Tutorial:Modding tutorial

This is a modding tutorial for Factorio version 0.17. 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:

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

Code tinted purple like this should not be included into the mod, it's just for educational/example purposes, and to boost understanding.

This tutorial was updated to version 0.17, 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, position in the player table, etc. All players have characters, but no characters have players within them.

Prototype

A prototype describes a kind of entity, a bit like a template. It defines stats, what the entity actually is, etc. A prototype is used to create an entity, and many functionally identical entities 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 game.surfaces[1], 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 on_entity_died, etc. More on this in the control scripting section.

Recipe

This is a data structure a bit like a prototype that defines a recipe for the internal crafting engine. A technology follows a similar setup.

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 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 at their API site. Get used to using this site, as it will become a frequent visit you will make while you develop mods. It contains information on Factorio's classes, information on concepts, and information on events that you can hook into. You will need to check this site often, so the author recommends bookmarking it. In addition to this site, there is 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. This author prefers Emacs, but it does not make a difference in the mod itself.

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 settings.lua. After settings.lua has been executed for all mods, each mod's settings-updates.lua is executed, and finally each mod's settings-final-fixes.lua is called. These 3 different phases of the settings stage allow to change settings of other mods without needing to rely on dependencies. 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 and entities. 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 data:extend({}), it expects a specific format, more on this later.

When running through this stage, the game looks through all mods for a file called data.lua. After data.lua has been executed for all mods, each mod's data-updates.lua is executed, and finally each mod's data-final-fixes.lua is called. These 3 different phases of the data stage allow to change data of other mods without needing to rely on dependencies. 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.

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.

Migrations

Migrations are scripts that are used to "fix" a save after a mod updates. Whenever prototypes change within a mod, migrations must be setup to correct 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 making changes to prototypes that effect prototype name, type, recipe, or technology. These things cannot be dynamically changed, and resetting techs or recipes may be necessary. Try to avoid these changes after shipping the mod out to the public. Try to come up with a finalized version of the prototype 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.

Control

Within most mods is a file called control.lua. 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. Because this 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 data life cycle page on the API site provides the best overview.

Runtime

At this stage, the mod is setup, and the save is running. Access to all tables provided by the game can be done inside of event handlers. (More on those below.)

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 data.lua, data-updates.lua, data-final-fixes.lua, or another file required by one of these three.

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

Mods with configurable user settings will use settings.lua to describe those settings.

Mods that define any game element with a readable name may also provide a locale 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.

Over time, the community has settled on some conventions for how a mod's directory structure should look. Following these to a T is not necessary, but can simplify things and make discussing mod bugs and improvements with other developers easier. More on directory structure below.

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, etc
• 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

Like this tutorial mentioned earlier, there is a somewhat community standard around for how a mod is laid out. This, combined with how the game expects mods to be laid out, limits us slightly. To start out, create a folder in your user data directory/mods folder. This folder must have a specific name, FireArmor_0.1.0. When you're finished, the mod directory should look like this:

• (user data directory, sometimes called .factorio)
  • mods
    • FireArmor_0.1.0

Then, inside FireArmor_0.1.0, create two files, info.json and data.lua. The directory should now look like:

• (user data directory, sometimes called .factorio)
  • mods
    • FireArmor_0.1.0
      • data.lua
      • info.json

The info.json file

Then, inside info.json, copy and paste the following into it:

{
    "name": "FireArmor",
    "version": "0.1.0",
    "title": "Fire Armor",
    "author": "You",
    "contact": "",
    "homepage": "",
    "factorio_version": "0.17",
    "dependencies": ["base >= 0.17"],
    "description": "This mod adds in fire armor that leaves behind damaging fire as you walk around."
}

To explain each field:

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 number. Some mods start at 0.0.1 or 0.1.0, while others follow Factorio versions and start at 0.17.0 (for Factorio version 0.17.x)
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.
contact	Put contact info here, so someone can find you in the event of a problem.
homepage	The homepage of your mod, put a website here if you have one for the mod. Not required.
factorio_version	This tells the game what version the mod is for, this must match the version you're developing the mod for, 0.17 in this case.
dependencies	Any dependencies of your mod. Some form of "base" should always be here, so base gets loaded first.
description	A short description of your mod.

And that's all for info.json! Next, in the data.lua file:

--data.lua

require("prototypes.item")

It's a pretty simple file, all we're doing here is just telling the game to execute the file called item.lua in prototypes, which we're about to create. Create a folder in FireArmor_0.1.0 called prototypes, then inside prototypes, create a file called item.lua. Your mod directory should now match this github snapshot.

