View source for Tutorial:Modding tutorial/Gangsir

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This is a modding tutorial for Factorio version 1.1. In this tutorial, the author will explain how Factorio works behind the scenes, how to modify Factorio, where to find documentation, and explain concepts.

== Overview ==
Before we start the tutorial, a few things to note:

<pre style="background-color:#AAFFAA!important; color:black">
Code tinted green like this should be included into the mod this tutorial is going to create; If the reader follows along with it. The best way to do this is to copy and paste, to ensure faithful reproduction.
Whenever code is added to the mod, a Lua comment with the file name will be at the beginning of the green box. Place the code in the box into that file. Eg:
--control.lua
</pre>

<pre style="background-color:#DDA0DD!important; color:black">
Code tinted purple like this should not be included into the mod, it's just for educational/example purposes, and to boost understanding.
</pre>

This tutorial was updated to version 1.1, so any ''viewers in the future should take note that some minor changes may have been made'', and should look at the changelogs up to the current version.

== Terminology used in modding ==

Before we start the tutorial, a few terms and definitions should be laid out, to ensure the reader understands.

; Mod : A script or series of scripts that allow modifications to the game through the API.
; Entity : An entity in Factorio is anything in the game that is not a concept, event, or tile. Examples of entities include the character, an assembling machine, a biter, etc. This can be 'machines' or free-moving objects like the character.
; Character : The actual entity that the player manipulates the world through.
; Player : All the data that defines a player, such as username, online time or the current zoom level.
; Prototype : A prototype describes an instance of an entity, item or recipe etc, a bit like a template. It defines stats, like what an entity actually is, an item's stack size, a recipe's ingredients etc. A prototype is used to create an instance of an entity/item/etc, and many functionally identical entities/items/etc will use the same prototype.
; Surface : A surface is a bit like a dimension. It is composed of terrain, such as grass, sand, and water, and all the entities on the surface. By default, there is only one surface in Factorio, referred to internally as "nauvis", or <code style="background-color:#DDA0DD; color:black">game.surfaces[1]</code>, but mods may create additional surfaces through the API.
; Event : An event is a recurring...event, that is triggered internally by the game. There are several events that mods may connect functions to, such as <code style="background-color:#DDA0DD; color:black">on_entity_died</code>, etc. More on this in the control scripting section.
; Item : Items are what is moved around in inventories, by inserters and on belts, etc. Each item in-game is an instance of the respective item prototype.

More terminology may be declared and defined later in this tutorial.

== Before beginning to mod ==

Before we can start modding Factorio, we must understand what Factorio is. You may be tempted to answer in lieu of the [[Factorio:About|about page]], but that is what a player would say. Since we are trying to become a modder, we need a more detailed explanation. Factorio is a game that is coded in the language C++, with an API provided by Wube (the developers of Factorio) to mod Factorio in the programming language Lua (version 5.2.1). This API allows adding scripts to the Factorio init process, to modify it without the source code of the base game being exposed, or modifying memory. This may be different than other games that offer modding, but this is a more professional and proper way of supporting modding.

To aid in the use of this API, the devs have kindly provided fairly comprehensive documentation of mods on at their [https://lua-api.factorio.com/latest/ API doc site]. Get used to using this site, as you will be frequently visiting it while you develop mods. The scripting API site contains information on [https://lua-api.factorio.com/latest/classes.html Factorio's classes] and information on [https://lua-api.factorio.com/latest/events.html events] that you can hook into. The [https://lua-api.factorio.com/latest/index-prototype.html prototype documentation] contains and links to information all around prototypes, listing their inheritance structure and their properties. You will need to check this site often, so the author recommends bookmarking it. In addition to this site, there are also many resources to be found created by the community, such as this tutorial.

=== Setup ===

The best way to develop a mod is to develop it in a place where it can be easily tested. When the tutorial gets to making the mod, this will be explained further. Additionally, using an editor that allows ease of typing and Lua language support is recommended. Emacs, Vim, Sublime Text, VSCode, and Notepad++ are all viable candidates.

== How Factorio loads mods ==

=== Load order ===
Within stages, mods are loaded by dependency, then by alphabetical order. This is ''very important'' to understand, as it can cause you problems if you neglect it and try to add inter-mod support to your mod.

