It's also a fascinating historical artifact of "when internet play meant orders of magnitude higher latency and jitter... as a base assumption."
Per memory, there's a bit where they've ferreted out all the sources of randomness, except one. And any source of randomness corrupted the downstream calculations, as they used global random generators.
There was some reason they couldn't just trace calls directly.
It turned out there was a line of code somewhere that said something like "if {rare condition} then rand.next {to figure out where to place an ornamental rock/tree}."
That was specifically related to their networking/syncing architecture -- deterministic lockstep simulation -- in which every client is running their own local instance of the simulation, and the only thing being passed around are interactions with the simulation.
So everyone has to have the same results and calculations at all times, because there's no authoritative simulation.
It's a common architecture for RTS's (interactions are far less numerous than individual entities) but not for general multiplayer games due to issues like everyone must be on the same timestep at all times -- so a single slow player slows everyone down. See https://gafferongames.com/post/deterministic_lockstep/
If you have an authoritative server, so the client is mostly a presentation/view of current game state, you don't need to be as careful about maintaining game state/randomness, since there's only one source of randomness (your authoritative server)
Ofc, if in an mmo you have multiple authoritative servers, you're back to the same problem, though I believe the more common solution is to have 1 server - 1 universe, and you just have more game instances (shards/servers) to connect to
There's a reason there were no complex-state MMOs at that time -- there simply wasn't the residential bandwidth to perform continual full state updates (latency aside).
E.g. grid & turn-based vs 3-D free-motion worlds
They say they specifically made the deterministic lockstep choice because it was the only option that (a) allowed the multiplayer gameplay they wanted and (b) fit within the computing and network resources of the time.
It's also a fascinating historical artifact of "when internet play meant orders of magnitude higher latency and jitter... as a base assumption."
Per memory, there's a bit where they've ferreted out all the sources of randomness, except one. And any source of randomness corrupted the downstream calculations, as they used global random generators.
There was some reason they couldn't just trace calls directly.
It turned out there was a line of code somewhere that said something like "if {rare condition} then rand.next {to figure out where to place an ornamental rock/tree}."