Notice how our earlier require used the folder and file name in it?

Prototype creation

Now, there are two ways to create prototypes in Factorio. There's the short way, and the long way. The long way requires copying an existing definition from one of the default lua files provided with an install of Factorio, and the short way just uses a lua function to copy and modify a definition. For the sake of this tutorial, we'll do it the short way.

In item.lua, copy and paste the following:

--item.lua

local fireArmor = table.deepcopy(data.raw.armor["heavy-armor"])

fireArmor.name = "fire-armor"
fireArmor.icons= {
   {
      icon=fireArmor.icon,
      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
   },
}

local recipe = table.deepcopy(data.raw.recipe["heavy-armor"])
recipe.enabled = true
recipe.name = "fire-armor"
recipe.ingredients = {{"copper-plate",200},{"steel-plate",50}}
recipe.result = "fire-armor"

data:extend{fireArmor,recipe}

What we've just done here is we've copied the definition of heavy armor, then changed it's properties, and injected it into the Factorio init with data:extend. The first line of code is probably the most interesting. table.deepcopy copies a table fully into another table. We do this from data.raw. The data part is a table, which will be used by game to setup the Factorio universe. In fact, it contains the function extend(self,prototypes) and a table called raw. The former is 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 /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.

At this point, the mod looks like this.

More on data.raw

When Factorio initializes, all prototypes are put into a table called data.raw. This table holds all types, and within those types, individual entities. 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:

local fireArmor = table.deepcopy(data.raw.armor["heavy-armor"])

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. For example, the player's prototype would be:

data.raw.player["player"]

Because the player is the player, his type matches his name. You could define a new type of player with a mod. You can see all the prototype fields for an entity in it's long declaration in the Factorio install, at (Install)/data/base/prototypes.

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, and change a property. For example, make the iron chest instead have 1000 health:

data.raw.container['iron-chest'].max_health = 1000

The reason why this code must be in data-final-fixes.lua or data-updates.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 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 periodically create fire on the ground as we walk with the armor on. The event we're going to use is called on_tick, since we want the fire to be periodically created.

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

Inside control.lua, copy and paste the following:

--control.lua

script.on_event({defines.events.on_tick},
   function (e)
      if e.tick % 30 == 0 then --common trick to reduce how often this runs, we don't want it running every tick, just every 30 ticks, so twice per second
         for index,player in pairs(game.connected_players) do  --loop through all online players on the server
            
            --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
      end
   end
)

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 is wearing, with defines.inventory.character_armor, which is an inventory constant. You can read the list of defines here.

At this point, the mod will look like this.

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 locale for our mod.

In the mod folder, create a folder called locale, then create another folder inside that called en, then a file called config.cfg.

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 config.cfg, paste the following:

[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.

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

Finally, the mod will look like this.

The finished tutorial mod

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

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:

Failed to load mods: __FireArmor__/data.lua:1:__FireArmor__/prototypes/item.lua:36: syntax error near 'true'

As of version 0.15, 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 mods:". So, we know that it's a mod that messed up, and by extension, we know it's our mod. 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 indirectly caused by line 1 of data.lua. There's no problem there, so it must be the next entry, line 36 of prototypes/item.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 36 of item.lua, we find:

recipe.enabled true

Hmm, that doesn't look right. Can you see what's missing? We left off an = between enabled and true. 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:

Error while running event FireArmor::on_tick (ID 0)
__FireArmor__/control.lua:3: attempt to index field '?' (a nil value)

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:

game.print(game.players[23])

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 game.players[23] 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. A player is a table of several values. Trying to print it will error, instead a function to print it is needed.

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:

Error while running event FireArmor::on_tick (ID 0)
Unknown entity name: fire-flam
stack traceback:
__FireArmor__/control.lua:6: in function <__FireArmor__/control.lua:2>

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 6 of control.lua, inside of an on_tick 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:

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:106: Map tick at moment of crash: 432029
696.148 Error Util.cpp:76: 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.

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 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

local globalLocal = 1
script.on_event(defines.events.on_player_built_item, function()
    globalLocal = math.random()
end)

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 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.

Selective requiring

if setting1 then
   require("settingOne.lua")
end

Selective requiring, aka requiring different lua files based on settings or other criteria will also cause desyncs, and in some cases can cause connection rejections as the checksum of the mods will not match, as they load different data. All clients' mods must require the same series of files.

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 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.

Extended learning

One of the best ways to learn how to mod beyond this is to look at other mods. As all mods can be opened and looked at, looking at the mods of experienced modders can help significantly when making your own mod.

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.

See also

• Modding
• Modding FAQ