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Structure And The “ilities”

In nature, structure is an enabler or disabler of functional behavior. No hands – no grasping, no legs – no walking, no lungs – no living. Adding new functional components to a system enables new behavior and subtracting components disables behavior. Changing the arrangement of an existing system’s components and how they interconnect can also trade-off qualities of behavior, affectionately called the “ilities“. Thus, changes in structure effect changes in behavior.

The figure below shows a few examples of a change to an “ility” due to a change in structure. Given the structure on the left, the refactored structure on the right leads to an increase in the “ility” listed under the new structure. However, in moving from left to right, a trade-off has been made for the gain in the desired “ility”. For the monolithic->modular case, a decrease in end-to-end response-ability due to added box-to-box delay has been traded off. For the monolithic->redundant case, a decrease in buyability due to the added purchase cost of the duplicate component has been introduced. For the no feedback->feedback case, an increase in complexity has been effected due to the added interfaces. For the bowl->bottle example, a decrease in fill-ability has occurred because of the decreased diameter of the fill interface port.

Ilities

The plea of this story is: “to increase your aware-ability of the law of unintended consequences”. What you don’t know CAN hurt you. When you are bound and determined to institute what you think is a “can’t lose” change to a system that you own and/or control, make an effort to discover and uncover the ilities that will be sacrificed for those that you are attempting to instill in the system. This is especially true for socio-technical systems (do you know of any system that isn’t a socio-technical system?) where the influence on system behavior by the technical components is always dwarfed by the influence of the components that are comprised of groups of diverse individuals.

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