Spotify HLD: Streaming Audio That Starts Fast and Never Stalls

You tap play as the metro dives underground. For four stops there's no signal at all — and the song keeps going, not a hair of a stutter, then snaps back to crisp quality the second you surface. Nobody streamed those four minutes live; your phone had quietly drunk ahead while the signal was good, and when the pipe went dry it sipped from a reserve. That one trick — play from a reserve, refill when you can, and when you can't, pour something thinner rather than stop — is the whole of a music-streaming system, and the thing an interviewer means by "design Spotify." The catalog, playlists, and li…