Determine which one is client or server

This is the hardest part: determining which one should run in the client or in the server. In theory, the server should do everything in the update section. But in practice, I think that updating the particle can be just done in the client because I don't want the server calculating the large particle and slow him self. This makes my code getting messier because I put every update in the server first and realized it was very slow. I need to refactor a lot of code. However whenever there is something special in that particular particle, server just need to tell to the client. But in this assignment, there is problem about that.