Function serve_node 

pub async fn serve_node(
    engine: Engine,
    config: ServerConfig,
    cluster: Option<Arc<Node>>,
    shutdown: impl Future<Output = ()> + Send + 'static,
) -> Result<()>

serve as a member of a cluster (orbit server --cluster): engine is the node’s standby engine (crate::cluster::Node::standby_engine); the node’s election thread swaps in a leader engine once elected.