Reservoir sampling: pick k random items from a stream of unknown length.

Drawing a uniform sample from a stream too large for memory (or whose length stays unknown) is a genuine data-engineering problem. What interviewers watch for: the replace-with-decreasing-probability trick and a telescoping proof. Here is the answer.

Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.