Async and sync
今天在阅读jQuery的ajax的代码的时候，看到了其中有非常多的**回调函数**等，显然这是**异步编程**的思想，于是我想，有必要总结Asynchronous programming的相关知识了。
Async
"async"即"异步"。asynchronous的思想是: 对于耗时操作，不等待它执行完成，让它在**main loop/flow 之外独立运行而不阻塞**main loop，这样main loop就可以在它执行的这段时间内去处理其他的事情，待耗时操作完成的时候(是一个**event**)，它再**通知**(notify)main loop它的完成，然后**main loop**再来处理它的结果。
当进行**asynchronous programming**的时候，往往需要表达"当 某个**event** 发生时，执行 某个 callback"，这个**callback**往往是用户注册的自定义函数，这种是可以使用fluent API来进行描述的，它其实非常类似于builder pattern，即由用户来设置**callback**，最最典型的就是jQuery，现代很多programming language都是支持这种paradigm的。
Wikipedia Asynchrony (computer programming)
https://en.wikipedia.org/wiki/Asynchrony_(computer_programming)asynchronous
Asynchrony, in computer programming, refers to the occurrence of events independently of the main program flow and ways to deal with such events. These may be "outside" events such as the arrival of signals, or actions instigated(发起的) by a program that take place concurrently with program execution, without the program blocking to wait for results.[1] Asynchronous input/output is an example of the latter cause(原因) of asynchrony, and lets programs issue commands to storage or network devices that service these requests while the processor continues executing the program. Doing so provides a degree of parallelism.[1]
NOTE: Asynchrony**的本质含义: 某个**event**的出现是独立于 **main program flow 的，这个event可能是inside的，也可能是outside的，下面是例子:
1) outside: signal是 asynchronous event，并且它是典型的 outside event，它的出现是独立于main program flow的。
2) inside: "Asynchronous input/output is an example of the latter cause(原因) of asynchrony"的意思是: asynchronous IO引发**asynchrony**的另外一个原因，因为IO完成的event是 独立于 main program flow的。
从control theory的角度来看，asynchronous event是不是由当前program控制的，它是由外部系统控制的，由外部系统进行通知。
A common way for dealing with asynchrony in a programming interface is to provide subroutines (methods, functions) that return to their caller an object, sometimes called a future or promise, that represents the ongoing events. Such an object will then typically come with a synchronizing operation that blocks until the operation is completed. Some programming languages, such as Cilk, have special syntax for expressing an asynchronous procedure call.[2]
NOTE: 上面这段话对future/promise的介绍是非常好的，从asynchrony的角度非常容易理解引入"future or promise"的目的。
Examples of asynchrony
1) Ajax
"Ajax", short for "asynchronous JavaScript and XML")[3][4][5] is a set of web development techniques utilizing many web technologies used on the client-side to create asynchronous I/O Web applications.
2) Asynchronous method dispatch
Asynchronous method dispatch (AMD), a data communication method used when there is a need for the server side to handle a large number of long lasting client requests.[6]
3) Asynchronous method invocation
https://en.wikipedia.org/wiki/Asynchronous_method_invocation
4) Interrupt
今天在阅读 Wikipedia的context switch 的时候，其中提及了interrupt，显然，interrupt是一种典型的asynchronous方式。
5) Unix signal
参见工程Linux-OS的Programming\IO\Blocking-and-non-blocking-VS-sync-and-async章节。
6) Asynchronous IO
参见工程Linux-OS的Programming\IO\IO-model章节。
Async to sync
本段的意思是: 将asynchronous API转换为synchronous API，因为在一些情况下，基于asynchronous + callback的模式是不方便使用的，
Examples
下面是我遇到的一些将asynchronous operation转换为synchronize operation的例子:
1、amust API，使用promise-future、condition variable
Synchronous API
The C library offers both a synchronous and an asynchronous API. This library offers only an asynchronous version. If you prefer a synchronous API, call
get()on the returnedfutureto block until you receive the response.
Provide golang's General powerful protocol, write code based on coroutine, can write simple code in a synchronous manner, while achieving asynchronous performance.
Blocking with timeout
显然，在没有收到响应之前，需要将caller阻塞，因此需要考虑的一个问题是: blocking with timeout，因为有可能一直无法收到响应，因此需要设置一个超时时间。
Sync to async
1、典型的实现方式是asynchronous method invocation + message queue(mailbox) + thread pool
例子包括:
a、libuv filesystem operation，参见 http://docs.libuv.org/en/v1.x/design.html