First of all: Sorry if I don't express myself very well, I'm not very fluent in english :'(

I'm developing a multiplayer tile-based game (imagine habbo) and having some trouble with the movement. The idea is to click on a tile (target tile) so your avatar moves there and everyone can see you moving there. In order to do that, there's a pathfinding function.

My first approach was to run the pathfinding function in the client and everytime the avatar moved a pixel (guided by the pathfinder), the client sent a message to the server which broadcasted this update. This was obviously crazy...

My second approach was to send to the server just the target tile. The server broadcasted this target tile, and each client ran the pathfinding function for each avatar in movement. I found problems with this because of synchronization: imagine two users moving that pass through the same tile at different times, maybe in one client with a little lag it happens that they pass through the same tile at the same time, so the pathfinder will stop one of them, as you can't walk over another avatar. This would lead to different positions in each client.

I've read some cases where the pathfinder runs in the server, the client just send the target tile and receives every x mseconds an update from the server with the pathfinding calculations for every avatar in movement (even himself). The problem I find here is how to set this "x mseconds" and that it might mean a lot of load to the server, handling all the pathfinding calculations...

So, has anyone deal with this problem? What would you do?

Thanks a lot, Carlos

I think it kind of depends on how you are modeling time and distance. If the client is already doing the path finding you can send the intended path to the server when they click the destination square. The server will be able to figure out if one player's path blocks another player's path before it happens and can notify the blocked player either right away or just before it happens (depending on how the game is supposed to be played).

If the players are snapped to the tile then they will be occupying that tile for whatever their tile per second speed is (assuming circular cows^H^H^H^H tiles) and the server will be able to sequence player movements and figure out when a collision has occurred. If players are not snapped to the tiles you will have do decide what an "occupied tile" is. Is it when the player has moved at least 30% across it or just barely snipped one of the corners?

Sending the intended path to the server will not get rid of the lag problem though. If the lagging client thinks there is a player on a tile and computes a path to get around them but the player has already moved (and the server knows they have moved) the lagging client will still send the round-about path. At that point it is about how often do you have the server update the client as to the world state (blocked tiles, etc) and will a client recomputed it's path if a quicker route suddenly opens up.

If an avatar is moving, ... as you send the reservation update.

Yeah, what he said!