There is NO rares spawn engine, the vast majority of the rares were and are programming oversights, items that were not locked down, or items that were take-able under special conditions (not talking about outrageous bugs). They exist today because OSI for the most part sees that its something that many players enjoy and that it is not detrimental to the game. There have been numerous instances when OSI employees (especially new ones) have in a befuddled stupor deleted rares. This isn't because there is a problem with the company, its due to the fact that Rares were not planned items, and most players that collect them know (not kidding) 3 to 4 times as much as the average GM, or Counselor. New OSI employees often don't know any better.
The present design and development team is now actively adding rares to the game like daemon bone armor, Glacial Staffs, and "the big fish". They are doing an awesome Job, and I can't wait to see what they have come up with in the Next year. Most rares however were not from this effort by OSI, there were just items that slipped between the cracks in this Giant Program.
At the last UO fair in Texas, a member of OSI stated that their is No "monthly rares spawning engine" and it was later clarified that even if they wanted to make monthly rares spawn that they couldn't because the of programming limitations, and that their really inst a time based spawn engine.
Unfortunately for many rares hunters they took this to believe that the "monthly rares" don't spawn any more. (in reality if they don't spawn anymore based on there not being a rares spawning engine, they never should have spawned in the first place so we know there is more to it than that).
Another one of the question/statements that I am most often asked is:
"I heard the monthly rares stopped spawning since trammel came out?"
Here is what I know.
1. All of the "monthly rares" are still spawning, and have spawned since Trammel came out.
I will say in advance I am probably wrong about parts of the "theory" but right or wrong
There are a lot of things that come into play, I'm not a "Jedi Programmer" by any means, but with some understanding about how UO works, and the things that are Known, this is my best interpretation of how it works.
The UO spawn engine.
Most players can relate to dungeon crawling, they know that the faster they kill off the monsters there, the sooner new monsters will spawn. (A fact that OSI has stated on numerous occasions). So to some degree that spawn engine is "event triggered" but also some dungeons (Ice dungeon and Destard come to mind) will have a spawn that gets out of control if no players have been there for a while killing it off. So there also seems to be a "time triggered" element.
However even with the most out of control spawn when it reaches its limit the spawn Stops. You will never walk into Destard and see wall to wall dragons, drakes, wyverns and Giant serpents (lol unless OSI is having a technical problem) They can get thick, but once they reach their ceiling they stop spawning.
The "Monthly rares" are exactly like that, there are only so many allowed by the server at one time and when they reach their ceiling they stop spawning until an existing one decays, is deleted, or in some cases moved to a different sub server (more on sub servers later). As far as an Extremely rare item decaying, that will probably only happen when the player who owns it leaves UO, deletions only happen when a player is banned, or some goose stepping GM sees the item and thinks he needs to do that, (as has happened with, the 2 story statue, numerous quest items, Miscolored food and ingots etc, awesome game, good people running it, but every now and then GRRRRRRRR.... lol )
"Monthly rare" is an Extremely Inaccurate term, it implies that some rares follow a 30 day cycle, as far as I know, (And I have invested a good bit of time on this) they don't. Even if the employee at OSI is correct that that its not possible for the game to track things on a monthly basis that way there are certainly mechanisms that exist in the game that would cause people to think that.
The shard "saves" several times a day, you may notice one when making purchases all the NPCs will close their "buy" menus, and appear towards the center (or wherever their default position in their shop is) the shard also saves and is stored for backup once a day during its maintenance cycle.
The Difference between the normal saves throughout the day, and the maintenance cycle is that the maintenance cycle will check a Large amount of items in the game, and if they are not there it will generate a new one in its place. This is how the daily rares like the fruit basket, large and small rock, tall candles etc work. This is also how other things like Dock barrels, stable water basins, and provisioners barrels are replaced when people take them.
Now for items that are Very rare like Tarot cards, they are not going to spawn unless an Existing set is deleted or decays. I'm not certain as to why so many people feel that its a "monthly rare", but because soo many people that have found more than 1 set of tarot cards agree on that, I have to assume there is something to it.
I have never been able to establish a 30 day, 31 day, 60 day or 90 day cycle on rare spawns, I establish spawn rates by counting the days myself or the people I hunt rares with camp server up spawn locations, its not a perfect system but its a little more scientific and action oriented than the guess work and speculation that is behind a lot of estimates I have seen. What I have noticed is that in the pre trammel times certain items spawn "about" monthly.
Attempts to count days are often also inaccurate because each time the server has a Hard core crash it will usually have a maintenance cycle done on it, which causes all the daily server up rares to spawn a second time that day (and any other rares that are going to spawn) to spawn whenever it comes back up. Most stories of rares being found during the day time, or early evening are due to a server crash and the shard being brought back online after a maintenance cycle. There are also times when the servers will not go down for maintenance, or end up having multiple maintenance cycles in a single day. All those things would make it difficult to track the amount of days in between spawns.