Factorio has three kinds of dependencies. There are required dependencies, and optional dependencies. The third kind, restrictive dependencies, does not affect mod order and instead prevents the game from loading if the other mod is found. Required dependencies are loaded first, always. The game will fail to initialize if one of these is not present. Optional dependencies are loaded first if present, but do not have to be present. This is useful for enabling bonus features if mods are used together. Required dependencies should be used for mod libraries, and similar infrastructure.

=== The settings stage ===
The very first mod stage that is loaded when Factorio initializes is the settings stage. This stage is used to define all mod settings that are later shown in the in-game mod settings GUI, and has no other functions or possibilities. When running through this stage, the game looks through all mods for a file called <code>settings.lua</code>. After settings.lua has been executed for all mods, each mod's <code>settings-updates.lua</code> is executed, and finally each mod's <code>settings-final-fixes.lua</code> is called. These 3 different phases of the settings stage allow to change settings of other mods without needing to rely on dependencies to load last. All other files to be loaded will need to be required. All the files run here should contain nothing but setting definitions and code to produce setting definitions.

The settings stage does not have access to prototype or runtime data because it is loaded before those stages. The settings are expected to have a certain format, and all additional code will be discarded once the stage is over.

Mod settings are not covered in this tutorial, see [[Tutorial:Mod settings]] for further info on them.

=== The data stage ===

This is the most restricted part of the Factorio init, there's not much you can do here other than declare prototypes for technologies, entities, items and more. Things like manipulating files, affecting the world, etc, are blocked/unavailable. In fact, any functions or changes made will be discarded, as the lua session is terminated. You also cannot mess with the data table, it will error or be ignored. When using <code>data:extend({})</code>, it expects a specific format, more on this later.

When running through this stage, the game looks through all mods for a file called <code>data.lua</code>. After data.lua has been executed for all mods, each mod's <code>data-updates.lua</code> is executed, and finally each mod's <code>data-final-fixes.lua</code> is called. These 3 different phases of the data stage allow to change data of other mods without needing to rely on dependencies to load last. For example, the base mod creates barrelling recipes for all (then present) fluids in data-updates.lua. This means that if you add a fluid in data.lua, the base mod's data-updates.lua will add barreling recipes for it, regardless of whether your mod depends on base. Of course this also means that if you add a fluid in data-final-fixes.lua, it is created after the barrelling code runs in data-updates.lua, so no barrelling recipe gets created, even when desired. Because of this and similar mod interactions, it is recommended to create prototypes as early as possible. So, don't use data-final-fixes.lua to exclude a fluid from barreling, instead create it in data.lua and utilize "auto_barrel = false" on the fluid.

All other files to be loaded will need to be required. All the files run here should contain nothing but prototype definitions and code to produce prototype definitions. More on requiring files later.

All prototypes are documented on the modding API documentation website: [https://lua-api.factorio.com/latest/index-prototype.html Prototype documentation].

=== Migrations ===

[https://lua-api.factorio.com/latest/auxiliary/migrations.html Migrations] are scripts that are used to "fix" a save after a mod updates. Whenever prototype names change within a mod, migrations must be setup to replace all the old instances of the prototyped entity in the world. This must be done for all updated entities, or the old entities will be removed from the world, which is an unprofessional fallback that makes users dislike you. While this tutorial will not discuss migrations, there are many resources on migrations to be found around the community, and the API site.

To avoid having to write migrations, avoid changing prototype names and technology unlocks. Prototypes names cannot be dynamically changed and technology unlocks of already researched technologies do not apply automatically, making migrations necessary. Try to avoid these changes after shipping the mod out to the public. Try to come up with a finalized version of prototype names that you can base the mod around. Of course, migrations are unnecessary if the user simply starts a new world with each mod update, but do not expect the community to do this.

