Build a streaming aggregation with exactly-once output, bounded state, and a defined policy for events that arrive an hour late.
Streaming interviews separate people who have run a job at 3am from people who have read the docs. The hard parts are not the windowed aggregate, they are bounding state so the job does not die, and choosing what a late event does to an already-emitted result.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
Streaming interviews separate people who have run a job at 3am from people who have read the docs. The hard parts are not the windowed aggregate, they are bounding state so the job does not die, and choosing what a late event does to an already-emitted result.
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.
