Design Trending Topics

X Trends is the canonical top-K streaming problem, and a strong answer is judged on one move: realizing you must not try to count everything exactly. Trends are scoped to a location, chosen historically by a "Where On Earth" id so a user in London and a user in São Paulo see different lists. Each entry shows an approximate post volume, which tells you the exact count was never the point. And the list refreshes on the order of minutes rather than seconds, which buys you room to approximate. Candidate reports on this shape describe the same arc every time. A generic answer says "increment a counter in Redis per hashtag" and gets walked straight into the cardinality wall. The differentiated answer names approximate counting, a Count-Min Sketch for volumes and a heavy-hitters summary for the top set, and treats exactness as something you deliberately trade away.