2- and 3-Tier Application Structures in Visual Basic .NET

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1011 2- and 3-Tier Application Structures
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In the chapter on WinForms applications, we brie y looked at the idea of a 2-tier structure, where the business layer of the program was developed independently of the presentation or user-interface layer In fact this is a simpli cation of what happens in most real-world applications, where the 2-tier structure would not be adequate because of the need to make objects persist over time In a simple application, we may have no need to store data for later retrieval (for example, calculating someone s tax), and so a 2-tier structure would be suitable for this However, in most applications we need to store object information from one run to the next (for example a word processor); in such applications, it is possible to use
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10 n Object Modelling
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3-Tier Application Services Structure
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a 2-tier structure by making objects responsible for the storage and retrieval of their own data, but it is sometimes organizationally preferable to introduce a 3rd tier to take care of the storage and retrieval of objects In most business applications, we need to work with a persistent store of object information which may contain the data of thousands or millions of objects (for example, a banking system), and in this case it is necessary to introduce a 3rd tier, responsible for storing and retrieving objects on demand Microsoft promotes the idea of services in application design, where three distinct application tiers are responsible for user services (ie the user-interface), business services (ie the main logic of the application that models the actual requirements) and data services (responsible for storing object data in a database and recreating objects from this data as necessary) Figure 101 shows this diagrammatically In a Visual Basic NET application, these services are usually developed as separate modules (possibly many in each tier) User services are normally realized as Form classes, possibly with some additional classes to support operations that are only relevant to the job of presenting data (for example, objects that can format data for easier presentation) Business services perform the main work of the application, and are normally realised as classes that de ne individual business objects (eg an email, a customer record or a bank transaction) and Collection classes that organize
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101 Application Structure
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these into meaningful groups (eg an email in-box, a list of customer invoices or all of the transactions in a bank account) Data services may be implemented by standard classes that know how to store data in and retrieve data from a database system (eg ADO NET, which is a set of classes for working with many different forms of database), or custom-built classes that will take care of converting raw data into business objects and storing business objects as raw data We will be examining database interactions and the ADO NET database classes in 12 For now, it will be enough to accept that a database is a way of storing information with a regular structure that you might conveniently think of as lists of items
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Exercise 101
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An application program manages multiple bank accounts for multiple users To create and manage this application, two ArrayList collections, colAccounts and colUsers are instantiated on the Main form of the program and given Public scope: a) b) c) Which tier have these collections been allocated to Which tier do you think they actually belong in Given that the two collections are only ArrayList instances, can you think of a better approach to creating and maintaining them so that they t better into the tier they belong in
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1012 Appointing Responsibilities
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One approach to designing object models is to concentrate on the responsibilities of the objects in the system In a 3-tier system, we can divide an application s responsibilities as follows
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