code: protected void addOperation(final MemcachedNode node, final Operation o) {
      if (!node.isAuthenticated()) {
        retryOperation(o);
        return;
      }
      o.setHandlingNode(node);
      o.initialize();
      node.addOp(o);
      addedQueue.offer(node);
      metrics.markMeter(OVERALL_REQUEST_METRIC);
  
      Selector s = selector.wakeup();
      assert s == selector : "Wakeup returned the wrong selector.";
      getLogger().debug("Added %s to %s", o, node);
    }
docs: Enqueue an operation on the given node.
  
  @param node the node where to enqueue the {@link Operation}.
  @param o the operation to add.