Build a streaming deduplicator: drop duplicate events within a sliding time window, bounded memory, late events
At-least-once delivery means duplicates are guaranteed, not hypothetical. Anyone can use a set; the round is whether your set grows forever, and what you do with an event that arrives after its window closed.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
At-least-once delivery means duplicates are guaranteed, not hypothetical. Anyone can use a set; the round is whether your set grows forever, and what you do with an event that arrives after its window closed.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
