Robustness & Resilience
Channel: Systems Innovation
Date: 2015-06-08

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In this module we will be discussing robustness and resilience within self-organizing systems, we will firstly talk about what we mean by robustness, we will go on to discuss adaptation as a mechanism for resilience and why self- organizing systems are typically considered robust, we will look at the theory of requisite variety and finish by talking about self-organized criticality. 

All systems exist within an environment and are to a certain extent dependent upon a specific range of input values from that environment, the system has a set of parameters to these inputs within which it can maintain its structure and functionality but outside of these critical parameters the system will disintegrate, i.e. become degraded to a lower level of integration or functionality. Resilience and robustness are then defined by this set of parameters, the lower the system’s dependency upon its environment and the broader this range of input values that the system can operate within the more robust it can be said to be.
For example in computer science, robustness is the ability of a computer to cope with errors during execution, that is to say the ability of an algorithm to continue operating despite abnormalities in input, in this way robustness defines its independence from a specific range of inputs or inversely its capacity to process a wider range input states. To illustrate this farther we might think about a tree with-standing the force of wind blowing against it, the tree has a certain tensile strength through its capacity to bend, within a certain range of input values to the force exerted upon it, it will be able to withstand this perturbation from its environment. The wider the range to these input values the more robust the tree will be.
There are fundamentally just two ways for a system to maintain its integrity given some perturbation, it can resist this change or adapt to it. By resist we mean it creates a boundary or filter condition that prevents the external influence from altering the internal configuration to the system and thus preserving its functionality and structure up to sum limit, in our tree example this might mean the organism developing a sturdy truck. Inversely the system can adapt by finding or generating the appropriate response required to counterbalance the perturbation, we might think of this as the tree bending over in response to the force exerted upon it.
Robustness and resilience are general characteristics of self-organizing systems both through their capacity to resist change and their capacity to adapt to it. Firstly we will talk about their capacity to resist change through distributed control and feedback loops. In centralized systems with top-down control there are specialized components required for regulating the system, these represent largely irreplaceable hubs that will affect the whole system if removed or degraded, within complex systems in contrary, control is distributed out on the local level meaning there is much less specialization, missing or damaged components can often be replaced by others and this gives them a much lower level of criticality.
Secondly self-organizing systems are held within their current configuration by a set of feedback loops that are also distributed out across the system on the local level. A good example of this might be a magnet which consist of many tiny magnetic spins that are all aligned to produce an overall magnetic force, if part of the spins are knocked out of their alignment, the magnetic field produced by the rest of the spins will quickly pull them back, this force maintaining the system within its current configuration is distributed out again giving it a low level of criticality and thus a higher level of robustness.

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