=== Runtime stage ===

Within most mods is a file called <code>control.lua</code>. This file contains scripting that makes the mod do things during the game, rather than just adding entities to the game. During this stage, each mod's control.lua is run, in it's own lua instance (this means no inter-communication without special setup) which it will own for the rest of the play session. During the play session, access to all tables provided by the game can be done inside of event handlers. (More on those below.) Because the control.lua is run every time a save file is created or loaded you don't need to restart the game to see changes made to the control.lua file. Simply restarting or reloading a save will re-run this stage. '''There are a few other caveats to this stage, reading the [https://lua-api.factorio.com/latest/auxiliary/data-lifecycle.html data life cycle] page on the API site provides the best overview.'''

The control stage documented is documented on [https://lua-api.factorio.com/latest/index-runtime.html lua-api.factorio.com].

== The major components to any Factorio mod ==

Within the average mod, there are several components that make the mod function.

Mods that define new entities will need to declare these entities in <code>data.lua</code>, <code>data-updates.lua</code>, <code>data-final-fixes.lua</code>.

Mods with in-game effects will also need a <code>control.lua</code> file, to add scripting.

Mods with configurable user settings will use <code>settings.lua</code> to describe those settings.

Mods that define any game element with a readable name may also provide a <code>locale</code> directory and subdirectories with names/descriptions in one or more languages.

The mod that we'll make in this tutorial will include both data.lua prototypes and control.lua scripting, to give you a feel for both.

== The tutorial mod ==

And now for the moment you've been waiting for. Let's start making your first mod. You'll need:

* A recent install of Factorio
* A text editor, such as Emacs, Vim, Sublime Text, VSCode, or Notepad++.
* An understanding of the tutorial above
* An understanding of Lua as a programming language. Enough to know the syntax and how it works. If you have prior programming experience, it should not be difficult to pick up.

Once you have all of these things, we can begin.

For this mod, we're going to make a set of armor that leaves behind damaging fire behind you as you walk. It will be fully resistant to fire, but weaker towards physical damage than heavy armor, making it an armor for hit and run attacks.

=== Creation of the directory structure ===

The game expects mod to be laid out [[Tutorial:Mod structure|in a certain way]]. To start out, create a folder in your [[Application directory|user data directory]]/mods folder. This folder must have a specific name, <code>fire-armor</code>. You don't need to zip anything for now, that will come later when you're done working on the mod. When you're finished, the mod directory should look like this:

* (user data directory, sometimes called .factorio)
** mods
*** fire-armor

Then, inside fire-armor, create two files, <code>info.json</code> and <code>data.lua</code>. The directory should now look like:

* (user data directory, sometimes called .factorio)
** mods
*** fire-armor
**** data.lua
**** info.json

=== The info.json file ===

Then, inside [[Tutorial:Mod_structure#info.json|info.json]], copy and paste the following into it:

<pre style="background-color:#AAFFAA!important; color:black">
{
  "name": "fire-armor",
  "version": "0.1.0",
  "title": "Fire Armor",
  "author": "You",
  "factorio_version": "1.1",
  "dependencies": ["base >= 1.1"],
  "description": "This mod adds in fire armor that leaves behind damaging fire as you walk around."
}
</pre>

To explain each field:

{| class="wikitable"
! Item
! Explanation
|-
| name
| This is the internal name of your mod, it is used to identify your mod in code.
|-
| version
| This is the version of your mod. This can be anything you want, provided it's a of the format "number.number.number".
|-
| title
| The pretty title of your mod, this will be displayed on the mods screen and when you submit it to the mod portal.
|-
| author
| Your name! You can change this in the example above.
|-
| factorio_version
| This tells the game what version the mod is for, this must match the version you're developing the mod for, 1.1 in this case.
|-
| dependencies
| Any dependencies of your mod.
|-
| description
| A short description of your mod, which appears in game. The mod portal is better for a long description.
|}

And that's all for info.json! 

=== Prototype creation ===

Now, there are two ways to create prototypes in Factorio. There's the short way, and the long way. One way requires to create a complete prototype definition based on [https://lua-api.factorio.com/latest/index-prototype.html the documentation]. Another way uses a Lua function to copy and modify an already existing definition. For the sake of this tutorial, we'll do it both ways.

