Visual Studio .NET指南：探索.NET开发技术全貌

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Three-Tier Development

The three-tier model (shown in Figure 2-3) improves on the two-tier model by dividing

applications into service tiers: User Services, Business Services, and Data Services. These

divisions allow for greater scalability and reliability.

Figure 2-3: The logical three-tier model.

User Services

The User Services tier, also referred to as the presentation layer, is made up of windows

executables and/or web pages, such as dynamic HTML or Active Server Pages. User Services

tier is the interface that is used to display data to the user and to receive input. In a three-tier

model, it is unnecessary for the client or the User Services tier to know anything about the

database or any other service that is provided by the Data Services tier.

Business Services

The second tier is the Business Services tier, which is responsible for knowing exactly how to

access data. These responsibilities include requesting data on behalf of the User Services tier

and returning query results. The Business Services tier can, and in many cases should,

maintain business rules.

The Business Services tier, as shown in Figure 2-4

, exposes all the functionality that the User

Services tier requires. While its main purpose is to decouple the User Services from the Data

Services, the Business Services tier does much more. Any function, including calculations and

other application-specific tasks, are available through the Business Services tier. All User

Services have access to the Business Services tier, which physically resides on a server

accessible via the network. Any new or modified business rule needs only be deployed to the

Business Services tier, thus eliminating the need to redistribute anything to the client

application.

Figure 2-4: Business Services tier components being used by all User Services.

Data Services

In a further effort to decouple application services so that they can be more easily managed

and supported, the Data Services tier provides data access to the Business Services tier, which

in turn passes this data on to the client application in the User Services tier. ADO.NET and

the Database Management System (DBMS) are both maintained in the Data Services tier.

ADO.NET is Microsoft's solution for universal data access, and SQL Server is Microsoft's

solution for the DBMS. Both provide data access: ADO.NET provides a method for getting at

data and SQL Server provides the database engine that is required for maintaining the data

itself.

Code Management

Code management in a three-tier application is much easier to support and less problematic

than with a two-tier application. Since the application is logically and physically divided,

there is no need for a single development team. Presentation developers can build the user

interface without having access to data, business tier developers no longer need to understand

user interface requirements, and database programmers can focus on relational data and the

implementation of known business rules. Because each service tier is physically separated,

they can each be compiled and/or reconfigured without involving the other services, thus

freeing up the developers and reducing coordination requirements.

Scalability

Scalability is greatly improved in three-tier applications because database connections can be

disconnected or maintained for use by other clients, reducing the number of concurrent

database connections that are needed to support an application. Processing is transferred from

the client to the Business Services server, and network performance can increase because the

Business Services server can communicate with the database server on the same network,

reducing the amount of network traffic.

Scalability is also enhanced through the clustering of the Business Services and database

servers. Business components loaded into memory can remain loaded through services

provided by COM+ and Enterprise Services-this increases the number of users that can be

supported by the Business Services server because the time required to load a component

from the hard drive into memory is significantly reduced.

Business Rules

With one exception, there is no real right way to implement business rules. That exception is

that you should never place business rules in the client application or User Services tier where

the client applications can easily bypass them. As a result, if a rule is placed in one client

application, it must be placed in every application accessing the database.

You can reliably enforce business rules in either the Business Services or Data Services tier.

When placing business rules in the Business Services tier, make sure that all applications

accessing your data are also using your Business Services tier's components. If an application

can bypass the Business Services component that enforces the business rule, the rule itself is

not enforced. When enforcing business rules in the Data Services tier, no application can

bypass the business rules, hence the advantage of placing business rules here.

Programming languages such as Visual Basic .NET, Visual C++, and C# are well suited to

implementing business rules. However, the objects and languages that are available in SQL

Server for enforcing business rules are column-level constraints, table-level triggers, and

Transact SQL (Microsoft's SQL Server programming language). Although SQL Server's

Transact SQL is not a particularly flexible language, with a little effort and a lot of practice

you can implement almost any business rule.

ote When Transact SQL is not sufficient, Microsoft provides stored procedures for calling

COM components. These stored procedures are sp_oacreate, sp_oagetproperty,

sp_oasetproperty, sp_oamethod, and sp_oadestroy. For more information on these

stored procedures, refer to Microsoft's online SQL Server documentation.

N-Tier Development

Strategically dividing an application into tiers can greatly improve an application's scalability,

performance, flexibility, and manageability. Giving each tier a specific task allows the

development and system configuration to focus on the needs of the tasks that are performed

by that tier. Further dividing these tiers can let you further separate out the tasks of the tier.

Any application with three or more tiers is considered an N-tier application. "N" represents

any number greater than two. For the purposes of this section, N-tier refers to a five-tier

model, which is the same as the three-tier model with the Business Services tier divided into

three tiers or tier classes (as shown in Figure 2-5

): Facades, Main Business, and Data Access.

Services form or web page will need to instantiate, or load, several components to perform

any task. With the Facades classes, the User Services developer needs only to instantiate the

Facades class, and the Facades class deals with the complexity of business-functionality

instantiation.

Main Business Class

The Main Business classes or Business Level Layer (BLL) provide actual business

functionality, including enforcing business rules, maintaining collection classes, ensuring

business functionality, and providing access to data components. The Main Business classes

provide the actual intelligence of the application. The Facades classes call the appropriate

main business components (Figure 2-7), and the Data Access components only access data

they are told to access. You'll learn about Data Access components in the next section.

Figure 2-7: The relationship of the Main Business classes to Facades and Data Access classes.

Data Access Class

Data Access components (Figure 2-8), or Data Access Layer (DAL), perform data access on

behalf of the Main Business classes. These components know about the Database Services

tier, and as long as this is the only tier that is required to know how to access data, the

transition from one type of database server to another is as simple as modifying this tier alone.

The rest of the application is shielded from the burden of knowing how to access data.

Figure 2-8: The Data Access component ready to access and pass data to the Business

Services tier.

ADO.NET is Microsoft's preferred method for accessing data, no matter what the database

provider is. ADO.NET provides access to a multitude of data sources and methods, including

SQL Server, Oracle, Sybase, MS Access, MS Word, MS Excel, and so on. In tier

development, the data source is typically a DBMS (Database Management System) that

provides a variety of data access methods. When using ADO.NET, application developers

tend to use the query components of ADO.NET, although other database-specific methods are

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Visual Studio .NET指南：探索.NET开发技术全貌

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