Conceptual Integrity and the Design of Design

The Mythical Man Month is Frederick P Brooks’ famous book about software engineering, read by computer science students around the world. Of course, it’s not immediately obvious from the title what the book is going to be about, but the title of his latest book, The Design of Design pretty much says it all. Like The Mythical Man Month, The Design of Design is a collection of essays, but unlike its predecessor, it is not a computer book per se. Rather than focus solely on the design of computer systems, Brooks talks about design in general, exploring the possibility that widely disparate design disciplines such as music and art share elements in common with software. Brooks brings to bear his many years of experience building everything from houses to computer architectures in this book, sharing his insight and opinions on a wide variety of fascinating topics such as open source, the design rational behind his vacation beach house, to empiricism versus rationalism. The one overarching theme that ties all these essays together is the idea of conceptual integrity – which Brooks defines as consistency, coherence, and uniformity of style. This is the glue that holds a design together.

Brooks argues that all great works have conceptual integrity and that this holds true across all design mediums, not just computer system design. He uses Reims cathedral to illustrate this point:

“most European cathedrals show differences in plan or architectural style between parts built in different generations by different builders… So the peaceful Norman transept abuts and contradicts the soaring Gothic nave… Against these, the architectural unity of Reims stands in glorious contrast. The joy that stirs the beholder comes as much from the integrity of the design as from any particular excellences. As the guidebook tells, this integrity was achieved by the self-abnegation of eight generations of builders, each of whom sacrificed some of his ideas so that the whole might be of pure design.” 1

This pattern of design purity being achieved through a single unifying idea can be found everywhere. The sonata form has existed in classical music for centuries, and has been one way for a composer to structure their music around a central theme that is first introduced in the exposition, explored and contrasted in the development, and finally revisited in the recapitulation. The most famous example of this form is the first movement of Beethoven’s fifth symphony. Any great piece of literature will also be structured around a few central underlying themes that underpin the entire story: Shakespeare explores the age old question of fate versus free will in Macbeth, as does Tolstoy in War and Peace. Likewise, a good photograph will capture a particular mood or moment in time. Attention will be drawn to the main subject of the composition by carefully cropping it in such a way so that the subject is not lost among the noise of a busy background. Everything that remains must visually support and complement this subject.

Similarly, good software (and hardware) is deliberately built around a set of strong underlying ideas. Eric Raymond talks at length in his book, the Art of UNIX programming, about the design philosophy that shaped UNIX:

“The UNIX philosophy originated with Ken Tompson’s early meditations on how to design a small but capable operating system with a clean service interface. It grew as the Unix culture learned things about how to get maximum leverage out of Thompson’s design. It absorbed lessons from many sources along the way… The UNIX philosophy is bottom-up, not top down. It is pragmatic and grounded in experience.” 2

The end result is a clean and elegant system centered around the command line interface. Programs were built to do one task, and to do it well. Furthermore, they were made to play well with others: The output of one tool would serve as the input to another. To facilitate this, programs were designed to run in batch mode, with file formats using transparent text formats (compared to the opaque binary formats favored by Windows applications); Scripting languages such as awk and sed were created to provide strong text processing capabilities. By adhering to these principles, a strong foundation was built that fostered extensibility and flexibility, allowing programs to be combined in powerful ways that would not otherwise have been possible.

One of the reasons conceptual integrity produces such good designs is because the systems themselves are easier to build. Conceptual integrity provides a rough roadmap of what the finished product will look like; In a sense, all the difficult decisions have already been made. If you think of a design as a tree, with the final product at the root and each branch representing a set of possible design decisions, then conceptual integrity is what allows us to prune this tree, which is theoretically infinite in size. Not only will the end product be easier to build and maintain, it will be easier to use as well. This is because it will be consistent in structure.

Why is this a good thing? States Brooks: “Consistency is reinforcing and self-teaching, because it confirms and encourages our expectations… It aids the comprehensibility of the design” 3. This allows an end-user to rely more on their own intuition to figure out how a program works rather than by having to read a thousand page manual. For instance, the desktop metaphor pioneered by Xerox PARC that we all take for granted today, allows any user to easily manipulate the files and folders on his machine with just a mouse. It is not a difficult metaphor to grasp, and as a result can readily be taught even to the most inexperienced computer user. However, imagine a system that does not consistently apply this metaphor. Dragging a file onto the recycle bin makes a copy of the file, and dragging one window on top of another closes the one on the bottom. This may sound like a contrived scenario, but there are such poorly designed systems out there. In a previous article, I rant and rage about the horribly inconsistent and illogical design decisions in VB script.

Products that lack conceptual integrity are everywhere and easy to spot: They are bloated, ill conceived, and difficult to use. Features are haphazardly slapped together, often by the clueless non- technical middle management that you’d find in a Dilbert comic. Corporate intranet sites, bloatware found on new PCs, and websites of non tech companies are rife with examples of this. A look at the code base is revealing as well. Take for example, a poorly built web application. Instead of a clean and modular architecture with orthogonal functions, we instead find code in the business logic layer that returns HTML, insert functions in the database layer that happen to delete and update rows in other tables. Needless to say, maintaining such a code base is a nightmare, and is something that should be avoided. The goal then, is to strive for conceptual integrity in our designs. Brooks latest book provides a great starting point for discussion, ideas, and inspiration.


1 Brooks, Frederick. “Achieving Conceptual Integrity” The Mythical Man Month
2 Brooks, Frederick. “Esthetics and Style in Technical Design” The Design of Design
3 Raymond, Eric. “Art of UNIX Programming”