P2P multiplayer for web games

A recipe for small-group (2 to 4 player) online play where one player's game hosts the session and the others connect straight to it over WebRTC. No game server to run or pay for. A free signaling server introduces the peers, then the game traffic flows browser to browser.

The recipe covers real-time co-op, turn-based and shared-object games, and nothing in it depends on one engine. The engine only has to give you a per-frame update and a way to spawn and move objects. references/engines.md shows the glue for the common web engines. Unity and Godot games use the same room module through a JavaScript bridge (the harmony-engines skill shows how); the protocol, security and sync rules here apply unchanged, and those games are tested in their web build with two browser windows.

On Harmony. Harmony hosts the game and adds its overlay to every upload with no code, but runs no game servers or invitation-sending service, so these rooms work the same there. The game needs neither the Harmony SDK nor a Game ID: players type a name, and inviting is copying the room code. A few things change: the game doesn't pause for the overlay during a session, and it runs in a frame inside Harmony's page, where it can't read the clipboard. A game that already uses the SDK can also start players with their Harmony name and open the overlay, where they can paste the code in chat. Read references/harmony.md when the game is, or will be, on Harmony.