Explain What an HLS Playlist for a Recorded Twitch Broadcast Contains

You are interviewing for a product manager role at Twitch, and the interviewer asks a technical question: what does the playlist behind a video-on-demand (VOD) recording, such as a past broadcast, contain? Describe its main components and what the video player uses each one for. Unless the interviewer says otherwise, assume they mean the streaming playlist a player downloads in HTTP Live Streaming (HLS), the format in which these index files are formally called playlists. ```hint Think about a finished recording A recording has a fixed start and end that will never change. Ask which parts of the playlist exist because the full set of segments is known up front, and which exist so the player can choose a quality and jump to any moment. ``` ### Clarifying Questions - Do you mean the streaming playlist file behind a recorded video, or a viewer-facing list of saved videos that play one after another? - Should I cover only the per-quality playlist that lists the video segments, or also the top-level playlist that lists the available qualities? - Should I stick to HLS, or also explain how DASH describes the same recording? ### What a Strong Answer Covers - The two-level structure: a top-level playlist of quality variants and a per-variant playlist of media segments. - The segment-level components (segment URIs, durations, target duration, sequence numbering, initialization data) and what the player does with each. - The markers that tell the player the recording is complete, and how they enable a full-length seek bar and stop the player from reloading the playlist. - Optional components such as alternate audio and captions, encryption keys, discontinuities and keyframe-only tracks for scrubbing. - Product implications: seeking speed, startup time, storage and CDN cost, and editing a recording (trimming, muting, highlights) by changing the playlist rather than re-encoding the video. ### Follow-up Questions - When a live broadcast ends and becomes a recorded video, what has to change in its playlist? - Viewers say scrubbing through long recorded broadcasts is slow and shows no preview images. Which playlist components would you look at to improve it? - How could a highlight be built from an existing recording without re-encoding the video, and what limits that approach?