Executing on Server or Client

How you can run certain code on only the server, or only a client, or only on a host.

Overview

Very often your code will need to know if it's running on a server or a client, or . There are several ways of knowing this, for example, code executed directly from client or server only events or methods, checking compiler directives to see the type of game build, marking methods to only allow running on a specific instance, or simply checking a Boolean value.

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Checking a Boolean

Description

You can easily check if the current code is running on the server with a Boolean check. FishNet's NetworkBehaviour and NetworkObject classes have easy to use properties for this exact reason. They are also accessible from the NetworkManager directly.

You can use IsClientStarted to detect if the instance running the code is a client (it could also be a server).

IsClientOnlyStarted can be used to detect if the instance running the code is a client only and not a server.

IsServerStarted detects if the instance running the code is the server (It could also be a client).

And finally, IsServerOnlyStarted returns whether the instance running the code is a server only and not a client.

When checking from a NetworkBehaviour or on a NetworkObject you will often want to know if the code is running on the client or server side AND the NetworkObject has already been initialized on the network and can be safely used for network functionality, such as RPCs or network properties. You can do that using the IsServerInitialized, IsServerOnlyInitialized, IsClientInitialized, and IsClientOnlyInitialized properties.

Instead of using something like IsClientInitialized && IsServerInitialized each time you want to check if the code is running on the host, you can simply check IsHostInitialized. There are Started, and Only variations of this as well.

Example

Simple example showing how a NetworkBehaviour could prevent code from running on the server:

Player.cs

using FishNet.Object;

public class Player : NetworkBehaviour
{
    public void OnStruck()
    {
        if (!IsClientStarted)
            return;

        // Play visual effect and sounds only on a client.
    }
}

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Method Attributes

There are a variety of attributes which can be used to ensure a method only runs on the appropriate game instance.

These methods are also used for Fish-Networking Pro's automatic code stripping. Importantly, methods marked with the Server attribute will have their method body stripped from non server builds.

Client Method Attribute

Description

Placing a Client attribute above a method ensures that the method cannot be called unless the local client is active and the NetworkObject is initialized (the latter of which can be disabled by enabling the UseIsStarted option). There are additional properties which may be added to the attribute for additional functionality, such as for disabling the warning that would be logged if you try to call the method with this and it doesn't pass the check.

Example

Simple example showing the Client attribute to prevent the method from running when not a client.

[Client]
void ShowUI()
{
    // This code will only run on a client, otherwise it will print a warning.
}

And following is an example of the options you can use with it: If this method were called with no client active a warning would be printed and the method would not run. You can change the logging type using the Logging property to change whether it should log a warning at all or not. The default value is LoggingType.Warning.

It's also possible to allow only the owner of the object to call the method by setting RequireOwnership to true; this value is false by default.

Finally, you can use the UseIsStarted property to cause this to ignore the initialized state of the NetworkObject and check if the client alone is started.

// The server does not need to play VFX; only play VFX if the client is active.
[Client(Logging = LoggingType.Off, RequireOwnership = true, UseIsStarted = true)]
void PlayVFX() 
{ 
    // Play VFX here...
}

Server Method Attribute

Description

The Server attribute provides similar features of the Client variant, except it will not allow the method to run if the server is not active and the NetworkObject is initialized (the latter of which can be disabled by enabling the UseIsStarted option). There are additional properties which may be added to the attribute for additional functionality, such as for disabling the warning that would be logged if you try to call the method with this and it doesn't pass the check.

Example

Here is an example of the Server attribute being used to prevent code from running anywhere except the server.

Like the Client attribute, a warning will be thrown if this is called while the server is not active. The warning can be changed or disabled by adjusting the Logging property.

You can use the UseIsStarted property to cause this to ignore the initialized state of the NetworkObject and only check if the server is started.

// The server would validate hit results from a client.
[Server(Logging = LoggingType.Off, UseIsStarted = false)]
private void ValidateHit() 
{
    // Hit validation code here that will only run on the server.
}