Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
- Create BookmarkCreate Bookmark
- Create Note or TagCreate Note or Tag
- DownloadDownload
- PrintPrint
4. Change > Incremental Development and Design
Incremental Development and Design
There is a method of software development that avoids the three flaws by its very nature, called “incremental development and design.” It involves designing and building a system piece by piece, in order.
It is easiest to explain by example. Here’s how we would use it to develop a calculator program that needs to add, subtract, multiply, and divide:
-
Plan a system that does only addition and nothing else.
-
Implement that system.
-
Fix up the now-existing system’s design so that it is easy to add a subtraction feature.
-
Implement the subtraction feature in the system. Now we have a system that does only addition and subtraction, and nothing else.
-
Fix up the system’s design again so that it is easy to add a multiplication feature.
-
Implement the multiplication feature in the system. Now we have a system that does addition, subtraction, multiplication, and nothing else.
-
Fix up the system’s design again so that it is easy to add the division feature. (At this point, this should take little or no effort, because we already improved the design before implementing subtraction and multiplication.)
-
Implement the division feature in the system. Now we have the system we started out intending to build, with an excellent design that suits it well.
This method of development requires less time and less thought than planning the entire system up front and building it all at once. It may not be easy at first if you are used to other development methods, but it will become easy with practice.
The tricky part of using this method is deciding on the order of implementation. In general, you should pick whatever is simplest to work on at each step, when you get there. We picked addition first because it was the simplest of all four operations overall, and subtraction second because it logically built on addition in a very simple way. We could possibly have picked multiplication second, since multiplication is just the action of doing addition many times. The only thing we would not have picked second is division, because stepping from addition to division is too far of a logical jump—it’s too complex. On the other hand, stepping from multiplication to division at the end was really very simple, so that was a good choice.
Sometimes you may even need to take a single feature and break it down into many small, simple, logical steps so that it can be implemented easily.
This is actually a combination of two methods: one called “incremental development” and another called “incremental design.” Incremental development is a method of building up a whole system by doing work in small pieces. In our list, each step that started with “Implement” was part of the incremental development process. Incremental design is similarly a method of creating and improving the system’s design in small increments. Each step that started with “Fix up the system’s design” or “Plan” was part of the incremental design process.
Incremental development and design is not the only valid method of software development, but it is one that definitely prevents the three flaws outlined in the previous section.
[5] This is the story of a file called process_bug.cgi from a product called “Bugzilla.” The story has been simplified somewhat from what actually happened, but the numbers (in terms of lines of code and the time it took to fix it) are roughly accurate. If you want to see the entire history of the redesign project to see how it was done, you can read the records listed here: https://bugzilla.mozilla.org/showdependencytree.cgi?id=367914&hide_resolved=0.