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As discussed earlier in the chapter, it is possible to deploy the same logical layer onto multiple physical tiers. Using this idea, the Data Access layer can be put on an application server, and the Business Logic layer on both the client workstation and the application server, as shown in Figure 1-4.
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Figure 1-4. The five logical layers with separate application and database servers
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Putting the Data Access layer on the application server centralizes all access to the database on a single machine. In .NET, if the connections to the database for all users are made using the same user ID and password, you ll get the benefits of connection pooling for all your users. What this means immediately is that there will be far fewer connections to the database than there would be if each client machine connected directly. The actual reduction depends on the specific application, but often it means supporting 150 to 200 concurrent users with just two or three database connections! Of course, all user requests now go across an extra network hop, thereby causing increased latency (and therefore decreased performance). This performance cost translates into a huge scalability gain, however, because this architecture can handle many more concurrent users than a 2-tier physical configuration. With the Business Logic layer deployed on both the client and server, the application is able to fully exploit the strengths of both machines. Validation and a lot of other business processing can run on the client workstation to provide a rich and highly interactive experience for the user, while non-interactive processes can efficiently run on the application server. If well designed, such an architecture can support thousands of concurrent users with adequate performance.
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As with a Windows Forms application, the best performance is received from a web-based application by minimizing the number of physical tiers. However, the trade-off in a web scenario is different: in this case, it is possible to improve performance and scalability at the same time, but at the cost of security, as I will demonstrate. To get optimal performance in a web application, it is desirable to run most of the code in a single process on a single machine, as shown in Figure 1-5. The Presentation layer must be physically separate because it s running in a browser, but the UI, Business Logic, and Data Access layers can all run on the same machine, in the same process. In some cases, you might even put the Data Management layer on the same physical machine, though this is only suitable for smaller applications. This minimizes network and communication overhead and optimizes performance. Figure 1-6 shows how it is possible to get very good scalability, because the web server can be part of a web farm in which all the web servers are running the same code.
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CHAPTER 1 s DISTRIBUTED ARCHITECTURE
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Figure 1-5. The five logical layers as used for web applications
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Figure 1-6. The five logical layers deployed on a load-balanced web farm
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This setup provides very good database-connection pooling because each web server will be (potentially) servicing hundreds of concurrent users, and all database connections on a web server are pooled.
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s Note
In COM-based technologies such as ASP and Visual Basic 6, this configuration was problematic, because running COM components in the same process as ASP pages had drawbacks in terms of the manageability and stability of the system. Running the COM components in a COM+ server application addressed the stability issues, but at the cost of performance. These issues have been addressed in .NET, however, so this configuration is highly practical when using ASP.NET and other .NET components.
Unless the database server is getting overwhelmed with connections from the web servers in the web farm, a separate application server will rarely provide gains in scalability. If a separate application server is needed, there will be a reduction in performance because of the additional physical tier. (Hopefully, there will be a gain in scalability, because the application server can consolidate database connections across all the web servers.) It is important to consider fault tolerance in this case, because redundant application servers may be needed in order to avoid a point of failure. Another reason for implementing an application server is to increase security, and that s the topic of the next section.
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