Conceptual Integrity in Large Systems

The central argument of the Mythical Man Month from Fred Brooks is that conceptual integrity is the most important consideration in system design, and that conceptual integrity will only be achieved if the design comes from one, or a few resonant minds.

I will contend that conceptual integrity is the most important consideration in system design. It is better to have a system omit certain anomalous features and improvements, but to reflect one set of design ideas, than to have one that contains many good but independent and uncoordinated ideas.


Conceptual integrity in turn dictates that the design must proceed from one mind, or from a very small number of agreeing resonant minds.

If you’ve been the creative force in a group work, you will have experienced these challenges. Core ideas are misunderstood, insoncistencies start to pop up, and the result is a patchwork.

For my part, I can confirm that consistency erodes quickly if you don’t pay close attention. Maintaining conceptual integriy is hard work.

This doesn’t happen because people are dumb, neglecting or malevolent. It happens because as soon as you specialize, you lose sight of the whole. Someone does a change here, someone a change there, and both changes end up not being fully consistent with each other.

Unfortunaltely, unlike Brooks suggests, doing all the design work alone is usually not realisitc.

With a good review culture you can scale your design team from one head to a few: let people design parts of the system even if their understanding of the whole system is lacunary, and have one central person review how well the contributions fit it.

It’s like having mutliple authors for an article but having one person in charge of doing a complete pass on the article at the end to ensure consistency.

But if you want to tackle bigger challenges, you will have to scale your design team even more.

Ensuring conceptual integrity at scale is hard because it requires not only scaling knowledge but also standardizing the decision making process.

This is what guidelines try to achieve. Guidelines encode the principles, maxims, constraints, and goals of the system in a way that different people reach similar decisions. It’s evidently impossible to encode the complete decision making process in guidelines, given that so much subjective, but they help achieve a basic overall consistency.

As for the subjectivity: just take one of your colleague and ask yourself “what would he decide?” You might have a hunch at his decision, but chances are, you don’t know enough about all the thinking that went in his previous decisions to predict this one accurately. If you do, well, you’re two “reasonant” minds, as Brooks would say.

If you know lots of people will be involved in the design process, you will need more than guidelines and reviews. You will have to decompose the problem in parts that can be solved individually. Each part can be assigned one “mind”. The whole might not be fully consistent, but the solution at each level of abstraction will at least be consistent.

Following the newspaper analogy, a newspapers has an editor in chief that sets the tone of the writings and the overall orientation (these are guidelines). He or she will review the topics of the individual articles to make sure they fit in the issue of the newspaper, but he or she won’t edit every article himself (the parts).

No large system will be fully consistent (think of Microsoft Office, that our dear journalists might be using), but it doesn’t hurt too much, because no user will ever use all of the system.

Evolution will also bring some inconsistencies in the system. Moving from one system paradigm to the next is like moving from one local maxima to the next one. In between things will be worse, that is, less consistent. But if you think there’s a superior design paradigm for the whole system, it’s worth challenging the current one and see if there’s a path.

Fred Brooks is right that conceptual integrity is the most important aspect in system design. He’s also right that the more designers there are, the harder it is to ensure concistency. But for large systems that evolve, some inconsistencies are inevitable. Address them like other risks in your project.


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