I am planning a tile based MMO, like Tibia or top down Minecraft. I'm using a client-server model. I've got almost everything figured out, with a notable exception of state updates.
When information about a tile changes (e.g. new item is added) all clients that see this tile need to be informed about it. How do I efficiently decide which clients needs to receive this update? I'd like to support on the order of 100 000s of tiles and 100s of players.