and straightforward and will consist of only two pages: the first page will show the list of authors from the pubs database and the second page will display the list of titles specific to a selected author.
Please note that this article is not aimed at providing an exhaustive coverage of the individual features of ASP.NET 2.0, instead it only focuses on helping the readers understand the features of ASP.NET 2.0 and SQL Server 2005 that are essential to building a N-Tier web application.
The following screenshot shows the different layers in the example application. It also highlights the important characteristics of the example application.
Some of the important characteristics of the sample application are as follows:
1.The stored procedures in the SQL Server 2005 database are created using C#. The ability to create stored procedures in managed code enables complex business logic to be executed close to the database resulting in performance improvements. The compiled nature of the stored procedure also results in increased performance.
2.The data access layer classes are generated using the new TableAdapter Configuration Wizard, which enables you to create data access layer classes without writing a single line of code.
3.ASP.NET Web forms in the user interface layer are generated using master pages, providing a consistent look and feel for the entire application.
4.Web forms utilize ObjectDataSource control to directly bind the output of the middle tier methods to data bound controls such as a GridView control.
5.Web forms also take advantage of caching of database contents to increase the performance and throughput of the web application. This is made possible by the use of the database cache invalidation mechanism that can automatically remove specific items from the cache when the data in the database table changes.
Implementation of the Application
I will discuss the implementation by discussing each of the above layers, starting with the database layer.
Database Objects using Managed Code
One of the neat features of SQL Server 2005 is the integration with the .NET CLR. The integration of CLR with SQL Server extends the capability of SQL Server in several important ways. This integration enables developers to create database objects such as stored procedures, user defined functions, and triggers by using modern object-oriented languages such as VB.NET and C#. In this article, I will demonstrate how to create the stored procedures using C#. Before looking at the code, let us understand the pros and cons of using managed language in the database tier to create server side objects.
T-SQL Vs Managed Code
Although T-SQL, the existing data access and manipulation language, is well suited for set-oriented data access operations, it also has limitations. It was designed more than a decade ago and it is a procedural language rather than an object-oriented language. The integration of the .NET CLR with SQL Server enables the development of stored procedures, user-defined functions, triggers, aggregates, and user-defined types using any of the .NET languages. This is enabled by the fact that the SQL Server engine hosts the CLR in-process. All managed code that executes in the server runs within the confines of the CLR. The managed code accesses the database using ADO.NET in conjunction with the new SQL Server Data Provider. Both Visual Basic .NET and C# are modern programming languages offering full support for arrays, structured exception handling, and collections. Developers can leverage CLR integration to write code that has more complex logic and is more suited for computation tasks using languages such as Visual Basic .NET and C#. Managed code is better suited than Transact-SQL for number crunching and complicated execution logic, and features extensive support for many complex tasks, including string handling and regular expressions. T-SQL is a better candidate in situations where the code will mostly perform data access with little or no procedural logic. Even though the example you are going to see in this article is best written using T-SQL, I will take the managed code approach and show you how to leverage that feature.
Creating CLR Based Stored Procedures
For the purposes of this example, create a new SQL Server Project using Visual C# as the language of choice in Visual Studio 2005. Since you are creating a database project, you need to associate a data source with the project. At the time of creating the project, Visual Studio will automatically prompt you to either select an existing database reference or add a new database reference. Choose pubs as the database. Once the project is created, select Add Stored Procedure from the Project menu. In the Add New Item dialog box, enter Authors.cs and click Add button. After the class is created, modify the code in the class to look like the following.
using System;
using System.Data;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
public class Authors
{
[SqlProcedure]
public static void GetAuthors()
{
SqlPipe sp = SqlContext.Pipe;
using (SqlConnection conn = new
SqlConnection("context connection=true"))
{
conn.Open();
SqlCommand cmd = new SqlCommand();
cmd.CommandType = CommandType.Text;
cmd.Connection = conn;
cmd.CommandText = "Select DatePart(second, GetDate()) " +
" As timestamp,* from authors";
SqlDataReader rdr = cmd.ExecuteReader();
sp.Send(rdr);
}
}
[SqlProcedure]
public static void GetTitlesByAuthor(string authorID)
{
string sql = "select T.title, T.price, T.type, " +
"T.pubdate from authors A" +
" inner join titleauthor TA on A.au_id = TA.au_id " +
" inner join titles T on TA.title_id = T.title_id " +
" where A.au_id = '" + @authorID + "'";
using (SqlConnection conn = new
SqlConnection("context connection=true"))
{
conn.Open();
SqlPipe sp = SqlContext.Pipe;
SqlCommand cmd = new SqlCommand();
cmd.CommandType = CommandType.Text;
cmd.Connection = conn;
cmd.CommandText = sql;
SqlParameter paramauthorID = new
SqlParameter("@authorID", SqlDbType.VarChar, 11);
paramauthorID.Direction = ParameterDirection.Input;
paramauthorID.Value = authorID;
cmd.Parameters.Add(paramauthorID);
SqlDataReader rdr = cmd.ExecuteReader();
sp.Send(rdr);
}
}
}
Let us examine the above lines of code. The above code starts by importing the required namespaces and then declares a class named Authors. There are two important classes in the Microsoft.SqlServer.Server namespace that are specific to the in-proc provider:
***SqlContext: This class encapsulates the extensions required to execute in-process code in SQL Server 2005. In addition it provides the transaction and database connection which are part of the environment in which the routine executes.
