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This page aims to explain notions and specificities of Multiplayer Scripting and its differences with Singleplayer Scripting.
It features three parts:

Notions and general knowledge
Structuring Multiplayer code
Testing locally

Notions and general knowledge

The basics

Client-Server relationship

In Multiplayer, players are connected to a server.
- The server is a machine that distributes information among clients, such as unit positions, vehicle speeds, etc.
- The server can be either a dedicated machine, or hosted by a player; in the latter it means the server can have a player unit.
A machine connected to a server is called a client.
- Clients can join a mission after it started: they are considered "JIP" and scripting should be adapted to them. See the Join In Progress chapter for more information.

ⓘ

In Singleplayer, the game acts as a player-hosted server (isServer returns true).

⚠

Some Multiplayer commands do not work in Singleplayer, such as netId !
The other way around, some Singleplayer commands do not work in Multiplayer (e.g setAccTime).

Locality

Definitions

LOCAL is an attribute for the machine where the command/script/function is executed or the AI calculated.
- to check if an argument is local, use the local command
REMOTE means non local - on another machine. All player-controlled soldiers are remote units to a dedicated server, for example.
- to check if an argument is remote, simply use not local myObject
commands arguments and effects can be either local or global:

Template:EffArg	a local argument means an argument that is processed on the machine where the command is executed
Template:EffArg	a global argument means any argument, even a remote one
Template:EffArg	a local effect means that the effect will only happen on the machine where the command is executed
Template:EffArg	a global effect means that all the computers will receive it

Different machines and how to target them

Valid for Arma 3
Machine	Conditions	Is server	Is player	Is JIP
Dedicated Server	`isDedicated`	Template:task/	Template:task	Template:task
Player Server	`hasInterface && isServer`	Template:task/	Template:task/	Template:task
Player Client	`hasInterface && not isServer`	Template:task	Template:task/	Template:task
JIP Player Client	`hasInterface && didJIP`	Template:task	Template:task/	Template:task/
Headless Client	`not hasInterface && not isServer`	Template:task	Template:task	Template:task
JIP Headless Client	`not hasInterface && not isServer && didJIP`	Template:task	Template:task	Template:task/

For an extended version for all games, click

ⓘ

Before

1.99 and isServer, server could be detected by placing a Game Logic in the editor that would remain local to the server, then using the following:local myLogic

Targeted machine	Operation Flashpoint	Arma 1	Arma 2	Arma 3
Dedicated Server A server without a human player behind it	isServer && isNull player	isServer && isNull player	isDedicated	isDedicated
Player Server A server hosted by a player	isServer && not isNull player	isServer && not isNull player	isServer && not isDedicated	hasInterface && isServer
Server Any server, dedicated or hosted	isServer	isServer	isServer	isServer
Player Client A player connected since the lobby	not isServer && not isNull player	not isServer && not isNull player	not isServer && not isNull player	hasInterface && not isServer
JIP Player Client A player connected in the middle of a mission	N/A	not isServer && isNull player	not isServer && isNull player	hasInterface && didJIP
Headless Client A client without a human player behind it (used to offload server calculations)	N/A			not hasInterface && not isServer
JIP Headless Client Headless client connected during the ongoing mission	N/A			not hasInterface && not isServer && didJIP

↑ Back to spoiler's top

If you want to know if the current machine is a server (Dedicated Server or Player Server), use isServer.
If you want to know if the current machine has a player (Player Server included), use hasInterface.

Template:Feature arma3

General information about locality

about player:
- a player's unit is always local to the (human) player's machine
- a dedicated server doesn't have a player unit (isNull player returns true)
- to know if a certain unit is a player, use isPlayer
groups and AI:
- an AI group with a player-leader will be local to the player's computer
- a subordinate player being in a player-lead group will remain local to its computer (see bulletpoint #1)
objects:
- a driven vehicle is always local to the machine of its driver (not the commander's, not the gunner's)
- terrain objects (buildings, vegetation, roads etc.) are local everywhere (local nearestBuilding player returns true on every client)
- editor-placed objects and empty vehicles are local to the server
- editor-placed triggers are created on every machine unless specified otherwise ("Server Only" Eden Editor option ticked)
- units created with createUnit will be local to the computer that issued the command
- objects and vehicles created with createVehicle will be local to the computer that issued the command

Locality changes

if the player-leader dies, the AI is transferred to the machine where the new leader is local (server in case of AI, client in case of another player)
joining AI units will change locality to the new leader's computer
locality will change from server to client when a player gets in an empty vehicle
locality can also change in the event of a Team Switch or usage of selectPlayer

