Cache Consistency Trade-offs

1. Staleness Is a Number, Not a Feeling

"We cache it for sixty seconds" sounds like a complete description of a design decision. It is not. It states a bound and leaves the three things that actually matter unstated.

• How stale, at worst? Sixty seconds is the ceiling. The average is around thirty, and the figure that matters for an incident is the ceiling.
• How often is anything stale at all? If the underlying value changes once a week, almost every read during that week is perfectly correct. If it changes every second, almost every read is wrong.