Building Business Objects, Part III
Although abstraction is crucial in any effort to build a class library of re-usable business objects it is important to not get carried away and become what is known in my book as a “purist”. Physical patterns don’t always match logical patterns. Something beautiful on paper may be a clusterduck to implement in the real world. Pardon my French but if you are reading this kind of dry material, you probably already speak French. Our goal here is flexibility and efficiency, not adherence to rigid dogma for the sake of purity.
As an example of the point I am trying to make above lets revisit the Data Access class for a moment. There are some purists who will say no business logic should be embedded in the data access layer and no data access functions should be found in the business layer. On paper that is correct and a proper design should reflect that. In the real world where some guy is trying to piece everything together one can either go through great contortions to satisfy the separation of concerns aspect of encapsulation as laid out by the purist, or one can have a boat load of stored procedures in the database to return pre-configured datasets.
The reason why the stored procedure scenario breaks the rules of encapsulation is because the business layer is required to know something about how to return certain sets of data from the back-end data stores. I have concluded that breaking this rule is preferable to maintaining a SQL command object in every business object. The end result is the same.
I don’t know if this is even technically breaking the encapsulation rule but I have read a couple of articles that suggest it is. In a logical diagram I can draw the stored procedures and views as business rules, which are clearly in the business layer but in a physical diagram the procedures are stored in the back end database and thus are retrieved using the connection object in the DAL. The best of both worlds.
So what I am saying here is that stored procedures will be used extensively in this project including the current Employee business object. As mentioned at the end of the previous article we need to figure out what all is needed in the employee object. One thing the Employee object will definitely need is a database connection so it will need to know how to ask the Data Access Layer (DAL) for a connection. This is likely a static property but probably should nonetheless be acquired from a system initialization mechanism, like an .ini file. The value of this property should never be the connection string as the connection string is the business of the Data Access Layer.
Also, the Employee object must know how to get data to satisfy every method and property that is to be made available according to the contract made at instantiation. If the employee object says it can produce a punch history list, the object has to know the secret words to get a punch history list back from the DAL.
The calling program says the secret words to get a new employee object with ‘1234′ as the employee id –> set myEmp = new Employee('1234')
Everything goes well and Bob Jones’ employee record is returned encapsulated in an employee object. To get Bob’s punch history, I just need to request it from the Employee object. Depending on implementation the punch history maybe stored inside the object or returned as a record set. Either way, when me.getPunchHistory() is called, the object has to know how to request it from the DAL –>
myDBConn.RunProc(’stored_proc_Punch_History’, ‘1234′)
That’s it in a nutshell. Every bit of data I may want to retrieve, such as schedule, absence, vacation, is made available through the employee object utilizing stored procedures. The basic employee stuff is populated when the employee object is instantiated, the rest is provided as needed. The good news is that from now on, whenever a new application is needed that will interact with employee data, all the data access functions are predefined and the programmer can focus on writing the needed application instead of worrying about database connections, security, SQL queries, and all the rest of the administrative stuff.
In the next article we will take a look a how to display our employee information to the user, which is in the presentation layer of an n-tier application. Decisions made here should not in any way affect what the Data Access Layer will ultimately look like. But this is one of those cases where the physical architecture of an actual enterprise environment dictates as to how the logical models are implemented. VB.net is not the best tool for implementing design patterns because of the code-behind-the-form method used in the product. Such a construct physically binds the presentation and business layers, but not necessarily logically.
As I mentioned above, this contradiction can be avoided if the developer eschews all the nice functionality provided by VB.net, namely the typed datasets available in the connection object which can bind data directly to the components on the web forms. The code is generated automatically and the SQL query results are shoved directly into the presentation layer…
Considering that we are mandated to use VB.Net as the development platform, what should we do? Use VB.Net datasets, or write our own SQL Command objects to maintain order with the model-view-controller design pattern evident in the n-tier model?