normalization techniques form the basis for most designs, the top-down versus bottom-up debate may be based on a theoretical distinction rather than an actual difference.9-9Centralized versus Decentralized DesignThe two general approaches to database design (bottom-up and top-down) can be influenced by factors such as the scope and size of the system, the company’s management style, and the company’s structure
(centralized or decentralized). Depending on these factors, the database design may be based on two very different design philosophies: centralized and decentralized.Centralized designis productive when the data component has a relatively small number of objects and procedures. The design can be carried out and represented in a fairly simple database. Centralized design is typical of relatively simple, small databases and can be successfully done by a single database administrator or by a small, informal design team. The company operations and the scope of the problem are sufficiently limited to allow even a single designer to define the problem(s), create the conceptual design, verify the conceptual design with the user views, define system processes and data constraints to ensure the efficacy of the design, and ensure that the design will comply with all the requirements. (Although centralized design is typical for small companies, do not make the mistake of assuming that it is limited to small companies. Even large companies canoperate within a relatively simple database environment.) Figure 9.15summarizes the centralized design option. Note that a single conceptual design is completed and then validated in the centralized design approach.Figure9.15Centralized DesignDecentralized designmight be used when the system’s data component has a considerable number of entities and complex relations on which very complex operations are performed. Decentralized design is also often used when the problem itself is spread across several operational sites and each element is a subset of the entire data set. (See Figure 9.16.)Figure9.16Decentralized DesignIn large and complex projects, the database typically cannot be designedby only one person. Instead, a carefully selected team of database designers tackles a complex database project. Within the decentralized design framework, the database design task is divided into several modules. Once the design criteria have been established, the lead designer assigns design subsets or modules to design groups within the team.
Because each design group focuses on modeling a subset of the system, the definition of boundaries and the interrelation among data subsets must be very precise. Each design group creates a conceptual data model corresponding to the subset being modeled. Each conceptual model is then verified individually against the user views, processes, andconstraints for each of the modules. After the verification process has been completed, all modules are integrated into one conceptual model.