单线程事件循环模型处理步骤:

Clients Send request to Web Server. Node JS Web Server internally maintains a Limited Thread pool to provide services to the Client Requests. Node JS Web Server receives those requests and places them into a Queue. It is known as “Event Queue”. Node JS Web Server internally has a Component, known as “Event Loop”. Why it got this name is that it uses indefinite loop to receive requests and process them. Event Loop uses Single Thread only. It is main heart of Node JS Platform Processing Model. Event Loop checks any Client Request is placed in Event Queue. If not then wait for incoming requests for indefinitely. If yes, then pick up one Client Request from Event Queue Starts process that Client Request If that Client Request Does Not requires any Blocking IO Operations, then process everything, prepare response and send it back to client. If that Client Request requires some Blocking IO Operations like interacting with Database, File System, External Services then it will follow different approach Checks Threads availability from Internal Thread Pool Picks up one Thread and assign this Client Request to that thread. That Thread is responsible for taking that request, process it, perform Blocking IO operations, prepare response and send it back to the Event Loop very nicely explained by @Rambabu Posa for more explanation go throw this Link

在node.js中，请求应该是IO绑定，而不是CPU绑定。这意味着每个请求不应该强迫node.js做大量的计算。如果在解决请求时涉及大量计算，那么node.js不是一个好的选择。IO界需要很少的计算量。请求的大部分时间都花在对DB或服务的调用上。

Node.js有单线程事件循环，但它只是一个厨师。在后台，大部分工作是由操作系统完成的，Libuv确保了与操作系统的通信。Libuv的文档如下:

在事件驱动编程中，应用程序表示感兴趣的 当某些事件发生时，对它们做出反应。的责任 从操作系统收集事件或监视其他 事件源由libuv处理，用户可以注册 事件发生时调用的回调。

The incoming requests are handled by the Operating system. This is pretty much correct for almost all servers based on request-response model. Incoming network calls are queued in OS Non-blocking IO queue.'Event Loop constantly polls OS IO queue that is how it gets to know about the incoming client request. "Polling" means checking the status of some resource at a regular interval. If there are any incoming requests, evnet loop will take that request, it will execute that synchronously. while executing if there is any async call (i.e setTimeout), it will be put into the callback queue. After the event loop finishes executing sync calls, it can poll the callbacks, if it finds a callback that needs to be executed, it will execute that callback. then it will poll for any incoming request. If you check the node.js docs there is this image:

从文档阶段概述

poll:检索新的I/O事件;执行I/O相关的回调(几乎 除了关闭回调，由 定时器，和setimmediation ());节点将在适当的时候阻塞在这里。

事件循环不断地从不同队列轮询。如果一个请求需要外部调用或磁盘访问，这将被传递给操作系统，操作系统也有2个不同的队列。一旦事件循环检测到某些事情必须异步完成，它就会将它们放入队列中。一旦它被放入队列中，事件循环将处理到下一个任务。

这里要提到的一件事是，事件循环持续运行。只有Cpu可以将这个线程移出Cpu，事件循环本身不会这样做。

从文档中可以看出:

The secret to the scalability of Node.js is that it uses a small number of threads to handle many clients. If Node.js can make do with fewer threads, then it can spend more of your system's time and memory working on clients rather than on paying space and time overheads for threads (memory, context-switching). But because Node.js has only a few threads, you must structure your application to use them wisely. Here's a good rule of thumb for keeping your Node.js server speedy: Node.js is fast when the work associated with each client at any given time is "small".

注意，小任务意味着IO绑定任务而不是CPU绑定任务。只有当每个请求的工作主要是IO工作时，单个事件循环才会处理客户机负载。

Context switch basically means CPU is out of resources so It needs to stop the execution of one process to allow another process to execute. OS first has to evict process1 so it will take this process from CPU and it will save this process in the main memory. Next, OS will restore process2 by loading process control block from memory and it will put it on the CPU for execution. Then process2 will start its execution. Between process1 ended and the process2 started, we have lost some time. Large number of threads can cause a heavily loaded system to spend precious cycles on thread scheduling and context switching, which adds latency and imposes limits on scalability and throughput.