In the data.lua file, copy and paste the following:

<pre style="background-color:#AAFFAA!important; color:black">
--data.lua

local fireArmor = table.deepcopy(data.raw["armor"]["heavy-armor"]) -- copy the table that defines the heavy armor item into the fireArmor variable

fireArmor.name = "fire-armor"
fireArmor.icons = {
  {
    icon = fireArmor.icon,
    icon_size = fireArmor.icon_size,
    tint = {r=1,g=0,b=0,a=0.3}
  },
}

fireArmor.resistances = {
  {
    type = "physical",
    decrease = 6,
    percent = 10
  },
  {
    type = "explosion",
    decrease = 10,
    percent = 30
  },
  {
    type = "acid",
    decrease = 5,
    percent = 30
  },
  {
    type = "fire",
    decrease = 0,
    percent = 100
  }
}

-- create the recipe prototype from scratch
local recipe = {
  type = "recipe",
  name = "fire-armor",
  enabled = true,
  energy_required = 8, -- time to craft in seconds (at crafting speed 1)
  ingredients = {{"copper-plate", 200}, {"steel-plate", 50}},
  result = "fire-armor"
}

data:extend{fireArmor, recipe}

</pre>

What we've just done here is we've copied the definition of the heavy armor item, then changed its properties, and injected it into the Factorio init with data:extend. The first line of code is probably the most interesting. <code>table.deepcopy</code> copies a table fully into another table. We do this from data.raw. The <code>data</code> part is a table, which will be used by the game to setup the Factorio universe. In fact, it contains the function <code>extend(self, prototypes)</code> and a table called <code>raw</code>. The former is a customary way to add new stuff to the latter. It is actually data.raw that holds the prototypes for the game. (You can view the implementation in the file [https://github.com/wube/factorio-data/blob/master/core/lualib/dataloader.lua /factorio/data/core/lualib/dataloader.lua]). It is important to note that data.raw only exists during the data loading stage of the game. During the control stage, when the game is running and being played, you cannot read this data; instead you read processed values through the API from the various types like LuaEntityPrototype.

In addition to defining the item prototype, we also define a recipe for it. This is necessary if you want to be able to craft the thing. We also set it to enabled so it doesn't need a technology to unlock.

=== More on data.raw ===

When Factorio initializes, all prototypes are put into a table called data.raw. This table holds all prototype types, and within those types, individual prototypes identified by name: <code>local prototype = data.raw["prototype-type"]["internal-name"]</code>. You saw earlier how we deepcopied from the definition of heavy armor, and modified some fields. In fact, let's go over each part of the deepcopy line:

<pre style="background-color:#DDA0DD!important; color:black">
local fireArmor = table.deepcopy(data.raw["armor"]["heavy-armor"])
</pre>

We assign a variable called fireArmor that holds our copy of the heavy armor definition. Notice how in data.raw, there is a type table that holds all armors, and the specific armor we're looking for is called heavy-armor. We can find [[heavy armor]]'s prototype type and internal name in the infobox of its page on this wiki and just copy it from there.<br>
Alternatively, we can find the items prototype type and internal name by opening the game, inserting the item into our inventory and then pressing {{Keybinding|shift|ctrl|F}} while hovering over the item. This will open the prototype explorer GUI, which has rows showing the name and type of the item.

As another example, the [[player|character]]'s prototype would be, according to the infobox on the page:

<pre style="background-color:#DDA0DD!important; color:black">
data.raw["character"]["character"]
</pre>

Because the character is ''the'' character, his type matches his name. You could define a new character with a mod. You can see all the available prototype fields of the character in the documentation: [https://lua-api.factorio.com/latest/prototypes/CharacterPrototype.html CharacterPrototype].

You may be thinking at this point, "Can I modify Factorio's existing prototypes without making new ones?" Well, the answer is yes! You would simply access the data.raw table during init, in data-final-fixes.lua if you want to run after all other mods, and change a property. For example, make the iron chest instead have 1000 health:

<pre style="background-color:#DDA0DD!important; color:black">
data.raw["container"]["iron-chest"].max_health = 1000
</pre>

The reason why this code is in data-final-fixes.lua is because that is the last file run, after all mod files have been run. This prevents (to a degree) your changes from being messed with by other mods. Of course, it is still possible to have incompatibilities. You should note any that you know of in your mod's description. Again, the [https://lua-api.factorio.com/latest/auxiliary/data-lifecycle.html dev's documentation] on this should be looked at.

