What is Uptime? — UK Software Development Guide
Direct Answer
Uptime is the percentage of time a software system is operational and available to users. Common uptime targets: 99% (87.6 hours downtime/year), 99.9% (8.76 hours/year — "three nines"), 99.95% (4.38 hours/year), 99.99% (52.6 minutes/year — "four nines"). The practical difference between 99.9% and 99.99% is significant — and the architecture cost to achieve four nines is substantially higher.
Uptime in the UK
Uptime targets have UK regulatory significance. FCA operational resilience (PS21/3) requires UK FinTechs to define impact tolerances for important business services — effectively minimum uptime requirements. NHS supplier contracts typically require 99.9% uptime during clinical hours. UK eCommerce Black Friday planning targets 99.95%+ during peak hours. ClickMasters designs systems to meet the uptime target agreed in the SLA — and recommends against over-engineering for uptime levels beyond business requirements, as each "nine" adds significant architectural complexity and cost.
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