Now, to reiterate my title, this is what is wrong. This is the huge modern heresy of altering the human soul to fit its conditions, instead of altering human conditions to fit the human soul…it is the huge heresy of Precedent.
Other issues that you will face include: Performance issues are difficult to predict. When you are working with a shared database, such as the situation implied in Figure 3you may find that the performance characteristics of your database are hard to predict because each application accesses the database in its own unique way.
However, this same application also performs batch loads of items available for sale via other companies that you have partnered with, items that you want to carry on your web site as soon as they are available.
These batch loads can take several minutes, during which period the Item table is under heavy load and thus your online application is potentially affected. Data integrity is difficult to ensure with shared databases. Because no single application has control over the data it is very difficult to be sure that all applications are operating under the same business principles.
For example, your application may consider an order as fulfilled once it has been shipped and a payment has been received. The original legacy application that is still in use by your customer support representatives to take orders over the phone may consider an order as fulfilled once it has been shipped, the payment received, and the payment deposited into your bank account.
A slight difference in the way that a fundamental business rule has been implemented may have serious business implications for any application that accesses the shared databases.
Less subtly, imagine what would happen if your online order taking application calculates the total for an order and stores it in the order table, whereas the legacy application calculates the subtotals only for order items but does not total the order. When the order fulfillment application sees an order with no total it calculates the total, and appropriate taxes, whereas if a total already exists it uses the existing figure.
If a customer makes an order online and then calls back a few hours later and has one of your customer service representatives modify their existing order, perhaps to add several items to it, the order total is no longer current because it has not been updated properly.
Referential integrity issues such as this are covered in detail in the Implementing Referential Integrity article. Operational databases require different design strategies than reporting databases.
The schemas of operational databases reflect the operational needs of the applications that access them, often resulting in a reasonably normalized schema with some portions of it denormalized for performance reasons.
Reporting databaseson the other hand, are typically highly denormalized with significant data redundancy within them to support a wide range of reporting needs. Every technology has its strengths and weaknesses, and RDBMS technology is not an exception to this rule.
Luckily there are ways that you can mitigate some of these challenges, and encapsulation is an important technique to do so.
Your Greatest Ally Encapsulation is a design issue that deals with how functionality is compartmentalized within a system. You should not have to know how something is implemented to be able to use it.
The implication of encapsulation is that you can build anything anyway you want, and then you can later change the implementation and it will not affect other components within the system as long as the interface to that component did not change.
For example, consider your bank. How do they keep track of your account information, on a mainframe, a mini, or a PC? What database do they use? You just walk up to a teller and do whatever transactions you wish. By encapsulating access to a database, perhaps through something as simple as data access objects or something as complex as a persistence framework, you can reduce the coupling that your database is involved with.
The Implementation Strategies for Persisting Objects in RDBs chapter compares and contrasts various encapsulation strategies that you have available to you.
For now assume that it is possible to hide the details of your database schema from the majority of the developers within your organization while at the same time giving them access to your database.
Some people, often just the Agile DBA s responsible for supporting the database, will need to understand and work with the underlying schema to maintain and evolve the encapsulation strategy. One advantage of encapsulating access to your database is that it enables application programmers to focus on the business problem itself.
The application programmers will work with these data access classes, not the database. This enables your Agile DBA to evolve the database schema as they need to, perhaps via database refactoringsand all they need to worry about is keeping the data access classes up to date.
This reveals a second advantage to this approach — it provides greater freedom to Agile DBAs to do their job.
Figure 4 depicts the concept of encapsulating access to your database, showing how the best case scenario of Figure 2 and the worst case scenario of Figure 3 would likely change. In the best-case scenario your business source code would interact with the data access objects that in turn would interact with the database.
The primary advantage would be that all of the data-related code would be in one place, making it easier to modify whenever database schema changes occurred or to support performance-related changes. However, from the point of view of the application programmer this is a much easier change to detect and act on — with the database encapsulation strategy in place the application programmers are only dealing with program source code e.
Java and not program source code plus SQL code. Although it is possible that all applications could take advantage of your encapsulation strategy the reality is that only a subset will be able to.
Perhaps some applications already have an encapsulation strategy in place if so, you might want to consider reusing the existing strategy instead of building your own. Perhaps you want to use technologies, such as a bulk load facility, that require direct access to the database schema.
Another advantage of encapsulating access to a database is that it gives you a common place, in addition to the database itself, to implement data-oriented business rules.
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