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Design out-of-order windowed stream processor

BoxSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteCoding & Algorithms

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Design out-of-order windowed stream processor evaluates algorithm design, data structures, correctness, complexity, edge cases, and implementation details in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.

Question

Design an event processor for an unbounded, infinite stream of events. Each event has the fields id, timestamp, payload (a string), and checksum. Events may arrive out of timestamp order, and they should drive a rolling aggregate over a 1-minute (60-second) sliding window.

Your processor must:

  1. Validate each event by its checksum and discard any event whose checksum does not match its payload.
  2. Drop late and out-of-window events — any valid event whose timestamp falls more than 60 seconds behind the current window (the latest observed timestamp) is discarded.
  3. Handle duplicates — if the same event id is seen more than once, it must only be counted once.
  4. Maintain and report the average payload length over the latest 60 seconds, updating continuously as new events arrive and as old events fall out of the window.
  5. Keep the overall time complexity no worse than O(n log n) across n events.

Describe the data structures you would use, how you keep the window current as the clock advances, and how you handle late arrivals and duplicates.