添加slebetman的答案，以便更清楚地了解在执行代码时发生了什么。

The internal thread pool in nodeJs just has 4 threads by default. and its not like the whole request is attached to a new thread from the thread pool the whole execution of request happens just like any normal request (without any blocking task) , just that whenever a request has any long running or a heavy operation like db call ,a file operation or a http request the task is queued to the internal thread pool which is provided by libuv. And as nodeJs provides 4 threads in internal thread pool by default every 5th or next concurrent request waits until a thread is free and once these operations are over the callback is pushed to the callback queue. and is picked up by event loop and sends back the response.

现在这里有另一个信息，它不是单一的回调队列，有很多队列。

NextTick队列 微任务队列 定时器队列 IO回调队列(请求，文件操作，数据库操作) IO轮询队列 检查Phase queue或setimmediation 关闭处理程序队列

每当有请求时，代码就按照这个回调队列的顺序执行。

它不像当有一个阻塞请求时，它会附加到一个新线程。默认情况下只有4个线程。这是另一个排队过程。

在代码中，无论何时发生像文件读取这样的阻塞进程，然后调用一个从线程池中利用线程的函数，一旦操作完成，回调被传递到各自的队列，然后按顺序执行。

所有内容都基于回调的类型进行排队，并按照上面提到的顺序进行处理。

单线程事件循环模型处理步骤:

Clients Send request to Web Server. Node JS Web Server internally maintains a Limited Thread pool to provide services to the Client Requests. Node JS Web Server receives those requests and places them into a Queue. It is known as “Event Queue”. Node JS Web Server internally has a Component, known as “Event Loop”. Why it got this name is that it uses indefinite loop to receive requests and process them. Event Loop uses Single Thread only. It is main heart of Node JS Platform Processing Model. Event Loop checks any Client Request is placed in Event Queue. If not then wait for incoming requests for indefinitely. If yes, then pick up one Client Request from Event Queue Starts process that Client Request If that Client Request Does Not requires any Blocking IO Operations, then process everything, prepare response and send it back to client. If that Client Request requires some Blocking IO Operations like interacting with Database, File System, External Services then it will follow different approach Checks Threads availability from Internal Thread Pool Picks up one Thread and assign this Client Request to that thread. That Thread is responsible for taking that request, process it, perform Blocking IO operations, prepare response and send it back to the Event Loop very nicely explained by @Rambabu Posa for more explanation go throw this Link

多线程阻塞系统的阻塞部分使其效率较低。被阻塞的线程在等待响应期间不能用于其他任何事情。

而非阻塞单线程系统则充分利用了它的单线程系统。

见下图: 在这里，在厨房门口等待或在顾客挑选食物时等待，是“阻塞”了服务员的全部能力。在计算系统的意义上，它可以等待IO，或DB响应或任何阻塞整个线程的东西，即使线程在等待时能够进行其他工作。

让我们看看非阻塞是如何工作的:

在非阻塞系统中，服务员只接单和上菜，不在任何地方等待。他分享了他的手机号码，以便在他们完成订单后给他们回电话。同样地，他将自己的电话号码分享给厨房，以便在订单准备就绪时回复。

这就是Event循环在NodeJS中的工作方式，并且比阻塞多线程系统执行得更好。

在slebetman的回答中补充: 当你说Node.JS可以处理10,000个并发请求时，它们本质上是非阻塞请求，即这些请求主要与数据库查询有关。

在内部，Node.JS的事件循环正在处理一个线程池，其中每个线程处理一个非阻塞请求，事件循环在将工作委派给线程池中的一个线程后继续侦听更多的请求。当其中一个线程完成工作时，它向事件循环发送一个信号，它已经完成，也就是回调。事件循环然后处理此回调并发回响应。

作为NodeJS的新手，请阅读更多关于nextTick的内容，以了解事件循环内部是如何工作的。 阅读http://javascriptissexy.com上的博客，当我开始使用JavaScript/NodeJS时，它们对我非常有帮助。