Its possible that in the past (pre trammel) only certain maintenance cycles would count -ALL- the items on a server and regenerate replacements for any that have been deleted. If they had that "complete maintenance cycle" only once a month, then it would stand to reason that would cause many rares to be considered "monthly rares". If that was the case I don't think it is any longer since trammel has come out.
In regards to possible differences in Maintenances cycles.
Veterans out there will remember the "clean up Britannia" Program, where prize tickets were issued to players for throwing out their "junk" and reducing the overall item count of the game. The short version of the program for the people who were not in UO at the time (Or didn't own a house or boat to store stuff in) is as follows.
There were special "Clean up Britannia" trash cans all over the place, people could bring their excess NON-Stackable items there, and toss them in, and receive prize tickets in return.
At the time, there was a (moronic) 400 Stone weight limit on bank boxes, and a reasonable 125 Item limit for Bank Boxes, There is still the item limit, but the weight limit was removed from the bank boxes as part of this program. So, if you wanted to Store more than 400 Stones worth of items (Ingots, Lumber, Hides for example) you had to 1-Have a house to store them in, or 2 Have a Boat to store them in. You could stack things on the deck of boat on the first tile in front of the mast with reasonable safety, in houses you could just leave them on the floor and they would not decay. As far as Items went its the same as now, no matter what they weigh you had a limit of 125 Items total in your bank box.
The old system had some serious problems, 1-In that only people with houses could own much of anything in the game, the only storage option at the time for most people were boats, and though they were reasonably safe, they were Buggy. I remember the time I cast Paralyze field on the deck of my boat once and 5 of the 6 backpacks representing my life's work in the game at the time disappeared, also there were several boat Break in Bugs and ways to steal off the deck. In addition to all of those problems, there was a huge problem with the overall Item count in the game. Some shards were down for maintenance for almost 2 hours sometimes, certain houses would cause unbelievable lag, and pockets that would "trap" a character occasionally. The vast amount of items being stored in the game was causing some major problems.
I -think- that to cope with the item count they had a mandatory daily maintenance, but about once a month ran extended maintenance cycles, and only on those cycles would the game actually "replace" certain Items. (Like Tarot cards, the large vase, and other monthly rares. So they didn't have "timer" built into the game, rather the rares spawn inadvertently revolved (as it still revolves) around the Maintenance schedule.
This does not mean that one of the "monthlies" is going to spawn right afterwards, It will only spawn if an existing copy is deleted or decays.
As some follow up information on the clean up Britannia program, Many items were invented, and many systems put into place to lower the item count, and be beneficial to the players. Potion Kegs, Recycling metal items, turning unwanted cloths into bandages by using scissors on them, adding to the number of "Secure containers" in houses, removing weight limits from them and the bank box, and making more things stackable.
The Trammel changes.
I believe the specific changes in the rares spawn since the coming of Trammel are NOT programming related, but affected by other factors, in this case population, demand for rares, and a MUCH better understanding of rares in general.
In the past it was not unheard of that a cache of rares would be discovered when a house decayed, or occasionally in a pack on a bank floor somewhere.
Now however more and more people recognize rare items for being "rare" and not just a curious Bauble. So, 1-They are not throwing their baubles away, and 2- If they do quit the game they generally give their valuable items to their friends. 3-There are presently not many established players quitting UO.
Before Trammel UO had a paying population of about 110,000 accounts, and the American shards were experiencing a slow decline in the total number of paying accounts. More people were quitting UO than were new ones coming in, which added to a healthy turnover in "monthly rares" (incidentally, most of the material written that supports the "monthly spawning cycle" was written during this period of time.)
When Trammel was about to be made a reality the amount of paying UO accounts started to climb, and now (late January 2001) UO reports just over 200,000 Paying accounts nearly doubling the size of the player base. There are certainly a lot of Issues other than Trammel that were an aid in the success, a more responsive design and development team, and a significantly less "SIEG HEIL!" customer support and GM (game master) staff are some of them.
All of this has added to the existing player base, and dramatically increased customer retention.
More players have added to the overall competition for rare items, and the increased customer retention has dramatically decreased the average amount of time a rare will be held by a person before it decays, respawns, and is then claimed by another rates hunter.
As to why some items categorized as monthly seem to spawn all the time, and others only once or twice a year per shard. It has to do with the total number of that type of item allowed on a shard. Again to use tarot cards and the large vase as an example, there are probably between 100 and 150 sets of tarot cards on each shard, but their are almost certainly less than 20 Large vases on each shard.
It would stand to reason that more Tarot cards will decay or be deleted that would Large Vases over the same period of time.
I'm not certain how items establish what their limit is in the game, I have an Idea, but not much evidence to go on, so for now I will keep the Idea to myself :)