Buffered Writers & Ring Buffers

After the filesystem walk, the buffered writer is the Datadog coding shape that comes up the most. The prompt barely changes between rounds. You are handed a File (or some sink) whose write pushes bytes straight to disk. You wrap it in a BufferedFile that holds writes in memory and flushes only when its buffer fills up or someone calls flush. The warm-up takes five minutes. What you are graded on is the ladder of follow-ups that comes after it.

One reported round had the interviewer reject a candidate's dynamic-length buffer and insist on a fixed-length array with pointers tracking how full it was. Another pushed the buffer into a circular array the moment write changed to take a byte count. Neither steer is random. Both shapes are code Datadog ships: a buffer that batches writes, a ring buffer that keeps only the last N items. The interviewer already has one in mind and walks you toward it.