This can also be applied to other mods, not just Factorio's base. You could mod a mod, as long as you add the mod (that you modified with your mod) to your dependencies so it gets loaded first.

=== The control scripting ===

And now, to finalize the mod, we have to make it be more than just simple armor. Let's think about what we want the armor to do. We want the armor to create fire on the ground as we walk with the armor on. The event we're going to use is called [https://lua-api.factorio.com/latest/events.html#on_player_changed_position on_player_changed_position], since we want the fire to be created when the player moves.

In our mod folder, create a file called <code>control.lua</code>. The game will automatically execute this file, so requiring it is not necessary.

Inside control.lua, copy and paste the following:

<pre style="background-color:#AAFFAA!important; color:black;">
--control.lua

script.on_event(defines.events.on_player_changed_position,
  function(event)
    local player = game.get_player(event.player_index) -- get the player that moved            
    -- if they're wearing our armor
    if player.character and player.get_inventory(defines.inventory.character_armor).get_item_count("fire-armor") >= 1 then
       -- create the fire where they're standing
       player.surface.create_entity{name="fire-flame", position=player.position, force="neutral"} 
    end
  end
)

</pre>

I've used lua comments in the code above to explain each step. It's fairly easy to understand, and it shows how you would get the current armor that the player character is wearing, with defines.inventory.character_armor, which is an inventory constant. You can read the list of defines [https://lua-api.factorio.com/latest/defines.html#defines.inventory here].

=== Locale ===

If you've already tried loading up Factorio and trying the mod so far (which you can at this point without it crashing), you may have noticed that the item name of the armor says "Unknown key". This means that Factorio has the internal name, but it doesn't know what it should look like to the user. So, we need to create a locale for our mod.

In the mod folder, create a folder called <code>locale</code>, then create another folder inside that called <code>en</code>, then a file called <code>any_name_can_be_here.cfg</code>.

If you know another language, you can also translate your mod by making other language code files inside locale, such as de for German.

Inside the .cfg file, paste the following:

<pre style="background-color:#AAFFAA!important; color:black">

[item-name]
fire-armor=Fire armor

[item-description]
fire-armor=An armor that seems to catch the ground itself on fire when you take a step. It's warm to the touch.
</pre>

Notice how this is not a lua file. Locale is handled with C config files, so the format is different.

== The finished tutorial mod ==

Well, the mod is finished. Since this mod is only a tutorial, there isn't much balance to it.

If you want to share a mod with other users, it needs to be a zip file. For that, simply zip the <code>fire-armor</code> folder and then rename the archive to <code>fire-armor_0.1.0</code> so that it follows the expected mod zip name pattern of <code>mod-name_version</code>. Keep in mind to not submit this tutorial mod to the mod portal as your own, since it's from the Wiki.

However, you're free to take this mod and modify it for your own use, changing recipes, adding technologies, whatever.

== Extended learning ==
One of the best ways to learn how to mod beyond this is to look at other mods. The [[Tutorial:Inspecting a live mod]] is a good starting point for touring a particularly well-commented mod. As all mods can be opened and inspected, looking at the mods of experienced modders can help significantly when making your own mod.

Something you'll see a lot in other mods or the base game are <code>require</code> statements. These load other files, so you can split up long code files and organize your mod however you like.

For example, if you wanted to put some of your data stage code into a file called "foo.lua" in a folder called "bar", your mod folder would look like this:

* fire-armor
** bar
*** foo.lua
** data.lua
** control.lua
** info.json

And you would need to add this to data.lua:
<pre style="background-color:#DDA0DD!important; color:black">
require("bar.foo")
</pre>

=== Keeping your mod working ===
As Factorio evolves, things will change. Previously, you probably ignored the modding part of the changelog, you now need to read it and see if any changes affect your mod(s). If so, you'll need to fix them. If there's something wrong with your mod, the game will fail to init and explain why.

== Resolving common errors in modding ==

As you continue to write mods from scratch instead of from a tutorial, you may encounter the infamous error. There are several types of errors that you can encounter in modding Factorio, and knowing how to deal with these errors will allow you to continue working.

