Reactor pattern

1、wikipedia Reactor pattern 对 Reactor pattern的描述是非常正式的， 初次阅读是难以理解的

2、Linux IO multiplexing就是用于实现Reactor pattern的

3、使用Reactor pattern的例子:

a、zhihu 如何深刻理解reactor和proactor？ # A

b、zhihu I/O多路复用技术（multiplexing）是什么？ # A

典型的例子是就是Redis ae，它的实现比较简单，结合它的实现来理解会非常快。

wikipedia Reactor pattern

The reactor design pattern is an event handling pattern for handling service requests delivered concurrently to a service handler by one or more inputs. The service handler then demultiplexes the incoming requests and dispatches them synchronously to the associated request handlers.[1]

NOTE:

1、如果使用中文来进行理解的话，reactor就可以翻译为反应器，它是对发生的event进行react，即反映；由于这篇文章并没有列举示例，所以我在第一次阅读的时候并没有完全理解，今天在阅读《redis设计与实现》的第12章事件的时候，其中对此的解释是非常到位的，所以要想完全理解reactor pattern，需要结合这本书的内容一起来进行理解。

2、上文中的**synchronously**非常重要，它是reactor和Proactor pattern之间的差异所在；

3、: 参见Redis 设计与实现，其中chapter 12文件事件¶对reactor pattern进行了介绍；

Structure

Resources

Any resource that can provide input to or consume output from the system.

NOTE: resource可以是socket，file等；

Synchronous Event Demultiplexer

Uses an event loop to block on all resources. The demultiplexer sends the resource to the dispatcher when it is possible to start a synchronous operation on a resource without blocking (Example: a synchronous call to read() will block if there is no data to read. The demultiplexer uses select() on the resource, which blocks until the resource is available for reading. In this case, a synchronous call to read() won't block, and the demultiplexer can send the resource to the dispatcher.)

NOTE:

1、上面这一段中的Demultiplexer其实是比较容易让人混淆的，它的实现往往是使用I/O multiplex技术，如select等。

2、其实上面这段话是非常任意让人混淆的，结合Redis 设计与实现，其中chapter 12文件事件¶对reactor pattern的介绍，已经redis的实现是非常任意的；

Dispatcher

Handles registering and unregistering of request handlers. Dispatches resources from the demultiplexer to the associated request handler.

Request Handler

An application defined request handler and its associated resource.

Properties

All reactor systems are single-threaded by definition, but can exist in a multithreaded environment.

NOTE: redis默认情况下就是single-thread

Benefits

The reactor pattern completely separates application-specific code from the reactor implementation, which means that application components can be divided into modular, reusable parts.

Limitations

The reactor pattern can be more difficult to debug[2] than a procedural pattern due to the inverted flow of control. Also, by only calling request handlers synchronously, the reactor pattern limits maximum concurrency, especially on symmetric multiprocessing hardware. The scalability（可扩展性） of the reactor pattern is limited not only by calling request handlers synchronously, but also by the demultiplexer.[3]

TODO

https://www.zhihu.com/question/26943938

https://www.codeproject.com/Articles/33011/Proactor-Pattern

http://didawiki.cli.di.unipi.it/lib/exe/fetch.php/magistraleinformatica/tdp/tpd_reactor_proactor.pdf

https://www.boost.org/doc/libs/1_47_0/doc/html/boost_asio/overview/core/async.html