Open for Extension and Closed for Modification

This is one of the fundamentally design thought that every designers should deeply analysis. When we say open, it does not necessarily carry the real meaning of “open” and close does not imply the real meaning of “close”. In the context of designing, these two words are related to level of flexibility given to programmers/developers. This fundamentally principle suggests that an application design should be flexible enough to accommodate features to the application.

But a sudden spike of thought that comes to our mind – for every product release, we add a lot of features. Before proceeding further, let us ask some questions.

1. Does your product have added few features for each release?
2. Do you feel that the base architecture remain fairly stable over many releases?
3. Do the developers know the consequences to their features when the underlying framework changes?
4. How easy or tough adding of features are? (Easy or tough is very relative J )
5. How this fundamental design principle helps the framework to be flexible?
   

There will be two scenarios where the developers will modify the code. When the software is buggy, they do not have any other way but to modify the code. The developers typically identify the root cause of the bug and fix them. But some of the developers open the code to add new features. The modification of the existing code for adding new features conveys that there is something wrong. It is a bad design for a framework if it forces it clients to modify the code when adding new features. Apart from forcing the modifications, the design should not force itself to undergo a massive code change to support new functionalities. When it comes to framework, the designers should also see the framework from another perspective apart from providing basic functionalities. The perspective is that the framework is just a contract or guidelines given to the clients. The bottom line is the framework is just contract that governs and facilitates proper functioning of the applications. When it comes to an application, the application should foresee some changes in requirements and burn flexibilities in the design to accommodate those requirements.

The design principle, “Open for Extension; Closed for Modification” is a great tool and thought for building software. This is just a thought and for achieving this one needs a lot of things to be done and known. In few of my next blogs, I ll be writing on the design principles and these design principles could be a one-liner, a design pattern that is commonly needed or concepts taken from Java library.