Collect and Query Recent User Activity

Design a user-activity collection system that supports queries over recent periods such as the last minute, hour, and day. Explain the path from collection API through middleware and storage to the query service. Clarify what an activity contains, whether queries return events or aggregates, how users and activity types are filtered, and which clock defines the requested period. The source supplies no fixed traffic target, schema, or lateness policy. ### What a Strong Answer Covers - Event identity, ownership, timestamps, validation, and ingestion durability. - Storage and indexing connected to the required time-range queries. - Duplicate events, late arrival, partial windows, and query freshness. - Partitioning, retention, backpressure, and failure recovery. ### Follow-up Questions - How would you avoid counting a retried event twice? - How would the result change when an event arrives late but belongs inside the requested period?