source code to generate barcode in vb.net 26: Windows Services in C#

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26: Windows Services
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controlling the application because the developer must be aware of the location of the services Scalability is also a big issue for 2-tier modeled applications The larger the application and user base becomes, the harder it is to manage
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Three-Tier Model
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Arising from the concerns of the 2-tier model came the establishment of the 3-tier model In this case, all three services are handled by different applications and, in most cases, by three different machines The client or front-end application provides the visual interface for the user Business logic is retrieved from components running on remote servers (although this is not absolutely necessary it can be found on the same machine) Business services attach to the back-end data store and retrieve the data for the presentation service The advantages of this model are many Different platforms can provide separate service functionality The client can be a browser-based application connecting to a middle-tier provided by an entirely different operating system The middle tier can be ignorant of the type of service on either side of it, meaning that the database engine is not important to it, and the vehicle producing the user interface is immaterial to it It is a separate and distinct component that can be used by multiple applications
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-Tier Model
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We now get to the crux of the matter by taking the services and separating them (making them completely distinct and non-cohesive), we now can take advantage of the scalability that this model provides The business service can be split amongst many different applications or servers Data services can be provided by multiple platforms and data vendors The interaction of these services brings us to the topic of the rest of this chapter Components provide the functionality and separation that we have been talking about in an n-tier model Life would be wonderful if everyone in the development world chose the same platform and techniques to produce these components, but that is just not the case We have spoken in earlier chapters about working with COM components in Windows and web applications In this chapter, we will look at implementing a COM+ component, creating interfaces that are visible to COM, and working with strongly named assemblies The goal is to make sure that the middle-tier (the business services) are dealt with in an n-tiered environment
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PART V
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As you may have been able to determine, one of the goals of multi-tiered applications (or applications that make use of middle-tier services) is to provide for code reuse You may have created a module that connects to a database and retrieves the information about a single customer Many different applications can make use of that generic type of code As you will see in 28, the NET Framework allows you to create XML web services that may eventually replace the COM (Component Object Model) architecture However, because the world will not change overnight, you need to understand and work with component architecture, such as COM and COM+
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COM is an object-oriented approach to components and code reuse In order for client code (any code making use of the component) to talk to components, COM provides standard interfaces to which developers must adhere Language-specific details are unimportant, since the client must communicate through these standard interfaces As a result, the component can be written in C++ and the client can get access to it through Visual Basic All COM objects implement the IUnknown interface This interface keeps track of the number of clients accessing the component and is responsible for determining when the component (object) can be removed COM components are installed and registered in the local machine s Registry When registered, the component is uniquely identified by a globally unique identifier (GUID) The GUID information is written to the Registry and provides the client with information as to the location of the component and some of the configuration information When a COM component is requested, the COM provider (which may be a middle-tier server) determines if the component needs to be instantiated or can be retrieved from the component pool (where there are previously instantiated objects ready for use) Once the services have been used, the component is either returned to the pool or destroyed The component can be involved in complicated transactions, can have extensive security applied to it, and can be pooled (as noted above) The biggest benefit to the development world of the release of COM was the standard, cross-language object model that it provides With a single definition of what a remote object looks like and acts like, the developer s life is made much easier COM+ was introduced in 1997 and furthered the ease of development COM+ hides many of the details from the coder, thereby making developers lives easier Vendors creating tools that work with COM+ have to use the common library Metadata (information about the component) is now stored with every COM+ object A common set of types is implemented for COM+, so the problems of different language types are alleviated Another huge benefit to COM+ is the abstraction (for the developer) of the Interface Definition Language (IDL) Ask anyone who has had to learn this technology, and they will tell you that they have to go to their happy place and they will disappear muttering COM, COM, and more COM Remember that XML web services are designed to replace the COM architecture and provide a method of accessing remote objects over common protocols such as SOAP and HTML However, until the time comes, if ever, that they totally replace COM and COM+, you need to understand how to integrate the older technology into your NET applications
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