OOS (Out of Sync)

work in progress, not comprehensive

OOS is an error that occurs in a Wesnoth scenario when two machines disagree (or one machine disagrees with a replay file) about the game state, i.e. which units are where, how much HP they have, how much gold each side has, etc. More precisely, OOS is announced when a client reads a command, such as "move Dwarf Fighter 8,4 -> 8,5 -> 8,6" or "recruit Gryphon Rider 9,7", which is illegal or nonsensical based on its understanding of the game state.

OOS could in principle be caused by network issues such as dropped packets, by players having mismatched game files, or even by cheating, but it is commonly caused by scenario designers / add-on makers writing unsafe code. This page is about how to write WML that won't cause OOS. If you want to know what you should do if you get OOS in an online game, see [here].

Why does OOS happen?

Game begins out of sync

This can happen if one of the players has modified their game files, or if the players don't have matching add-on versions.

The OOS may still not appear until a mismatched resource actually appears and does something.

Game becomes out of sync

During a Wesnoth MP scenario with add-on content, the cfg files for the add-on reside on the host. As the game proceeds the host evaluates this WML and transmits new events / changes to the game state, through the server, to the other clients.

Network economy demands that not only is not all data transmitted, but also data is only transmitted at certain times in response to certain events ("synchronized events").

A scenario designer will cause OOS if she runs WML during an unsynchronized time which requires user input / randomness to evaluate properly.

Generally speaking a replay file receives the same kind of information about the game that a multiplayer observer of a game would receive, so any technique which would cause OOS for multiplayer will also cause corrupted replays.

A list of unsynchronized events can be found here: EventWML#Multiplayer_safety.

Some examples

If Player A changes the White Mage's magic attack from 9-3 to 10-3 while Player B did not, that would result in an OOS since as the game progresses each client would see a different amount of damage being done.

Say that the White Mage attacks a Spearman with 30 HP left and hits all 3 times. To Player A, it would appear that the Spearman died. However Player B would see that unit as having 3 HP left. As a result...

What if Player A then tried to move another unit to where the Spearman was? To Player A's client, it would work fine. To Player B's client, that wouldn't make any sense, since it's not possible to have multiple units on the same hex.
What if Player B tried to attack one of Player A's units? To Player B's client it seems like a normal move, but to Player A's client it seems like an empty hex is trying to attack him.

Konrad is writing a scenario in which a small number of random units are spawned for various sides in a start event, to try to add replay value.

However when he watches replays of his scenario he always gets OOS messages and notices that each time he watches the replay he sees different units.

Konrad is writing an MP scenario in which players survive attacks from an AI enemy. At the beginning of the scenario he wants to ask Player 1 what the difficulty level should be, so he uses a [message] dialog for this with several [option]s. He puts this in a start event so that it happens on turn 1.

Unfortunately for Konrad, start is not synchronized -- traditionally in SP wesnoth campaigns, start events would only do things like place doodads on the map and initialize variables, so it isn't synchronized and the different players won't get the same results of the query. After a few turns all the players get OOS errors.

How to make your code safe

Make the WML run at a synchronized time instead, i.e. after a moveto event.
Use Lua wesnoth.synchronize_choice to make sure that all clients match (this is always necessary when using Lua math.random). Note that this function doesn't work right in or before prestart.
End the scenario and proceed to another before the OOS is detected. The clients are resynchronized on load. (Note: This is used in at least one add-on but is a bit of a hack and only makes sense to do if e.g. you need to get randomness during prestart.)

Additional tricks and tips

It is a known bug that the Wesnoth rand function is not properly seeded during start and prestart. You can use the Lua random number generator, or the timestamp function on the host, together with synchronize_choice to achieve this. See [here].
You can modify some things. So while you shouldn't change the damage of a unit's attack, there's nothing wrong with changing it's portrait.