proactor 与 actor 模式

Proactor和Reactor都是并发编程的设计模式。不同之处在于，Proactor是异步，Reactor是同步阻塞。

相同点：demultiplexor负责提交IO操作(异步)、查询设备是否可操作(同步)，然后当条件满足时，就回调handler。
不同点：异步情况下(Proactor)，当回调handler时，表示IO操作已经完成；

        同步情况下(Reactor)，回调handler时，表示IO设备可以进行某个操作(can read or can write)，handler这个时候开始提交操作。

摘抄一些关键的东西：

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Two patterns that involve event demultiplexors are called Reactor and Proactor [1]. The Reactor patterns 
involve synchronous I/O, whereas the Proactor pattern involves asynchronous I/O.
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关于两个模式的大致模型，从以下文字基本可以明白：

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An example will help you understand the difference between Reactor and Proactor. We will focus on the read 
operation here, as the write implementation is similar. Here's a read in Reactor:

* An event handler declares interest in I/O events that indicate readiness for read on a particular socket ;
* The event demultiplexor waits for events ;
* An event comes in and wakes-up the demultiplexor, and the demultiplexor calls the appropriate handler; 
* The event handler performs the actual read operation, handles the data read, declares renewed interest in 
  I/O events, and returns control to the dispatcher .

By comparison, here is a read operation in Proactor (true async):

* A handler initiates an asynchronous read operation (note: the OS must support asynchronous I/O). In this 
  case, the handler does not care about I/O readiness events, but is instead registers interest in receiving 
  completion events;
* The event demultiplexor waits until the operation is completed ;
* While the event demultiplexor waits, the OS executes the read operation in a parallel kernel thread, puts 
  data into a user-defined buffer, and notifies the event demultiplexor that the read is complete ;
* The event demultiplexor calls the appropriate handler; 
* The event handler handles the data from user defined buffer, starts a new asynchronous operation, and returns
  control to the event demultiplexor.

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