You only need to keep that in memory for players who are online at the time the rest of the time it can be on disk , where there ' s more space . You probably only need to keep that in memory for players who are in the same area as the monster , or who are currently checking their stats . Your numbers are very dubious . 8 bytes per monster times 100 , 000 monsters is 800 kB , not 8 MB . That ' s a very small amount of data how much do you think they spend on tracking every player ' s inventory , auction house , quests finished , NPC relations , etc . ? At least this will only grow linearly with the number of players a simple friends listblocked list between players is quadratic , which surely would be much more of a problem in a big MMO . Some games already have a bestiary that keeps track of every monster killed by the player , so it ' s definitely doable !