Above and Below the Line

People working above the line of representation continuously build and refresh their models of what lies below the line. That activity is critical to the resilience of Internet-facing systems and the principal source of adaptive capacity. ... A cursory look inside [Internet-facing systems] shows a situation more like an intensive care unit: continuous monitoring, elaborate struggles to manage related resources, and many interventions by teams of around-the-clock experts working in shifts. Far from being hale and hearty, these are brittle and often quite fragile assemblies that totter along only because they are surrounded by people who understand how they work, how they fail, what can happen, and what to do about it. Communications of the ACM, Volume 63, Issue 3. https://dl.acm.org/doi/10.1145/3379510 pdf

At IT Revolution conference in 2018, Richard Cook explained the above the line / below the line framework, clarifying complexity, mental models and emphasizing that continuous partial failure is the normal state of our software systems. The study of incidents is the revealing thing about how systems actually work. See Working in the Cyclone.