Good software design principles part 2

For the benefit of this blog we will call the module mentioned in the previous post the SM for successful module. Remember that the objective for this module was to make our software capable of acquiring and analysing data from any third party instrument. When thinking about a problem like this many of you might immediately think about the Open Close Principle. If you did not think about this and/or if you do not know what the OCP is, here is a definition:

software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification

Here is a link to the Wikipedia article: OCP
Trying to satisfy the OCP has a number of implication. One of them is that you should not have to modify the application code each time you want to support a new instrument. So of course the first thing that was decided was that the module should be loaded dynamically at runtime based on information in the application configuration. This is not earth shattering or revolutionary but it is a very useful concept. What we will be doing with the SM is to build a plugin infrastructure for our application instrument interface. Plugin are used in Eclipse, Netbeans and a whole menagerie of applications. In Java this is easy to do you just pack your SM in a .jar. Now the tricky par is to define the API that this jar file should implement. Another more immediate task is to define how the whole thing will be structured in terms of module. Now from the discussion we know that the starting point is this:

Now with a project like this you want to get your dependencies right from the beginning. In the case of this project it is easy to understand that has illustrated on the diagram communication between the two module goes both way. The Application supplies the SM with parameters and possibly other information and the SM returns statuses and results to the Application. We will probably need to allocate objects and possibly implement interfaces on the application side as well as in the SM. The question then is: where do we define those classes and interfaces ? The answer is: in a third module. Now the high level view of the project looks like this:

Now except for the weird looking arrow the elements on that diagram are packages with their dependencies. You have:

1) Application.
2) SM(I). This is a specific SM Instance (I added the I between parentheses to highlight that).
3) SMF the Successful Module Framework

As you can see there is no circular dependency in the standard UML elements. At compile time the APplication depends only on the SMF and the SM(I) also knows only the SMF. Now I added a non UML element (the weird broken arrow not quite connected) to represent the runtime dependencies in the system. I think having this extra arrow makes everything obvious and clean. In my next blog entry we will continue on our analysis of the SM and SMF. In fact for a while the emphasis will turn on the SMF and the key patterns used in that module the most important being:

- Abstract Factory
- Strategy

Now a closing comment. It goes without saying that before you start on a project like this a good analysis and requirements definition phase is in order. This is beyond the scope of the current thread but we might come back to this or insert a few blog entries about this phase later.