Template:Feature arma3

Code examples

Code	Argu-ments	Effect	Description
`remoteUnit setDamage 1;`	Template:EffArg	Template:EffArg	Any unit (local or remote) will die, and all the computers will know
`localUnit addMagazine "30Rnd_556x45_STANAG";`	Template:EffArg	Template:EffArg	Only a local unit can have its inventory edited with this command, but all the machines will be notified: if a player looks into the localUnit inventory, the added magazine will be there
`remoteUnit setFace "Miller";`	Template:EffArg	Template:EffArg	Any unit (local or remote) can get its face changed, but the new face will not be propagated through the network. Only the local machine's player will see the effect of the command ⓘ This command should ideally be executed on every machine, for example with: [remoteUnit, "Miller"] remoteExec ["setFace", 0, true]; See Remote Execution paragraph for more information.
`localCamera cameraEffect ["Internal", "Back"];`	Template:EffArg	Template:EffArg	Only a local camera can be entered, and of course only the local machine will see through this camera

Network ID

Network IDs identify machines and network objects. They can be used to target proper machines with remote execution; see Remote Execution chapter for more information.

⚠

A machine ID (ownerID) is not to be confused with an object's network ID (netId).

Machine network ID

Every machine, including the server, has a network ID.
A server has an ID of 2, every new client has the next number (the first client will be number 3, second client number 4, etc.)

To get the current machine's ID, use clientOwner.
To get the machine's ID of an object's owner, use owner object (MP only).
To get the machine's ID of a group's owner, use groupOwner object (MP only).

Object network ID

Every object synchronised through the network has a network ID, shortened to netId.

To get an object's netId, use netId (MP only). SP & MP variant: BIS_fnc_netId
To get an object from a netId, use objectFromNetId (MP only). SP & MP variant: BIS_fnc_objectFromNetId
To get a group from a netId, use groupFromNetId (MP only). SP & MP variant: BIS_fnc_groupFromNetId

Sending information across network

Remote Execution

Since Arma 3 v1.50, remoteExec and remoteExecCall efficient engine network commands are available:

["Message to everyone, including JIP players!", 0, true] remoteExec ["hint"];

See Arma 3 Remote Execution for more information.

Arma 3 alpha introduced BIS_fnc_MP (obsolete since Arma 3 v1.50, use the above version):

["Message to everyone, including JIP  players!", "hint", true, true] call BIS_fnc_MP;  // obsolete since Arma 3 v1.50, use remoteExec or remoteExecCall

Arma 2 introduced the Multiplayer framework (not present in Arma 3):

// having the Functions module placed in the editor
waitUntil { not isNil "BIS_MPF_InitDone"; };
[nil, nil, rHINT, "Message to everyone!"] call RE;

PublicVariable commands

PublicVariable commands allow to send global variables to specified machines.

Network reception is guaranteed
Short variable names should be used for network performance
These commands shouldn't be used intensely for the same reason

Command	Effect	JIP synchronised
publicVariable	Set/update a variable value from the local machine to all the other machines (including server)	Template:task/
publicVariableServer	Set/update a variable value from a client to the server	Template:task
publicVariableClient	Set/update a variable value from the local machine (not necessarily the server) to a specific client	Template:task

How it works

The server has the variable ABC_Score to broadcast:

ABC_Score = 5;				// sets the value
publicVariable "ABC_Score"	// publishes the variable (do not forget the quotes!)

This sends the variable name and value to the clients.

If the variable is not yet declared on their machine, it will declare it as well.
If the variable already exists, the value is overridden by the server's value.
If the variable changes again on the server, it has to be manually publicVariable'd once again!
A JIP player will synchronise server's publicVariable'd variables before executing init.sqf. See Initialization Order for more information.
- A JIP player will not synchronise publicVariableClient-received variables after he disconnects/reconnects. Only server-known public variables sent with publicVariable will be synchronised.

setVariable command

Available since Arma 2, the public version of the setVariable command (alternative syntax) allows you to store a variable into an object and spread this variable across the network. Template:Feature arma3

Player connection events

The following commands will execute given code when a player is connecting or disconnecting. This code will run for players connecting in the lobby and for JIP players!

ⓘ

Before Arma 3 v1.57, use BIS_fnc_addStackedEventHandler for onPlayerConnected/onPlayerDisconnected.

Client state

A client state is the client's connection state. It can be obtained with getClientState and getClientStateNumber commands on both server and clients.

getClientStateNumber	getClientState	Description
0	"NONE"	No client (or singleplayer)
1	"CREATED"	Client is created
2	"CONNECTED"	Client is connected to server, message formats are registered
3	"LOGGED IN"	Identity is created
4	"MISSION SELECTED"	Mission is selected
5	"MISSION ASKED"	Server was asked to send / not send mission
6	"ROLE ASSIGNED"	Role was assigned (and confirmed)
7	"MISSION RECEIVED"	Mission received
8	"GAME LOADED"	Island loaded, vehicles received
9	"BRIEFING SHOWN"	Briefing was displayed
10	"BRIEFING READ"	Ready to play mission
11	"GAME FINISHED"	Game was finished
12	"DEBRIEFING READ"	Debriefing read, ready to continue with next mission

Join In Progress

A JIP player will have many information synchronised (publicVariable'd and setVariable server variables for example) by the game before accessing the mission itself. JIP was introduced in Operation Flashpoint: Elite on XBox and Arma on PC.

