这个 servlet 作为所有 HTTP Web 服务请求的主入口点，它把请求和响应对象实例分派给请求处理器进行进一步处理。
This servlet serves as the primary entry point of all the HTTP Web service requests and delegates the request and response object instances to the request processor for further processing.
白盒子是可互相交换的，主应用程序可能把集群中的任何机器指派为控制者，而这个机器就把map和reduce任务分派给其它连接的白盒子。
White boxes are interchangeable, and the main application may assign any machine in the cluster as the controller, and this one in turn assigns map or reduce tasks to other connected white boxes.
然而，在模式循环期间，系统会收到、分派消息，但是并不给应用程序机会去过滤主消息循环中的消息。
However, during a modal loop, the system retrieves and dispatches messages without allowing an application the chance to filter the messages in its main message loop.
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