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Data access code interacts with the Data Management layer to retrieve, insert, update, and remove information. The data access layer doesn t actually manage or store the data; it merely provides an interface between the business logic and the database. Data access gets its own logical layer for much the same reason that the presentation is split from the UI. In some cases, data access will occur on a machine that s physically separate from the one on which the UI and/or business logic is running. In other cases, data access code will run on the same machine as the business logic (or even the UI) in order to improve performance or fault tolerance.
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It may sound odd to say that putting the data access layer on the same machine as the business logic can increase fault tolerance, but consider the case of web farms, in which each web server is identical to all the others. Putting the data access code on the web servers provides automatic redundancy of the data access layer along with the business logic and UI layers. Adding an extra physical tier just to do the data access makes fault tolerance harder to implement, because it increases the number of tiers in which redundancy needs to be implemented. As a side effect, adding more physical tiers also reduces performance for a single user, so it s not something that should be done lightly.
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Logically defining data access as a separate layer enforces a separation between the business logic and any interaction with a database (or any other data source). This separation provides the flexibility to choose later whether to run the data access code on the same machine as the business logic, or on a separate machine. It also makes it much easier to change data sources without affecting the application. This is important because it enables switching from one database vendor to another at some point. This separation is useful for another reason: Microsoft has a habit of changing data access technologies every three years or so, meaning that it is necessary to rewrite the data access code
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CHAPTER 1 s DISTRIBUTED ARCHITECTURE
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to keep up (remember DAO, RDO, ADO 1.0, ADO 2.0, and now ADO.NET ). By isolating the data access code into a specific layer, the impact of these changes is limited to a smaller part of the application. Data access mechanisms are typically implemented as a set of services, with each service being a procedure that s called by the business logic to retrieve, insert, update, or delete data. Although these services are often constructed using objects, it s important to recognize that the designs for an effective data access layer are really quite procedural in nature. Attempts to force more object-oriented designs for relational database access often result in increased complexity or decreased performance. I think the best approach is to implement the data access as a set of methods, but encapsulate those methods within objects to keep them logically organized.
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If you re using an object database instead of a relational database, then of course the data access code may be very object-oriented. Few of us get such an opportunity, however, because almost all data is stored in relational databases.
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Sometimes the data access layer can be as simple as a series of methods that use ADO.NET directly to retrieve or store data. In other circumstances, the data access layer is more complex, providing a more abstract or even metadata-driven way to get at data. In these cases, the data access layer can contain a lot of complex code to provide this more abstract data access scheme. The framework created in this book doesn t restrict how you implement your data access layer. The examples in the book will work directly against ADO.NET, but you could also use a metadata-driven data access layer if you prefer. Another common role for the data access layer is to provide mapping between the objectoriented business logic and the relational data in a data store. A good object-oriented model is almost never the same as a good relational database model. Objects often contain data from multiple tables, or even from multiple databases; or conversely, multiple objects in the model can represent a single table. The process of taking the data from the tables in a relational model and getting it into the objectoriented model is called object-relational mapping (ORM), and I ll have more to say on the subject in 2.
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