Watching without playing
Spectator and replay systems let people view a match they are not playing, either live or afterward. The elegant trick is that a deterministic simulation needs only inputs, not full frames, to reproduce the match.
Replays as input logs
- The server records the stream of inputs and the initial seed for the match.
- To replay, a client re runs the same deterministic simulation, feeding the logged inputs.
- This produces a tiny file compared to recording video, since one match is just a list of inputs.
Determinism, the same property that powers prediction, is what makes this faithful. Any nondeterminism would desync the replay.
Live spectating
- Live viewers can receive a delayed snapshot stream, often minutes behind, to prevent stream sniping.
- A dedicated relay fans out the spectator stream so player facing servers are not loaded.
- Spectators get a free camera, so interest management must send a wider or full view.
Key idea
Replays store only inputs and a seed, replaying through the deterministic simulation, while live spectating fans out a delayed stream via a relay.