def process(operation, &callback)
      if executing?(:pipeline)
        queue.push([ operation, callback ])
      else
        flush([[ operation, callback ]])
      end
    end
def process(operation, &callback)
      if executing?(:pipeline)
        queue.push([ operation, callback ])
      else
        flush([[ operation, callback ]])
      end
    end
Processes the provided operation on this node, and will execute the
 callback when the operation is sent to the database.

 @example Process a read operation.
   node.process(query) do |reply|
     return reply.documents
   end

 @param [ Message ] operation The database operation.
 @param [ Proc ] callback The callback to run on operation completion.

 @return [ Object ] The result of the callback.

 @since 1.0.0