Information	Arma 1	Arma 2	Arma 3
date/time	Template:task/	Template:task/	Template:task/
date/time + setDate/skipTime	Template:task	Template:task	Template:task/
weather (overcast, fog)	Template:task	Template:task/	Template:task/
weather + setOvercast/setFog/setRain/setLightnings	Template:task	Template:task	Template:task/
time passed since mission start (time command)	unknown	unknown	Template:task/
publicVariable-sent variables (Number, String, Text, Array, Code)	Template:task/	Template:task/	Template:task/
publicVariable-sent variables (nil)	Template:task	Template:task	Template:task/
setVariable-assigned variables (when alternative syntax's public parameter is set to true)	N/A	Template:task/	Template:task/
remoteExec- and remoteExecCall-executed code if JIP prerequisites are met	N/A		Template:task/

Structuring Multiplayer code

Responsibilities

The server initialises and works with variables
A connecting or connected player-client gets the mission data from the server
Once an objective is reached, the server tells the players via remoteExec/remoteExecCall, publicVariable or setVariable
Players can get their client-side UI code triggered by the server

See earlier chapter about how to target specific machines.

Server

The Server must be considered as the one ruler of the mission. He is the game's referee and all the decisions should come from it. All the values on the server should be considered as true.
Consider the server as a powerful machine where you can put all the mission conditions and heavy scripting on. In case of a very heavy mission, the server can be assisted by Headless Clients (this involves a per-mission specific design).

Server-side code must gather or wait all the variables (potentially from client-side scripts and publicVariableServer) and take and broadcast its decisions accordingly using publicVariable.

Server-side code should ideally not make any reference to a player variable considering a server can potentially be dedicated, unless you want to force this mission to be player-hosted (such as Arma 2's campaign).

ⓘ

Use allPlayers, BIS_fnc_listPlayers, playableUnits (and switchableUnits for SP compatibility) to refer to players.

Client

The Client must be considered as an average machine that can leave the game at any moment. No mission-vital code must be executed on its side and no decisions must be taken client-side.
Again, one must expect that any client can disconnect at any moment.

The client executes all the (local) UI code (Post process effects, Dialogs, etc.)
The player's machine should not deal with a great piece of code.

⚠

Do not forget that a server can be a player, and therefore should execute player code in that case.

The proper way to check for player-side condition is

if (hasInterface) then { (…) };

Note that the following example is incorrect:

if (isServer) then
{
	(…)
}
 else // WRONG: a server CAN be a player
{
	(…)
};

Code writing

The usual four steps of coding are the following:

Think it well
- If you cannot figure out how to write your code, say what you want to do out loud or write it down in your language, then replace it by code little by little.
Make it work
- use the -showScriptErrors startup parameter to see your code errors.
Make it readable
- name your variables properly, as if you had to give it to someone that should get it without the need of explaining
Optimise then
- use more performance-friendly commands, reduce your search radius and loop frequencies once everything works.

See Code Optimisation - Rules for advices and more information.

Testing locally

To properly test MP scripting locality issues, it is recommended to run a dedicated server and connect with two clients. In order to do so:

In your arma3server.exe's server.cfg:

loopback = true;	// force LAN-only server
kickDuplicate = 0;	// disable duplicate Arma kick

// needed in case of Headless Client test only
headlessClients[]	= { "127.0.0.1" };
localClient[]		= { "127.0.0.1" };

Run Arma 3 twice from Steam and connect each (with -showScriptErrors flag)
- If you use one screen for the first client and the other screen for the second client, use -noPause -window launcher options. This will not pause render if the window doesn't have focus.
Try your mission and check for script errors:
- on client's screens and RPT files
- in your server's RPT files
To ensure all cases, your mission should ideally work properly:
- in Singleplayer
- in Player-hosted Multiplayer
- in a Dedicated Server-hosted game

ⓘ

Of course, nothing prevents you to invite friends to play your mission and report bugs!

Multiplayer Scripting

Contents

Notions and general knowledge

The basics

Locality

Definitions

Different machines and how to target them

General information about locality

Locality changes

Code examples

Network ID

Machine network ID

Object network ID

Sending information across network

Remote Execution

PublicVariable commands

How it works

setVariable command

Player connection events

Client state

Join In Progress

See also

Structuring Multiplayer code

Responsibilities

Server

Client

Code writing

See also

Testing locally

See Also

Navigation menu

Multiplayer Scripting

Notions and general knowledge

The basics

Locality

Definitions

Different machines and how to target them

General information about locality

Locality changes

Code examples

Network ID

Machine network ID

Object network ID

Sending information across network

Remote Execution

PublicVariable commands

How it works

setVariable command

Player connection events

Client state

Join In Progress

See also

Structuring Multiplayer code

Responsibilities

Server

Client

Code writing

See also

Testing locally

See Also

Navigation menu

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