The art of thinking · Parts, wholes and systems
Near-decomposability
Also known as: Nearly decomposable system, Near-decomposability (Herbert Simon), Architecture of complexity
In systems theory, near-decomposability is Herbert A. Simon’s term for complex systems built as hierarchies of subsystems in which interactions within a subsystem are much stronger and faster than interactions between subsystems, so that each subsystem can be understood largely on its own.
- Thinking models
- Systems theory
In one sentence
Simon’s near-decomposability: complex systems are hierarchies of subsystems with strong inner and weak outer interactions.
Example
Each station of a packaging line can be documented largely on its own because most interactions happen inside the station; the few signals between stations get their own interface pages.
How it applies
- The watchmakers: Simon’s parable compares two watchmakers, Hora and Tempus. Tempus assembles a watch of a thousand parts in one go and has to start over at every interruption; Hora builds stable subassemblies of ten parts and loses little when interrupted. Hierarchic, nearly decomposable structures are more likely to evolve and easier to build and understand.
- Short and long run: In the short run each subsystem behaves almost independently; in the long run the subsystems influence each other only in aggregate.
- Design rule: The idea underlies modular design in engineering and software (Parnas’ information hiding, 1972) and the study of product architectures.
- Technical documentation: Documentation can be modular to the extent that the product is: topics follow the subsystems, interface topics cover the weak interactions between them. Where a product is not nearly decomposable — tightly coupled functions — topic boundaries cut through real dependencies.
Near-decomposability vs. modularity
Modularity is a design property chosen for a system; near-decomposability is Simon’s description of why complex systems in nature and technology tend to have that property, and of the degree to which they have it.
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