Server

A server is an entity with the Server role marker, a connection component such as NetcodeServer, and an accepting transport component such as UdpEndpoint. ServerUdpIo is shorthand for UdpEndpoint plus Server.

Server requires Endpoint, which owns the per-peer link collection and optional receive conditioner. A P2P peer can own an Endpoint without being a Server.

Every accepted link has LinkOf { endpoint } pointing at its owning endpoint. In an application that also has P2P endpoints, filter for the Server role before applying server-specific behavior:

pub(crate) fn handle_new_client(
    trigger: On<Add, LinkOf>,
    links: Query<&LinkOf>,
    servers: Query<(), With<Server>>,
    mut commands: Commands,
) {
    let Ok(link_of) = links.get(trigger.entity) else {
        return;
    };
    if !servers.contains(link_of.endpoint) {
        return;
    }
    commands.entity(trigger.entity).insert((
        ReplicationSender,
        Name::from("Client"),
    ));
}

At that point the client is only linked, not connected: netcode authentication still has to succeed. Only when the Connected component is added is the client real, and that’s where game behaviour starts (spawn a player, etc.).

The server’s per-frame jobs mirror the client’s: read inputs and step simulation in FixedUpdate, replicate the world in PostUpdate (ReplicationSystems::Send) at the rate set by the ReplicationMetadata resource.