Friday, February 22, 2008

Bottom-Up Software Design is for Sheep

Considering the devisive issues of our day: Vim vs Emacs, Ruby vs Python, .Net vs Java, Mac vs PC, and REST vs SOAP, there are plenty of opportunities to spark a holy war. Lets consider another polarizing issue:

Is it better to design and develop software from the top-down or from the bottom-up?
I am fortunate to have the opportunity to work across a wide variety projects and I am always curious to discover how my newly met colleagues are inclined regarding design. This evening I came across a compelling blog, by Gustavo Duarte, that took a stance opposed to my own. Here is my take on the issue.

Design by decomposition is the quintessential technique of the engineer

One of Gustavo's main points is that Feynman believed that software engineering has much in common with the other engineering disciplines. Considering this point, the most important concept in engineering is modularization. Most products and devices are made up of replaceable parts. Modularization is achieved by decomposing a system into sub-systems. System decomposition is top-down design.

Certainly we can agree that some of the first decisions we might make include the selection of a platform (Linux, Windows), technology stack (LAMP, Java, .NET), or language/framework (Ruby on Rails, Django).

Our choices will determine the composition of our system, how functionality is divided among system components, and usage of key patterns (e.g Model-View-Controller).

Do we even realize that we are doing top-down design?

Perhaps the availability of powerful platforms and frameworks is removing the importance of the top-down design perspective from our collective conscience.
None-the-less, software system design by decomposition into sub-systems implies a top-down approach to design.

The Space Shuttle Challenger was not a victim of top-down design.

Gustavo's blog walks us through some evidence that suggests that Dr. Feynman believed that top-down design doomed Challenger. The Challenger Disaster illustrates that a likely point of system failure is at sub-system interfaces. I don't see this as an indictment against top-down design.

For a analysis of the mulitude of events that led to the Challenger disaster, I recommend the book NO DOWNLINK. Essentially, the Challenger and her crew where the victims of pork-barrel politics, post-space race budget cuts, and the Challenger Syndrome.

Poor design choices such as the use of Solid Rocket Boosters (SRBs) and then the segmenting of the SRBs to aid in transportability where driven more by budgetary and political concerns then by faulty engineering practices.

Defending up-front software design

Another point made by Gustavo is that "Big up-front design is foolish". The word "Big" implies a non-incremental approach. Up-front design is not a foolish idea. However developing software in a non-incremental fashion (i.e. the waterfall method) is a bad idea.

Top-down software design leads to effectivly modularized code that fosters many good things such as reuse, testability, and maintainability.
Gustavo drags UML into this discussion and I agree with some of his points. The idea of designers churning out UML blue-prints to throw "over the wall" to "implementors" has no appeal to me and in fact, simple does not work.

UML is not implicitly evil and the use of UML does not have to imply some type of bureaucratic software factory where coder slaves toil away, mindlessly "implementing" designs.

UML is just a tool that lets you work on your system at a high level. UML is just a white board.

Using UML to create diagrams (which are essentially pictures) to foster communication is a beautiful thing.
To summarize, UML should be used to open a new communication channel, never to replace face-to-face discussion.


The main problem with up-front design from a developer's perspective is that it doesn't address many of the underlying risks to the success of the project. The term "risk" is a nice way to articulate that feeling you get in your stomach when you know you are doomed.

The best way to handle "risk" is to start working on the risky parts of the system as soon as possible, so that you have plenty of time to handle all the foo bar.

Risk is what leads developers to favor the bottom-up approach. This is because the risky sections of a system exist deep within the system and at sub-system interfaces such as APIs.
Up-front design may add risk because some design decisions are based on assumptions that can only be validated with working code.

Managing Risk

All forms of engineering require lab testing, research and development and prototyping to determine how elements of a system will actually perform and software is no different.

Risk-reduction prototyping is a technique used to mitigate risks, validate design assumptions, and to develop techniques that will be used during the production of a software system.


As software developers, we are also software users. We use frameworks and tool-sets that allow us to focus on the specific application that we are creating. If we don't maintain the ability to design software from the top down, we function more as users and less as developers. We become in effect, sheep walking the well-worn path instead of being being the pioneering trail-blazers we had hoped to be.

I believe that a pragmatic combination of both top-down (up-front design) and bottom-up (risk-reduction prototyping) techniques will greatly increase a software project's chances for success.

Dr. Feynman believed that software engineer has much in common with the other engineering disciplines. This is certaintly true, but 'how much' is less certain. Thinking of software engineering as just another engineering discipline comes with its own pitfalls.

What is your take on this topic?