Remote Execution – Arma 3
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This page explains Remote Execution, one of the cornerstones of Multiplayer Scripting in Arma 3. Remote execution is primarily needed to properly use commands that have local effect (LELocal) or only take local arguments (LALocal) in multiplayer.
Remote Execution Framework
The Remote Execution Framework consists of two commands, remoteExec and remoteExecCall, and an optional config entry, CfgRemoteExec (used for security, see Security).
Use Case Example
To understand why remote execution is needed and what it is used for, consider the following use case example: Let's say we want to use hint to display an announcement to all the players in our MP mission at the same time.
We already know how to use the hint command:
hint "You have 5 minutes left!"; // Normal use of hint.
The problem is that hint only has local effect. This means that only the machine where hint is executed will display our "You have 5 minutes left!" message. But we do not want to display our message to just a single player, we want to display our message to every player.
So in order to solve this problem, we need to make use of remote execution:
"You have 5 minutes left!" remoteExec ["hint", 0]; // Remote execution of hint.
Now all machines that are taking part in our MP session will execute hint "You have 5 minutes left!";, which means that our message will be displayed to every player at the same time - and we have achieved our goal.
History
Prior to 
1.50 there was no engine based remote execution. The only officially supported way to execute code on a non-local machine was provided in the form of BIS_fnc_MP. This scripted framework suffered from several issues, mainly poor network traffic optimization and insufficient security control. The network traffic was optimized in 1.46, but the security issues of the scripted framework could not be fixed properly.
To fully address these issues, remote execution was implemented directly into the game engine, with 1.50 introducing two new script commands - remoteExec and remoteExecCall - as well as a new CfgRemoteExec, finally allowing for proper and secure remote execution from all machines.
As of 1.54, BIS_fnc_MP has been rewritten to internally use remoteExec and remoteExecCall, retaining full backwards compatibility and enabling it to work with the updated CfgRemoteExec. Despite this, BIS_fnc_MP is now deprecated and should no longer be used!
remoteExec and remoteExecCall
remoteExec
Content of MP message sent by remoteExec command is executed on target machine in scheduled environment and as such abides to the same rules and limitations as other scheduled code.
If you want to execute more complex code that needs to be processed as separate spawn use remoteExec command.
More specifically:
- If your code includes any delays (commands like sleep or waitUntil) you MUST use remoteExec.
- If your code contains more CPU demanding operations that will take some time for the game to process, you SHOULD use remoteExec, otherwise you might experience performance drops.
remoteExecCall
Unlike remoteExec, MP messages send by remoteExecCall command are executed sequentially outside of the scheduled environment. This makes the remoteExecCall command very useful, if you need to execute multiple codes/commands in a set sequence.
The code sent by remoteExecCall MUST NOT contain any delays and SHOULD NOT be too complex and CPU demanding.
Security
The biggest issue of previous scripted remote execution was the lack of control over what can and cannot be executed from local clients. To address this we’ve created system where content authors can define through CfgRemoteExec config how the remote execution should operate on clients.
Because remoteExec and remoteExecCall can be used to remotely execute script commands (like for e.g. setDamage) as well as scripted functions (e.g. BIS_fnc_setRank) we have separated the security settings into two groups: Functions and Commands. This allows people to quickly set different rules for functions and commands separately.
The security rules consist of 3 security settings - operation mode, allowed targets and jip. The operation mode is very important as it defines if functions/commands can be executed remotely from client or not and ev. allows you to whitelist specific functions/commands. The allowed targets and jip are very optional and you really do not need to use them, unless you want to be super safe.
To get more info about those settings and the CfgRemoteExec config, check the subsections below.
Operation modes
Operation mode is numeric value describing how functions or commands should be treated on server on client.
0: remote execution is blocked 1: only whitelisted functions/commands are allowed for remote execution 2: remote execution fully opened
Allowed targets
In addition to the operation mode, in client subclass, allowed targets can be defined for individual whitelisted commands and functions. This adds another layer of security and control to the system, as it allows you to define not only who can send the execution request, but also where it can be executed.
0: can target all machines (default) 1: can target only clients, execution on server is denied 2: can target only server, execution on clients is denied
JIP
To control who can add JIP message into JIP queue, we have added an optional parameter jip. This parameter can be defined in whitelisted function/command class, at the same place as allowed targets are defined or in the Functions and/or Commands classes. This parameter affects only clients (no effect if defined within Server subclass).
If it is defined on both levels, the more local definition takes precedence (which is the value defined in the whitelisted function/command).
0: JIP flag cannot be set 1: JIP flag can be set (default)
Config location
The CfgRemoteExec class can be defined in mission description.ext, campaign description.ext or global (addon) config. As usual the more local config takes precedence. In case of more global configs exist, the mode attribute will be overridden by the last parsed config and whitelisted commands and functions will be merged.
Sample CfgRemoteExec definition:
class CfgRemoteExec
{
class Commands
{
mode = 1;
class setFuel { allowedTargets = 2; }; // execute only on server
class hint { jip = 0; }; // jip is not allowed for this command
};
class Functions
{
mode = 0;
jip = 0; // no functions can use jip
class BIS_fnc_setRank { allowedTargets = 1; }; // execute only on clients, server execution denied
};
};
Advanced Techniques & Functionality Insight
If you want to know more about how the remote execution works and handles some more complicated issues and edge cases continue reading, otherwise feel free to skip.
JIP queue
When command or function is executed through remoteExec or remoteExecCall, it can be flagged as persistent through the isPersistent attribute. If it is flagged, the request is being stored on server in the JIP queue under the unique JIP id. When new player joins the MP session that is currently running (we call this Join-In-Progress, JIP) all the MP messages that were stored in the JIP queue are executed on him.
JIP id
The JIP id is a unique key under which the JIP MP message is being stored in the JIP queue on the server. When you set isPersistent flag to true, the JIP id is auto-generated on the machine the remote execution was initiated. If you set the isPersistent flag to specific string, that string is considered to be the JIP id.
The auto-generated JIP is always unique. If the JIP is manually supplied, the content author needs to make sure it is unique, otherwise he/she will overwrite another MP msg that is currently stored in the queue under the JIP id.
Overwriting JIP message in the queue
Every JIP message is stored in the queue. If you need to overwrite the JIP message with another message, use the same JIP id for the new message. This way the new message will be stored in the queue under the provided id and will overwrite the previously stored message that used the same id.
Deleting JIP message from the queue
To remove a specific JIP message from the queue call the remoteExec with function/command name set to an empty string, the JIP id of the message you want to remove and no other params.
remoteExec ["", "JIPid"];
Validity verification
Validity of remote execution request is verified in 2 steps:
- When the request is initiated (issued from a client machine)
- When the server is going to broadcast the request.
If the request is initialized directly from the server, step 1 is skipped (this applies for hosted server admins as well).
For a remote execution request to be valid following criteria need to be met:
- input parameters must be properly defined
- function/command must exist on the machine
- function/command remote execution request must be allowed in CfgRemoteExec
- remote execution must be either fully open (mode = 2)
- or the particular function/command must be whitelisted
- if JIP is used (isPersistent flag is not false), JIP must be allowed in CfgRemoteExec (jip = 1)
If any of the aforementioned is not met remote execution is blocked.