Spectating

osu!stable

One player watches another play in real time: the watched player's (the host's) gameplay frames are relayed to every spectator, and several people can watch the same host and see each other as "fellow spectators". Nothing needs setting up.

How it works

When you click Spectate on an online player, your client sends START_SPECTATING. The server:

1. records you as a spectator of that host;
2. tells the host SPECTATOR_JOINED, the host client's cue to start streaming its frames;
3. introduces you and the other spectators to each other (FELLOW_SPECTATOR_JOINED both ways).

While the host plays, its client streams SPECTATE_FRAMES, and the server relays each bundle as it is to every spectator. If you don't have the host's map, your client sends CANT_SPECTATE; everyone is told, and your client starts watching again once the map has downloaded. The host stops streaming when its last spectator leaves.

You do	Client → server	Server → others
Start watching	START_SPECTATING(16)	SPECTATOR_JOINED(13) → host; FELLOW_SPECTATOR_JOINED(42) between spectators
Stop, or log out	STOP_SPECTATING(17)	SPECTATOR_LEFT(14) → host; FELLOW_SPECTATOR_LEFT(43) → the other spectators
Host plays	SPECTATE_FRAMES(18)	SPECTATE_FRAMES(15) → every spectator
Missing the map	CANT_SPECTATE(21)	SPECTATOR_CANT_SPECTATE(22) → host and spectators

The #spectator channel

While a player is watched, the server opens a #spectator channel for the host and everyone watching them, the same tab osu!stable shows while spectating. Each host has its own. The host joins it when the first spectator arrives and leaves when the last one goes; spectators join and leave as they start and stop watching. ! commands work there too. Nobody joins or leaves it by hand.

Limits

• Spectating lives in the one TomYum process, like the rest of online state, so it doesn't span several server copies.
• Spectate state is cleaned up on logout.

osu!lazer

lazer's spectating (and multiplayer and presence) is ppy's spectator server, the spectator service of the production stack, reached at spectator.<domain> or osu.<domain>/signalr/. It saves lazer replays to the website's uploads, where the website and TomYum read them.

The tournament client

The tournament client's windows spectate players like a game does, each showing its own player. See tournaments.

For plugins

Plugins can read spectator frames (SpectateFrameEvent), and can have BanchoBot spectate an osu!stable player (IHostSpectating) so their frames flow without a human watching. See plugins.