System design interview question
"Design a web analytics platform. Millions of websites embed a tracking snippet; it sends events for every page view and interaction. Site owners then ask arbitrary questions — 'unique visitors from mobile in Germany last week, segmented by landing page' — and expect answers in seconds. 20 million sites, 500 billion events a day."
arbitrary-dimension analytics at massive scale, where the query space is unbounded and the data is enormous. The distinguishing content is: sessionization (turning a stream of events into user sessions), the impossibility of pre-aggregating every dimension combination, approximate distinct counting, and multi-tenant isolation across millions of tenants.
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