Decomposition as a Tool for Managing Complexity
Systems Architecture · Chapter 13
Figure 13.1 · A Real (but Unwieldy) Complexity Measure
A vehicle manufacturer’s attempt to define complexity via part-sharing across a 13-level product hierarchy. The measure was not incorrect — but it was so cumbersome (the decomposition itself was never agreed upon) that it went unused. Source: Crawley, Cameron & Selva (2016), Fig. 13.1.
Box 13.2 · Principle of Apparent Complexity
Figure 13.2 · Complicatedness in Civil Architecture
The pyramid is less complicated than the Taj Mahal, even though both perform the same function — entombing royalty. In civil architecture there are aesthetic reasons for “complicatedness.” Source: Crawley, Cameron & Selva (2016), Fig. 13.2.
Figure 13.3 · A Complicated Network
The same function is delivered by Figures 13.3 and 13.4, and the inputs/outputs at an electrical layer are identical — but one architecture is far less complicated and more amenable to analysis and modification. Source: Crawley, Cameron & Selva (2016), Fig. 13.3.
Figure 13.4 · The Same Network, Organized
Decomposing the network into cables, routers, and racks — and abstracting a bundle of cables into a single “bus” — makes the organization legible without changing what the network does. Source: Crawley, Cameron & Selva (2016), Fig. 13.4.
Box 13.3 · Principle of Essential Complexity
Figure 13.5 · Is This Complexity Essential?
A powered liftgate’s logic: circles are processes, lines are signals. A minimum system (switch + reversing logic) wouldn’t capture the function that prevents opening while the vehicle moves. It’s hard to argue this diagram contains gratuitous functions or forms — but does it also carry gratuitous complexity inherited from legacy architectures? Source: Crawley, Cameron & Selva (2016), Fig. 13.5.
Figure 13.6 · Apparent ≠ Essential
A system’s essential or gratuitous complexity is independent of its apparent complexity. The Concorde’s flight deck is visually overwhelming and required extensive training — high apparent complexity. Given the Concorde’s success, that complexity could be argued essential to delivering its performance with the technology of the era. Source: Crawley, Cameron & Selva (2016), Fig. 13.6.
Figure 13.7 · Low Apparent, High Actual
The smartphone’s touchscreen interface has very low apparent complexity — little or no training to operate — while masking substantial internal hardware complexity. No one would argue the Concorde’s interface should be condensed to a menu, four buttons, and scrolling — but is the phone’s hidden complexity essential, or gratuitous? Source: Crawley, Cameron & Selva (2016), Fig. 13.7.
Figure 13.8 · Decomposition Has a Price
Five entities, six connections (left). Grouping into two clusters of three and two apparently simplifies the system to one connection (right) — but the decomposition introduces an interface between the groups: the system now has six entities and nine connections (middle)! Reducing apparent complexity can increase actual complexity. Source: Crawley, Cameron & Selva (2016), Fig. 13.8.
- The architect must decide whether to invest in actual complexity (to bring down apparent complexity), or not to invest at all
Box 13.4 · Principle of the 2nd Law
Box 13.5 · Principle of Decomposition
- A bad example: decomposing operations by nominal get-ready/get-set/go/get-unset/get-unready focuses on the typical case, omitting fault conditions, contingencies, and commissioning
Figure 13.9 · Illustration of “2 Down, 1 Up”
Box 13.6 · Principle of “2 Down, 1 Up”
Figure 13.10 · Potential Decomposition Planes
Box 13.7 · Principle of Elegance
- The choice of decomposition plane is a strong lever on the elegance of the resulting architecture
Figure 13.11 · Form vs. Function
Saturn V: The Launch Vehicle
The Saturn V, developed by NASA 1962–1968, launched the Apollo Command Service Module, Lunar Module, and crew from Earth’s surface into orbit and toward the Moon. Its solution-neutral function: accelerate a payload by a change in velocity (Delta-ν) in each of three stages. Source: Crawley, Cameron & Selva (2016), Fig. 13.12.
Space Station Freedom
Space Station Freedom (SSF), a semi-permanent Earth-orbiting station announced in 1984, was never built — over a decade it underwent design changes and eventually evolved into the International Space Station (1993). Source: Crawley, Cameron & Selva (2016), Fig. 13.13.
- Solution-neutral function: a laboratory for micro-gravity science and Earth observation, a base for space operations, and — a strong Cold-War political function — cementing international relationships (elements from Europe, Canada, Japan)
