简单地说，

function sum(a,b){
return a+b;
}

为非阻塞。而异步则用于执行阻塞任务，然后返回阻塞任务的响应

同步/异步是用来描述两个模块之间的关系。 阻塞/非阻塞是描述一个模块的情况。

一个例子: 模块X: I。 模块Y:“书店”。 X问Y:你有《c++入门》这本书吗?

blocking: before Y answers X, X keeps waiting there for the answer. Now X (one module) is blocking. X and Y are two threads or two processes or one thread or one process? we DON'T know. non-blocking: before Y answers X, X just leaves there and do other things. X may come back every two minutes to check if Y has finished its job? Or X won't come back until Y calls him? We don't know. We only know that X can do other things before Y finishes its job. Here X (one module) is non-blocking. X and Y are two threads or two processes or one process? we DON'T know. BUT we are sure that X and Y couldn't be one thread. synchronous: before Y answers X, X keeps waiting there for the answer. It means that X can't continue until Y finishes its job. Now we say: X and Y (two modules) are synchronous. X and Y are two threads or two processes or one thread or one process? we DON'T know. asynchronous: before Y answers X, X leaves there and X can do other jobs. X won't come back until Y calls him. Now we say: X and Y (two modules) are asynchronous. X and Y are two threads or two processes or one process? we DON'T know. BUT we are sure that X and Y couldn't be one thread.

请注意上面两个加粗的句子。为什么题目中的粗体字包含两种情况，而题目中的粗体字只包含一种情况?这是区分非阻塞和异步的关键。

让我试着用另一种方式来解释这四个词:

blocking: OMG, I'm frozen! I can't move! I have to wait for that specific event to happen. If that happens, I would be saved! non-blocking: I was told that I had to wait for that specific event to happen. OK, I understand and I promise that I would wait for that. But while waiting, I can still do some other things, I'm not frozen, I'm still alive, I can jump, I can walk, I can sing a song etc. synchronous: My mom is gonna cook, she sends me to buy some meat. I just said to my mom: We are synchronous! I'm so sorry but you have to wait even if I might need 100 years to get some meat back... asynchronous: We will make a pizza, we need tomato and cheeze. Now I say: Let's go shopping. I'll buy some tomatoes and you will buy some cheeze. We needn't wait for each other because we are asynchronous.

下面是一个关于非阻塞和同步的典型例子:

// thread X
while (true)
{
    msg = recv(Y, NON_BLOCKING_FLAG);
    if (msg is not empty)
    {
        break;
    }
    else
    {
        sleep(2000); // 2 sec
    }
}

// thread Y
// prepare the book for X
send(X, book);

You can see that this design is non-blocking (you can say that most of time this loop does something nonsense but in CPU's eyes, X is running, which means that X is non-blocking. If you want you can replace sleep(2000) with any other code) whereas X and Y (two modules) are synchronous because X can't continue to do any other things (X can't jump out of the loop) until it gets the book from Y. Normally in this case, making X blocking is much better because non-blocking spends much resource for a stupid loop. But this example is good to help you understand the fact: non-blocking doesn't mean asynchronous.

这四个字确实很容易让我们困惑，我们应该记住的是，这四个字是为建筑设计服务的。学习如何设计一个好的架构是区分它们的唯一方法。

例如，我们可以设计这样一种架构:

// Module X = Module X1 + Module X2
// Module X1
while (true)
{
    msg = recv(many_other_modules, NON_BLOCKING_FLAG);
    if (msg is not null)
    {
        if (msg == "done")
        {
            break;
        }
        // create a thread to process msg
    }
    else
    {
        sleep(2000); // 2 sec
    }
}
// Module X2
broadcast("I got the book from Y");


// Module Y
// prepare the book for X
send(X, book);

在这个例子中，我们可以说

X1是非阻塞的 X1和X2是同步的 X和Y是异步的

如果需要，还可以用这四个字来描述在X1中创建的线程。

再说一次:这四个字是为建筑设计服务的。所以我们需要的是一个合适的架构，而不是像语言律师一样区分这四个词。如果你遇到一些情况，你不能很清楚地区分这四个词，你应该忘记这四个词，用你自己的话来描述你的架构。

所以更重要的事情是:我们什么时候使用同步而不是异步?什么时候用阻塞代替非阻塞?X1的阻塞性比非阻塞性更好吗?X和Y是同步的还是异步的?为什么Nginx是非阻塞的?Apache为什么阻塞?这些问题是你必须弄清楚的。

为了做出正确的选择，您必须分析您的需求并测试不同体系结构的性能。没有这样一种体系结构可以满足各种需求。

阻塞:在原语(同步或异步)处理完成后，控制返回到调用进程

非阻塞:调用后控制权立即返回进程

同步意味着按顺序在另一个结果之后启动一个。

异步意味着一起开始，结果上不保证顺序

阻塞是指某些东西导致执行下一步的阻碍。

无阻塞是指不等待任何东西就继续运行，克服障碍。

我敲门，等着他们开门。(我在这里无所事事)

我敲门，如果他们马上开门，我就和他们打招呼，然后走进去等等。如果门没有立刻打开，我就去下一户人家敲门。(我正在做某事，没有闲着)

只有在下雨的时候我才会出去。(依赖关系存在)

我要出去。可能会下雨。(独立事件，发生时间无关紧要)

同步或异步，两者都可以是阻塞或非阻塞的，反之亦然

block/non-block描述了初始化实体本身的行为，它意味着实体在等待I/O完成期间所做的事情 同步/异步描述了I/O初始化实体和I/O执行器(例如操作系统)之间的行为，它意味着这两个实体是否可以并行执行

它们只在拼写上有所不同。它们所指的内容没有区别。从技术上讲，你可以说它们的重点不同。非阻塞指的是控制流(它不阻塞)。异步是指当事件\数据被处理时(不是同步的)。