***SqlPipe: This class enables routines to send tabular results and messages to the client. This class is conceptually similar to the Response class found in ASP.NET in that it can be used to send messages to the callers.
The Authors class contains two static methods named GetAuthors and GetTitlesByAuthor. As the name suggests, the GetAuthors method simply returns all the authors from the authors table in the pubs database and the GetTitlesByAuthor method returns all the titles for a specific author.
Inside the GetAuthors method, you start by getting reference to the SqlPipe object by invoking the Pipe property of the SqlContext class.
SqlPipe sp = SqlContext.Pipe;
Then you open the connection to the database using the SqlConnection object. Note that the connection string passed to the constructor of the SqlConnection object is set to "context connection=true" meaning that you want to use the context of the logged on user to open the connection to the database.
using (SqlConnection conn = new SqlConnection("context connection=true"))
Here open the connection to the database using the Open() method.
conn.Open();
Then you create an instance of the SqlCommand object and set its properties appropriately.
SqlCommand cmd = new SqlCommand();
cmd.CommandType = CommandType.Text;
cmd.Connection = conn;
cmd.CommandText = "Select DatePart(second, GetDate()) " + " As timestamp,* from authors";
Finally you execute the sql query by calling the ExecuteReader method of the SqlCommand object.
SqlDataReader rdr = cmd.ExecuteReader();
Then using the SqlPipe object, you then return tabular results and messages to the client. This is accomplished using the Send method of the SqlPipe class.
sp.Send(rdr);
The Send method provides various overloads that are useful in transmitting data through the pipe to the calling application. Various overloads of the Send method are:
*Send (ISqlDataReader) - Sends the tabular results in the form of a SqlDataReader object.
*Send (ISqlDataRecord) - Sends the results in the form of a SqlDataRecord object.
*Send (ISqlError) - Sends error information in the form of a SqlError object.
*Send (String) - Sends messages in the form of a string value to the calling application.
Both the methods in the Authors class utilize one of the Send methods that allows you to send tabular results to the client application in the form of a SqlDataReader object. Since the
GetTitlesByAuthor method implementation is very similar to the GetAuthors method, I will not be discussing that method in detail.
Now that the stored procedures are created, deploying it is very simple and straightforward. Before deploying it, you need to build the project first. To build the project, select Build->Build
from the menu. This will compile all the classes in the project and if there are any compilation errors, they will be displayed in the Error List pane. Once the project is built, you can then deploy it onto the SQL Server by selecting Build->Deploy from the menu. This will not only register the assembly in the SQL Server but also deploy the stored procedures in the SQL Server. Once the stored procedures are deployed to the SQL Server, they can then be invoked from the data access layer, which is the topic of focus in the next section.
Before executing the stored procedure, ensure you execute the following sql script using SQL Server Management Studio to enable managed code execution in the SQL Server.
EXEC sp_configure 'clr enabled', 1;
RECONFIGURE WITH OVERRIDE;
GO
Data Access Layer using TableAdapter Configuration Wizard
Traditionally the process you employ to create data access layer classes is a manual process, meaning that you first create a class and then add the appropriate methods to it. With the introduction of Visual Studio 2005, Microsoft has introduced a new TableAdapter Configuration Wizard that makes creating a data access logic layer class a breezy experience. Using this wizard, you can create a data access logic layer component without having to write a single line of code. This increases the productivity of the developers to a great extent. Once you create those classes, you can consume them exactly the same way you consume built-in objects. Before looking at an example, let us briefly review what a TableAdapter is. A TableAdapter connects to a database, executes queries, or stored procedures against a database, and fills a DataTable with the data returned by the query or stored procedure. In addition to filling existing data tables with data, TableAdapters can return new data tables filled with data. The TableAdapter Configuration Wizard allows you to create and edit TableAdapters in strongly typed datasets. The wizard creates TableAdapters based on SQL statements or existing stored procedures in the database. Through the wizard, you can also create new stored procedures in the database.
This section will discuss the creation of a data access component that will leverage the stored procedures created in the previous step. To start, create a new ASP.NET web site named NTierExample in Visual C# by selecting New Web Site from the File menu as shown below.
To create a data component, begin by right clicking on the web site and selecting Add New Item from the context menu. In the Add New Item dialog box, select DataSet from the list of templates. Change the name of the file to Authors.xsd and click Add.
When you click Add, you will be prompted if you want to place the component inside the App_Code directory. Click OK in the prompt and this will bring up the TableAdapter Configuration Wizard. In the first step of the TableAdapter Configuration Wizard, you need to specify the connection string and in the second step you will be prompted if you want to save the connection string in the web.config file. In this step, save the connection string to the web.config file by checking the check box.
In the next step, you will be asked to choose a command type. Select the Use existing stored procedures option as shown below and click Next.
Clicking Next in the above screen brings up the following screen wherein you select the stored procedure to use.
Click Next in the above dialog box and you will see the Choose Methods to Generate dialog box wherein you can specify the name of the method that will be used to invoke the stored procedure selected in the previous step. Specify the name of the method as GetAuthors as shown below:
