Most of the in-game aspects of this problem have been discussed to death on MUD-Dev long ago. If you're really curious, make sure to go read it. This is a good quick summary, but there's more out there. Always.
The phrase 'persistent world' means a world that exists more-or-less 24⁄7, whether you're there or not. The implication is that some of the changes that you make to the world may persist.
For a persistent world, it can be useful to have a persistent server, though that's not mandatory. You can have a non-persistent world on a persistent server (EverQuest is almost this), or a persistent world on a non-persistent server (certain MUDs are exactly this). You may find it's easier to use a persistent server if you're going to host a persistent game, though.
Current 'persistent worlds' aren't especially persistent. Respawning tends to occur so rapidly that if you leave for an hour, there's hardly any way to tell you were ever there except by looking at your character. There are a few exceptions like house deeds, but not many. Changes to persistent worlds tend to occur in small amounts, and in rare locations.
A fair number of MUDs will let objects sit around for essentially forever. If they're removed, it's by the actions of other players, or of the world itself in some way. This gives a more immediate sort of persistence, though respawns mean that the effect of the player is still pretty limited. Monsters you kill will be back in a few minutes, items you take are the same way. Overall, it feels a bit like a syndicated television show where no problem is allowed to take more than thirty minutes to fix.
Persistence comes in degrees. The game existing after you leave (a la EverQuest) is the simplest degree of persistence. As the player's actions have greater and greater long-term effect on the world, the degree of persistence is increased. A somewhat persistent game will leave objects lying around until they're moved or cleaned up. A more persistent world will fail to respawn monsters and items when they are killed or moved. An exceptionally persistent world would allow the player to dig a hole in the ground, or build a building, or burn down a whole town, and the results of those actions would remain until the next person changes the area again by building, altering or destroying.
But why would you do this in a MUD? When the old stuff isn't going to be kept around for any time anyway, why bother to make it persist for years? Why would you care?
One answer is that you can make a much better game with persistence, but only if you abandon the constant resets that define so much of current hack-and-slash MUD-dom. Imagine a game where monsters are spawned in some more dynamic way and wander around more freely. Imagine a game where a monster, when you killed it, were to actually die and not pop back up in the same spot a few minutes later.
That's the concept. It's not the only concept. There are other alluring possibilities that persistence allows, including simple things like burying a treasure in the forest and being able to dig it up months or years later.
In a MUD, you reset objects and monsters constantly. What if you didn't?
But wouldn't your world be entirely depopulated after a few days? Not if there were another way to spawn monsters. Normally, MUD monsters spawn in a single location and stay there unless they're molested. However, if animals were to wander randomly in from the far edges of the forest, if orcs were to spawn in tunnels and only come to the surface from there, if townsmen had to train to become guards rather than having guards spring fully-formed from the dust... Then a few clever players, by carefully placing NPCs and traps, would be able to change the balance of the world. Those orcs could be beaten back and the tunnels guarded by powerful NPCs or players, and there would be no orcs for some time (until villainous players intervened, perhaps). Or the orcs could be herded toward the forest, making it more dangerous. There are many variations, and the interesting combinations would increase exponentially as the world grew larger and more varied.
If a player logs out or disconnects, something must be done with his body. If you make the player disappear, then there's no change in a persistent MUD. But if you have the player become a 'linkdead statue' or the like, remember that those statues will persist forever unless cleaned up. So make sure there is some way for the players to go 'off camera' when they log out. Disappearing, player houses, gnomes carrying the statues away, whatever, as long as there's something.
I suppose it could be argued that the 'most' persistent solution would be to have the characters remain logged on as NPCs, running shops, accomplishing tasks, or otherwise being part of the world. However, AI is simply not good enough at this point for that to be a reasonable and common solution, and players tend to take a very dim view of their character losing experience or items by being killed when they can't control the character.
There are other concerns with DGD's style of persistence, but they have nothing to do with game design. You can read more about that here if you're interested.
Standard MUDs have horrendous inflation. Items and money just keep respawning with monsters or respawning separately. As more and more money is poured into your game over weeks and years, it just keeps accumulating and object prices (usually) stay constant, so players quickly have far more buying power than they'll ever need. There are only a few good items that are buyable, and more experienced players have, almost without exception, already bought them, then given enough money to your new players to let them buy them.
A persistent MUD, by requiring more planning, may actually help with this problem. If you just have creatures respawn or wander in and you never have any money leaving your economy, you've got the same problem. But by having a more complicated spawning system, you also leave the possibility of merchants moving on, creatures stealing money and leaving the game area, and other money sinks. If you have sensible ways to introduce creatures and valuable items into the economy, make sure you have a sensible way to remove them as well, or at least you understand enough economics to understand the effects on your game.
Other standard 'sinks' include taxation, robbery, items wearing out, unattended items rotting away, and various NPCs scavenging or otherwise disposing of objects.
A problem that nearly every MUD has to deal with is the Old Players' Club — the set of players who've been on your MUD for a very long time, and have ultrapowerful characters with all the best loot. Some of them may even have older 'discontinued' items and skills that no longer appear in your MUD for balance reasons, but you just didn't have the heart to take from them.
To a (very) small extent, these characters will go away on their own as the players quit your MUD. And to a larger extent, these players are doing your MUD good, not harm. But it's annoying for a new player to realize that it will take years for him to come close in power to these characters, especially if he'll be competing with them for equipment and experience, and even more so if player versus player combat is at all common in the MUD. Also, you may not like them being able to casually give away equipment of tremendous power to newbies or helping them powerlevel.
In that case, you again need sinks, but in this case for power or for players rather than for items or money. One possibility is a 'hall of fame' so that players can trade in their old characters for newer ones, but in return have their very powerful characters immortalized somewhere in the MUD. If you're willing to risk player dissatisfaction, you can also have skill decay, like item decay, so that characters who can find no greater challenges in the MUD will still have to stay active if they want to maintain their great in-game power. The combination of skill decay and item decay can also solve the problem of 'discontinued' items and skills remaining in the MUD, especially if discontinued items can't be repaired and discontinued skills can't be improved.
Standard LPMUDs have initialization code in every object to set their datafields. Rooms set exits this way, objects set their weight, et cetera. A persistent MUD in the DGD sense, or one that saves and loads object data in some functionally similar way, allows a new and different trick for MUDs that don't work this way.
If objects have data that persists over time, then builders can alter it. For instance, you could give builders items or commands that would make objects heavier or lighter, which would be useful for fine-tuning puzzles involving weights and scales. Builders could directly alter the description of an object when a typo was found with a "set" command on the command-line, if they so chose, rather than having to edit the LPC file.
Standard initialization code prevents this. If the object is initialized (for instance, by a reboot) then the modified weights or descriptions or whatever are lost. Since reboots, if they occur at all, will occur daily or weekly, that would remove all use of this modification as a tool for builders. So if your MUD is going to use persistence as a builder tool, you'll need to get rid of standard initialization code, or you'll need to make sure that it only ever runs once for a given object, and that the data is loaded and saved across reboots after that. DGD-style persistence, mentioned above, does this pleasantly and transparently.
There is also the matter of player-created content. Having a policy on such content is necessary in a persistent world where the players can make direct modifications to the game world and those modifications might reasonably 'stick'. If builders can use persistence for building, in some sense so can players.
@@INCLUDE re_gurbalib_1@@
@@INCLUDE re_gurbalib_2@@
@@INCLUDE persistence_1@@
@@INCLUDE persistence_2@@
@@INCLUDE persistence_3@@
@@INCLUDE persistence_4@@