Design a Rolling Event Counter
Company: Uber
Role: Software Engineer
Category: Software Engineering Fundamentals
Difficulty: medium
Interview Round: Technical Screen
Design an in-memory rolling event counter.
The counter should support two operations:
- `record(timestamp)`: record one event at the given timestamp, measured in seconds.
- `count(timestamp)`: return the number of events that occurred in the last 300 seconds, including events whose timestamps are in the interval `[timestamp - 299, timestamp]`.
Discuss and implement an approach for the single-machine case. Then discuss trade-offs around:
- when to remove expired events,
- whether to use a background cleanup thread,
- how stale data can affect correctness,
- memory usage under high traffic, and
- concurrency concerns.
Quick Answer: This question evaluates understanding of data structures and time-windowed counting, performance and memory trade-offs, expiration policies, and concurrency when implementing an in-memory rolling event counter.