Live streaming - latency against the buffer that saves you

Live is not video-on-demand with a shorter deadline. It is a different system, because the thing every other design leans on - the ability to fetch a byte again - does not exist. A second that is lost is gone.

Functional

• A broadcaster pushes one stream over RTMP or SRT and it is playable worldwide within seconds.
• The stream is transcoded to a ladder in real time, so viewers on poor connections are not excluded from a live event.
• A viewer joining halfway in starts at the live edge, not at the start.
• Chat and stream events are delivered alongside the video.
• The broadcast is recorded: a rolling DVR window during the event, and a permanent replay afterwards.
• A broadcaster can see, live, whether their own upstream is the problem.

Non-functional

• Glass-to-glass latency target of 8 seconds in standard mode, 3 seconds in low-latency mode. It is a target, not a guarantee, and the difference is the point of this board.
• Rebuffer ratio under 0.5 percent - a looser target than video-on-demand, because there is no buffer to hide behind.
• Ingest availability 99.99 percent during a broadcast. A dropped ingest is not a degraded experience, it is the event ending.
• One segment must serve a million viewers. Any per-viewer element in a manifest or URL breaks that and is treated as an outage, not a feature.
• No single-segment loss may stall playback: a missing rung falls back to a lower one within one segment.

Questions to ask before designing this

• What is the latency REQUIREMENT, in the product sense? Betting and auctions need under 2 seconds. A concert needs 10 and would rather never stutter. These are two different systems and the answer changes every box on the diagram.
• How many concurrent channels, and how peaked? 50,000 channels at 4 hours each is a fleet-sizing problem; one channel with 20 million viewers is a fan-out problem. They are not the same build.
• Is chat in scope? If yes, the spoiler problem is in scope, and either chat must be delayed to match video or video must be dragged down to match chat.
• Is DVR required during the broadcast, or only a replay afterwards? Seek-during-live turns a rolling window of three segments into a full manifest of thousands and changes the cache behaviour completely.
• What upstream do broadcasters actually have? Designing for 6 Mbps when the median is 2 means the ladder starts below where you thought.
• Who pays for the egress - the platform or the broadcaster? That answer decides whether the top rung exists.