Tuesday, December 8, 2015

Layers of redundancy

Over at HerdingCats, there's a posting about the various ways you can think about and employ redundancy in systems design.

As a former Director of System Engineering, I certainly lend my endorsement to what was written, reproduced here as wrought there. [Disclosure: I don't agree with some of the stabs at Agile, but I certainly would add a spear re "no estimates"]

Here's the opening proposition, as posed by HerdingCats:

Redundancy  provides resiliency to the system to withstand disruption within acceptable degradation parameters and to recover within an acceptable time and composite costs and risks

Here are the rules:
  • Absorption rule - is a buffering characteristic that prevents overload of the system. Redundancy can provide this protection.
  • Limit Degradation support rule - provide a lower limit to which the system can degrade before failing. This is he circuit breaker for your home. Also the circuit breaker for the stick exchange.
  • Margin Support Rule - margin is added to the system to protect from disruptions. This can be schedule margin, cost margin, technical performance margin, operational margin. Any kind of margin that allows the system to continue to operate properly inside the range of parameters.
  • Physical Redundancy rule - buy two in case one breaks .... Fault-Tolerant System Reliability in the Presence of Imperfect Diagnostic Coverage, describes how ... triple redundancy was protected through real-time fault detection and dynamic reconfiguration of the hardware components.
  • Functional Redundancy rule - is sometimes called design diversity and avoids the vulnerabilities of Physical Redundancy.
  • Layers of Defense rule - states that for a failure to occur a disturbance has to penetrate a series of layers simulate to layers of Swiss Cheese. The system has holes like the holes in Swiss Cheese, that allow the failure to penetrate to the next level, where they can be handled.

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