Delta encoding multiplayer game state

Old Light is a browser strategy game where a tab can stay open for days. The client holds a full copy of the galaxy state it is allowed to see, and the server keeps that copy honest by sending patches: every change arrives as a world.delta message the client merges into what it already has. Sending changes instead of resending state is textbook delta encoding. What that leaves open is what a game state patch actually holds, and why the patch a rival receives is not the one you receive. I covered how the stream starts (one snapshot on connect, then deltas) and the time-math traps inside it in the networking post . This post is about the delta itself. What goes in a game state patch When people say delta encoding they usually mean byte diffs: compare two versions of a blob, ship the difference. That requires the sender to know which version the receiver holds. A game server broadcasting to thousands of sockets can't afford that; tracking a per-client "last known state" and diffing against it on every change would be more expensive than the update. So an Old Light delta states facts about players and sectors instead: interface WorldDelta { added ?: { players ?: Player [] }; removed ?: { playerIds ?: string [] }; updated ?: { players ?: Player []; sectors ?: Sector []; dirtySectors ?: SectorCoord []; // map data here went stale, refetch it tradeBoard ?: TradeBoardDelta ; // the market board moved deals ?: DealsDelta ; // a negotiation moved; only its two parties get this }; serverNow : number ; } A delta says a player joined, an id is gone, a player's row changed, or a sector's public map data went stale. The last two fields carry no payload. They say a surface moved, a client with that surface open goes and reads it, which keeps a busy marketplace off every socket that isn't looking at one. The server can emit the identical message to every socket without knowing what any of them currently holds, and the client can apply it to whatever it has. It also tells the rendere