reactor和proactor模式的比较

在研究Java AIO的时候理解到reactor和proactor模式，顺便研究了一下。

这里面会提到blokcing/non-blocking,synchronous/asynchronous的对比，要仔细体会才行。

如果实现一个高性能的服务器端，比如web server，有以下几种方式：

详细的可以参考http://www.cs.wustl.edu/~schmidt/PDF/Reactor1-93.pdf

1、Non-blocking I/O solution

One method for handling I/O on multiple descriptors involves the use of "polling." polling operates by cycling through a set of open descriptors,checking each one for pending I/O activity.

The primary disadvantage with polling is that it consumes excessive CPU cycles by making unnecessary system calls while "busy-waiting". For instance,if input occurs only intermittently on the I/O descriptors,the server process will repeatedly and superfluously poll descriptors that do not have any pending logging records. On the other hand,if I/O is continuously received up all descriptors,this approach may be reasonable. In addition,an advantage with polling is that it is portable accross OS platforms.

2、Multi-Process Solution - master/slave模式

有一个master进程负责监听client请求，然后对于每个请求调用fork方法创建一个单独的进程去处理。

3、Multi-threaded solution

和2类似，只不过将process换成了更轻量级的thread

4、The Event Demultiplexing Solution

主要是用到了操作系统的事件多路分用（event demultiplexing）功能：通过调用select和poll。这一类里根据实现的不同又分为几类：

4.1 select-based

4.2 poll-based

以上两种存在着以下一些缺点：

-> Complicated and Error-Prone Interfaces

-> Low-Level Interfaces

-> Non-Portable Interfaces

-> Non-Extensible Interfaces

为了解决这些问题，我们在更高层面引入了模式，把这些底层的信息屏蔽掉了，使得更易于使用。

4.3 reactive event dispatching model

4.4 proactive event dispatching model

Java AIO的实现者Alan Bateman解释以上两种模式的区别是：

proactive：

-> Initiate non-blocking I/O operation

-> Notification when I/O completes

-> Proactor pattern

reactive:

-> notification when channel ready for I/O (S elector)

-> perform non-blocking I/O operation

-> Reactor pattern

说实话，还不是很能理解其中的奥秘。

（编辑：李大同）

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