=== Syntax errors ===

The Lua programming language expects things to be laid out a certain way. If you miss a bracket, = sign, or dot, you will encounter a syntax error. As an example, see the error below:

<pre style="background-color:#DDA0DD!important; color:black">
Failed to load mod "fire-armor": __fire-armor__/data.lua:39: unfinished string near '"fire-armor,'
</pre>

You'll see an error like the one above whenever you make a syntax error within the prototype definitions. The game will offer to restart, disable the troubling mod, disable all mods, or exit. Let's dissect the error, shall we?

Right away, we see the reason why Factorio didn't start normally. "Failed to load mod "fire-armor":". So, we know that it's our mod that messed up. Whenever the Lua engine of Factorio has a syntax error, it will print a mini stack-trace that follows through all requires, listing the call order. First, we see that the problem was caused by line 39 of data.lua. After stating where it is line-wise, it will attempt to give you an estimate of where in the line the problem is. Don't trust this estimate, only roughly trust the line number, plus or minus a few lines.

Going to line 39 of data.lua, we find:

<pre style="background-color:#DDA0DD!important; color:black">
  name = "fire-armor,
</pre>

Hmm, that doesn't look right. Can you see what's missing? We left off an " after armor before the comma. Thus, syntax error. Fixing these can be difficult for new programmers, who don't know what to look for.

=== Illogical actions, indexing nil ===

In lua, "nothing" is defined as the keyword nil. This is similar to null in other programming languages. Whenever the programmer tries to access something in a table that is nil, they will get an error like the following:

<pre style="background-color:#DDA0DD!important; color:black">
Error while running event fire-armor::on_player_changed_position (ID 82)
__fire-armor__/control.lua:3: attempt to index field '?' (a nil value)
</pre>

The "attempt to index field ..." error is often caused by the modder making an assumption that didn't work out. These types of errors will always be identifiable by their signature line, "attempt to index field". If we look at line 3 of control.lua (where the error is), we see:

<pre style="background-color:#DDA0DD!important; color:black">
game.print(game.players[23])
</pre>

What assumption has the modder made here? Well, there's actually two problems with this line. The first thing is that the modder has assumed that <code>game.players[23]</code> is a valid player, which isn't the case; this is why we get the "index field '?'" bit. The game doesn't know what the field is that we tried to index, because it hasn't been created yet. These errors are difficult to debug unless you know the ins and outs of the modding API well.

The second issue is a lot more subtle, and won't work. The modder is attempting to print a userdata table. [https://lua-api.factorio.com/latest/LuaPlayer.html A player] is a table of several values. Trying to print it simply print "LuaPlayer" instead of providing useful data.

=== Error while running event ===

Another common type of error in Factorio is the "Error while running event" error. This type of error only happens in control.lua scripting, and it happens when something goes wrong in an event function, such as a syntax error. '''Note that syntax errors in control.lua do not stop the game from starting, but may trigger after a save is loaded'''. There are a great deal of errors under this broad category, here's an example:

<pre style="background-color:#DDA0DD!important; color:black">
Error while running event fire-armor::on_player_changed_position (ID 82)
Unknown entity name: fire-flam
stack traceback:
__fire-armor__/control.lua:7: in function <__fire-armor__/control.lua:2>
</pre>

As you saw with the prototypes syntax error, Factorio gives a small traceback and the error name itself. In this case, we've attempted to spawn an entity called "fire-flam" on line 7 of control.lua, inside of an on_player_changed_position event hook. Fire-flam isn't a real entity type, so we crashed.

These types of errors can range from being a simple fix (like the one above, add the missing e), or can be very difficult.

=== Internal errors ===

The most rare form of error and the worst form is the internal error. This is an error with the C++ code of the game, and there's nothing you can do but report it to the devs. Mods occasionally cause these, and almost all of them are considered bugs, as mods ''should not'' be able to cause these, if that makes sense. They often get thrown into the logs.

