CHAPTER 1 Understanding object/relational persistence manipulate data, the (Web hosting contract)
CHAPTER 1 Understanding object/relational persistence manipulate data, the tables and columns involved must be specified at least three times (insert, update, select), adding to the time required for design and implementation. The distinct dialects for every SQL database management system don t improve the situation. To round out your understanding of object persistence, and before we approach possible solutions, we need to discuss application architecture and the role of a persistence layer in typical application design. 1.3 Persistence layers and alternatives In a medium- or large-sized application, it usually makes sense to organize classes by concern. Persistence is one concern; others include presentation, workflow, and business logic.1 A typical object-oriented architecture includes layers of code that represent the concerns. It s normal and certainly best practice to group all classes and components responsible for persistence into a separate persistence layer in a layered system architecture. In this section, we first look at the layers of this type of architecture and why we use them. After that, we focus on the layer we re most interested in the persis tence layer and some of the ways it can be implemented. 1.3.1 Layered architecture A layered architecture defines interfaces between code that implements the various concerns, allowing changes to be made to the way one concern is implemented without significant disruption to code in the other layers. Layering also determines the kinds of interlayer dependencies that occur. The rules are as follows: Layers communicate from top to bottom. A layer is dependent only on the layer directly below it. Each layer is unaware of any other layers except for the layer just below it. Different systems group concerns differently, so they define different layers. A typical, proven, high-level application architecture uses three layers: one each for presentation, business logic, and persistence, as shown in figure 1.4. Let s take a closer look at the layers and elements in the diagram: 1 There are also the so-called cross-cutting concerns, which may be implemented generically by framework code, for example. Typical cross-cutting concerns include logging, authorization, and transaction demarcation.
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