public LoadBalancerCommand<T> build() {
            if (loadBalancerContext == null && loadBalancer == null) {
                throw new IllegalArgumentException("Either LoadBalancer or LoadBalancerContext needs to be set");
            }
            
            if (listeners != null && listeners.size() > 0) {
                this.invoker = new ExecutionContextListenerInvoker(executionContext, listeners, config);
            }
            
            if (loadBalancerContext == null) {
                loadBalancerContext = new LoadBalancerContext(loadBalancer, config);
            }
            
            return new LoadBalancerCommand<T>(this);
        }
Pin the operation to a specific server.  Otherwise run on any server returned by the load balancer

@param server
ribbon-loadbalancer/src/main/java/com/netflix/loadbalancer/reactive/LoadBalancerCommand.java