Presentation tier in .NET framework

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Figure 6.1 A three-tier architecture: Presentation layer + Business layer + Database layer
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SOA as a precursor to the cloud
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of application is the one most responsible for the success of application servers.
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Tightly coupled (clustered) architectures are a form of parallel processing. This refers typically to a cluster of machines that work closely together, running a shared process in parallel. The task is subdivided in parts made individually by each one and then put back together to make the final result. Peer-to-peer is clientless and has no single point of failure that can cause total failure. This type of architecture has no special machine or machines that provide a service or manage the network resources. Instead, all responsibilities are uniformly divided among all machines, known as peers. Peers can serve both as clients and servers.
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Application server
Frameworks API Web services
Integration services
JDBC Service gateways
Data services
Database File servers Communication
This book focuses on the multitier architectures (three-tier and N-tier) because they apply best to the web and to the cloud. This is because the browser is the definition of a thin client presentation layer where the work has to be done on an application server on its behalf. A SOA falls into the same category. The next section will drill down into the SOA style of distributed application.
Figure 6.2 An N-tier architecture. Many variations are possible. But generally, an application server is involved. From the application server, many different logical layers can be accessed. At the application server level, you can begin to interact with the cloud. Any or all of these layers can operate in the cloud effectively.
Loose coupling
In computer science, coupling refers to the degree of direct knowledge that one component has of another. It s the degree to which components depend on one another. What does this have to do with reliability or the cloud Loose coupling affects reliability because each component that operates somewhat independently from all other objects can be built, tested, and replaced separately from all other objects. It s easier to build the other components such that they can handle when this component fails, either by failing gracefully themselves or by accessing another instance of this component running somewhere else. Earlier, you learned about humans interacting with websites through a browser, and one machine at one site interacting with another machine at another site. Loose coupling is the only application architecture that can provide reliable web applications, because one site never knows when another may be out, slow, or have made an unannounced change in its interface.
Achieving high reliability at cloud scale
Strong Coupling
Figure 6.3 Strong coupling. Changes in A impact B, C, and D. Changes in B impact A, C, and D.
As some have pointed out, the ultimate way to make two components loosely coupled is to not connect them at all; short of that, make sure the communications between components don t depend on internals of the component and only access an abstract interface layer. Loose Coupling At the class level, strong coupling occurs when a dependent class contains a pointer directly to a concrete A class that provides the required behavior. This is shown abstractly in figure 6.3. Loose coupling occurs when the dependent class contains a pointer only to an interface, which can then be implemented by one or many concrete C B classes. Loose coupling provides extensibility to designs (see figure 6.4). You can later add a new concrete class that implements the same interface without ever having to modify and recompile the dependent class. Strong D coupling doesn t allow this. Tight coupling leads to a situation where a change Figure 6.4 Loose coupling. in one module forces a ripple effect of changes in Modifications in A s behavior other modules. Further, assembly of modules requires don t impact B, C, or D. Modifications in B s behavior more effort and time due to the increased intermodule may affect A but nothing else. dependencies. One module may be harder to reuse because dependent modules must be included with it. Loosely coupled systems benefit from the negation of each of these characteristics. Tight versus loose coupling is an important concept for application reliability, SOAs, and ultimately reliable cloud applications. Table 6.1 lists a series of important characteristics of applications that can be measured against an application tightly
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