Desynchronization

A desynchronization (desync) is when during multiplayer a player's game becomes desynchronised from the Factorio server. Their game has become different from the server's game.

What are desyncs

Desyncs can be caused either by a bug in the core Factorio game code or by a bug in a mod/scenario. They should not occur during regular activity.

Factorio multiplayer architecture

Factorio multiplayer code uses Deterministic Lockstep to synchronize clients, this is a method of synchronizing a game from one computer to another by sending only the user inputs that control that game, rather than networking the state of the objects in the game itself. It means that all players need to simulate every single tick of the game identically. If any computer does something ever-so-slightly different, a desynchronization (desync) occurs. The game includes processes to ensure any issues with network traffic does not cause desyncs.

Player experience of desyncs

Upon a desync between a player and server occurring that player immediately disconnects from the server and starts to reconnect. This reconnection isn't to allow the player to play the game, but to obtain the server's copy of the save game (map and game state) to include in a desync report. After the reconnection is complete the player is offered the options to save the report, quit or reconnect to the server to play.

The desync report can be provided to either the mod/scenario makers or the Factorio game developers to help them resolve the bug. Depending upon the exact cause of the desync the player may be able to reconnect to the server and continue playing, or they may find they immediately desync on reconnecting repeatedly.

Desync Report

The desync report contains both the player's and server's copy of the save game (map and game state). The player's copy is from when the desync occurred and titled desynced-level. With the server's copy being from when the automatic reconnection was made and titled reference-level.zip. A copy of the players Factorio log file is included titled log.txt. As the report contains two copies of the save game it may have a large file size.

The desync report does not state the cause but is a record of the game state at the time of the desync.

Next Steps

The next steps after a desync occurs is to start a desync investigation. In many situations restarting the server will fix the desync in the short term and allow all players to rejoin until the conditions for the bug to occur are re-created. As the server will load from its save and the players on re-joining will load from a new save, reducing the time for the cause of the desync to occur. However, a server restart isn't guaranteed success and may lead to no players being able to rejoin the server without desyncing.

Desync Investigations

In most cases when desyncs occur and mods or scenarios are being used they are the cause and need to be handled first. If they are found to not be the cause then it should be reported as a core Factorio desync error. In both cases identifying the activity that triggers the desync is an important step. If it is tied to a player's action or activity that happens in the game (i.e. a train arriving at a station) or is a time-related event. In some cases, the desync may not be reproducible and so only a code investigation can work towards a fix.

When desyncs occur and mods/scenarios are in the use there are 2 different approaches that can be taken to help get the issue fixed. The code investigation approach requires you to be familiar with Factorio modding and its coding rules. The less technical method is to identify the mod and activity that causes the desync through trial and error so it can be reported to the mod author.

Identifying desync causing mod through trial and error

This is a less technical method aiming to identify the mod and its activity that seems to cause the desync so it can be reported to the mod/scenario author. The activity that causes the desync should be established prior to these activities being done to confirm if they are having an impact. These steps may affect the gameplay and should be done on a test save.

• Removing any newly added mods and rolling back (return to the previous version) any updated mods to see if the issue still occurs.
• Remove mods from the game to rule them out as the cause.

Once the desync can no longer be reproduced the suspected mod can be re-added/updated and the desync recreated to confirm the mod and activity are the cause of the desync. As these steps require the server to be restarted this may in itself fix any desyncs in the short term until the conditions for the bug to occur are re-created.

Code investigations with mods/scenarios involved

The desync report is a useful reference for investigating desyncs but it does not state the cause of the desync. Comparing the desync files and use of heavy mode can assist in identifying which mods and activities to review initially. Ideally, it is supplemented by an account of what was occurring and being done by players at the point of the desync occurring to assist with narrowing down the activities of code to review. However in many complicated scenarios this may not be known prior to the investigation.

Code that causes desyncs

There are a number of ways mod and scenario code can cause desyncs due to incorrect variable scoping, requiring or API event handling. Examples of these can be found in the Gansir's modding tutorial multiplayer and desyncs section, with the full technical definition of usage constraints for API events in the Factorio API data lifecycle. Not all instances of non-compliance will lead to desyncs or errors, but most will. For this reason, it is recommended to always maintain compliance to avoid risk and time-consuming testing. Instances of non-compliance found should be raised to the code author.

Comparing game state & save files

The desync report files and heavy mode dumps contain a number of files that can be compared by any text comparison tool to look for differences. These files are generally sequelized dumps of Lua tables or c++ game objects and so are only partially human readable. The non-human readable item can include table ids (0x.NUMBER) and magic numbers related to the c++ game state. These ids and references should generally be the same between files as the in-game entity they reference normally won't have changed between the 2 save game states being obtained. When the data differs between files the source code of that mod/scenario must be examined for all interactions with that data item for any instances of non-compliance.

Examining the desync files

Within each savegame's zip in the desync report are a number of files that when compared can show data changes. As the server copy of the save is obtained after the desync there may be genuine differences between the files. So differences aren't hard evidence of the cause of a desync, but only a pointer on where to investigate further.

• script.dat - contains the scenario and mods Factorio global tables. Generally, the saves persistent data should be the same between the 2 saves. On rare occasions, there can be a difference due to how the information is written out to the file. Should a mod causes a client to desync it can cause the clients game to stop process other mods activity mid tick. This can lead those mod's global tables to have differences between the client and the server despite having no code defects themselves.
• level_with_tags_tick_XXXX.dat - contains the current game state including messages printed to players. This can show if entity states differ and if messages written to players differ. level.dat is the same information, but without the descriptive tags for data values.

Using heavy mode command

Heavy mode is an in-game option that can be enabled to make Factorio save and load the game very tick. This allows Factorio to highlight if there are any changes in the game state between the save and subsequent load. Should a state change occur a message is shown in-game and the save and load game states recorded to a dumps folder in the players Factorio Data folder. Typically this process will trigger for incorrect changing of data as part of the on_load event or local/global variables being changed and not persisted. The recorded game state is a dump of the internal game state, including entities, but does not include the mod's global table. This allows the checking if an entity's state has become different between the load and save state.

This process can be used to partially test a mod for multiplayer compliance without the need of multiple players or a server.

Desync reporting of core Factorio game

If no mod or scenario were involved in the desync, the desync report and activity information should be posted to the Factorio forums bug report section.