code: public void insertOperation(final MemcachedNode node, final Operation o) {
      o.setHandlingNode(node);
      o.initialize();
      node.insertOp(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: Insert an operation on the given node to the beginning of the queue.
  
  @param node the node where to insert the {@link Operation}.
  @param o the operation to insert.