Broadcasting Client Updates via Multicast RPCs

In this lesson, you will learn how to leverage Multicast RPCs to propagate non-authoritative visual and auditory feedback from the server to every connected client simultaneously.

The Role of Multicast RPCs

While RepNotify variables are ideal for state synchronization (like current health), they aren’t designed for transient events like explosion particles or impact sounds. When the server performs an action that needs to be "felt" by everyone—but doesn't change the game state—a Multicast RPC is your tool of choice.

A Multicast RPC originates on the server and executes on the server, as well as on every client currently connected to that actor. Because they are "fire and forget," they are perfect for non-critical cosmetic effects.

Implementation Pattern

To implement this, define your function in your class header with the NetMulticast keyword:

UFUNCTION(NetMulticast, Unreliable)
void Multicast_PlayImpactEffect(FVector Location);

void AMyActor::Multicast_PlayImpactEffect_Implementation(FVector Location)
{
    // Trigger particle systems or sounds locally on all machines
    UGameplayStatics::SpawnEmitterAtLocation(GetWorld(), ImpactTemplate, Location);
}

Critical Considerations

1. Unreliable vs. Reliable: For visual effects like sparks or sounds, always use Unreliable. If a packet is dropped, it is better to skip that one specific particle burst than to clog the network buffer trying to retransmit an outdated effect.
2. Authority Requirement: Only the Server can trigger a Multicast. If a client calls this function, the engine will ignore the request. If you need a client to initiate an effect, you must first call a Server RPC, which then performs the logic and triggers the Multicast RPC.
3. Relevance: Multicasts are only sent to clients who currently have the actor relevant (within their network dormancy and relevancy range). If a player joins after the explosion has finished, they will not see the effect, which is exactly the behavior you want for transient visual feedback.

Key Takeaway: Use Multicast RPCs for "fire-and-forget" cosmetic events that do not impact game state, ensuring you prioritize Unreliable replication to keep your network traffic optimized.