Internet Download Manager LLD: Byte Ranges, Parallel Couriers, and a Crash-Proof Journal

"Design a download manager." The kind that makes a 2 GB file land in a quarter of the time and survives you closing the lid mid-download. This is the first Phase-3 question where concurrency and durability sit in the same room — and the interviewer is watching whether you can hold both without dropping either. The pretty part (parallel speed) is easy to gesture at; the part that earns the offer is the ugly question nobody asks at home: what's on disk the instant the power dies? The hinge is one idea the thread pool article set up and this one cashes in: a unit of work that owns its own range c…