An example:

<pre style="background-color:#DDA0DD!important; color:black">
696.148 Error FlowStatistics.cpp:236: FlowStatistics attempted to save value larger than uint16 as uint16. Exiting to prevent save corruption.
Logger::writeStacktrace skipped.
696.148 Error CrashHandler.cpp:190: Map tick at moment of crash: 432029
696.148 Error Util.cpp:97: Unexpected error occurred. If you're running the latest version of the game you can help us solve the problem by posting the contents of the log file on the Factorio forums.
Please also include the save file(s), any mods you may be using, and any steps you know of to reproduce the crash.
</pre>

== Multiplayer and desyncs ==

The reader may be wondering at this point how Factorio handles multiplayer with mods. It's fairly simple, but is still worth considering.

Factorio is [https://en.wikipedia.org/wiki/Deterministic_algorithm deterministic], which means that when you provide a constant input, you get a constant output, with no variance. Every client and the server all reach the same points at the same time in simulation, so they all agree on what happened. When this differs, the players experience a ''desync''.

; Desync : Misalignment with server and clients. Client 1 expected A, but got B. All other clients got A. Thus, Client 1 will desync. Desync can also happen when all clients have information (for example a variable) but a client that recently joined the game doesn't. That client will be desynced.
: ''See also: [[Desynchronization]]''

Desyncs happen a lot to new devs of Factorio mods, because they are unaware that a particular piece of code they used causes desyncs. As a general rule, there are a few things that should never be done.

=== Use local variables that are not final outside of event hooks ===

<pre style="background-color:#DDA0DD!important; color:black">
local globalLocal = 1
script.on_event(defines.events.on_player_built_item, function()
    globalLocal = math.random()
end)
</pre>

If the modder places a local variable outside of an event hook that gets changed during runtime, desyncs will happen when that variable is utilised to modify the game state (i.e. manipulate an entity, print text to players). If making a "global" variable is necessary, place the variable in the [https://lua-api.factorio.com/latest/auxiliary/global.html global] table instead. The game syncs this table between all clients, so they can all be aware of and reach the same conclusion as each other.

=== Conditional event subscribing ===

Mods in factorio may subscribe to events in order to be notified when they happen. This allows mods to react to events when they occur. Typically, event subscription is done at the top level of a lua file. 

Doing event subscription inside of a conditional, function, or other event is dangerous, as doing it incorrectly will lead to desyncs. Basically, since both the server and client need to reach the same conclusion after running code, conditional subscription can lead to certain clients or the server being subscribed to an event when the others are not, causing desyncs. 

=== Improper use of on_load ===

Another way to cause desyncs is to make improper actions inside of an on_load call, which some players new to modding might try to do. According to the [https://lua-api.factorio.com/latest/LuaBootstrap.html#LuaBootstrap.on_load documentation], the on_load functionality is meant for 3 purposes '''only''':

* Re-register conditional event handlers
* Re-setup meta tables
* Create local references to tables stored in the global table

Doing anything else will cause desyncs. The game will catch most attempts, crashing instead and terminating the mod.

=== Comparison by reference ===

Be cautious of comparing tables by reference. In multiplayer syncing, tables deserialized from the server state will be new objects, not equal by reference to any table initialized by client code.

<pre style="background-color:#DDA0DD!important; color:black">
if a == b then
</pre>
<pre style="background-color:#DDA0DD!important; color:black">
if a ~= b then
</pre>
If <code>a</code> and <code>b</code> are tables in the above conditionals, there will for example be different results between server and client if <code>a</code> is created locally and <code>b</code> is downloaded from the server.

Note that LuaObjects provided by the game have their equality operator overwritten to prevent this behaviour, so code such as <code>LuaEntityA ~= LuaEntityB</code> will not desync.
However, this does not apply when LuaObjects are used as keys in tables:
<pre style="background-color:#DDA0DD!important; color:black">
if table[LuaObject] then
</pre>
This will desync in the same way as described for the plain tables <code>a</code> and <code>b</code> above. For entities it is recommended to use [https://lua-api.factorio.com/latest/LuaEntity.html#LuaEntity.unit_number LuaEntity.unit_number] as the table key instead of the whole entity.

== See also ==
* [[Modding]]
* [[Tutorial:Modding tutorial|Modding tutorial overview]]

[[Category:Modding]]