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This article mainly explains "how to use eventloop in Node.js". The content in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "how to use eventloop in Node.js".

The main thread reads events from the "task queue", which is a recurring process, so the whole running mechanism is also called Event Loop (event loop).

In fact, I also talked about eventloop in browsers in the previous article. However, there is a difference between eventloop in NodeJs and browser. Mastering eventloop is a very important skill for nodejs writers. Because it means that you can not only write js, but also have research on NodeJs.

Why is there an eventloop?

We know that the essence of NodeJs is to move the browser's v8 to the operating system, so we also bring the browser's event loop over. But why is there a design like eventloop?

For historical reasons, js was designed as a simple language to manipulate dom on the page (I'm sure you've all heard the story that js designed in 10 days). To this end, we certainly want js to be as simple, lightweight and lightweight as possible. The rendering engine as light as js runs in a single thread.

So the problem is that if you are running js on a thread, there is no problem when the contemporary code is linear. But on the page, we need user interaction, and we don't know why it happened. What about js? If there is running code in front of you, how should the program react when a user interacts? If you deal with the user's interaction first, the original program will be paused (that is, blocked). To avoid this blocking, js uses a message queue to hold this user interaction. After all the programs have run, go to the message queue to get the interactive events, and then execute them. This solves the problem of congestion.

Eventloop of the browser

We all know that when browsing the page, user interaction can occur at any time, in order to respond to the user immediately. Js won't shut down, he won't stop cycling. It is roughly as follows:

Get tasks from message queue-- > execute tasks-- > finish execution-- > get tasks from message queues-- >.

Of course, as we mentioned in the previous event loop article, in order to classify different asynchronous tasks, there is actually a distinction between macro tasks and micro tasks in the event loop. Their execution is roughly as follows

Get micro task from message queue-> execute micro task-> finish micro task-> get macro task from message queue-> execute macro task-> finish macro task-> get micro task from message queue-- >... eventloop of NodeJs

In fact, the general idea of node's event cycle is similar to that of browsers, but nodeJs makes a distinction between different macro tasks at different times. Here is the official flow chart:

You can see that each event loop in nodeJs is divided into six specific periods, each with a specified macro task. Then before each period of macro task execution, priority will be given to the execution of the micro-task queue.

Overview timers execution callback pending callbacks triggered by setTimeout () and setInterval () is delayed to the next loop iteration I / O callback idle, prepare is only used internally, developers can not pay attention to poll to retrieve new I / O events Perform I / O related callbacks (almost all callbacks are performed, except for close callbacks and timers scheduled callbacks and setImmediate () scheduled callbacks, which will block at this stage when appropriate) check executes setImmediate () close callbacks such as socket.on ('close',..)

In fact, from the above table, we already have a clear idea of the execution order of the whole event loop mechanism. But you may still have some questions. Let's talk about it in more detail.

Pending callbacks

This stage is actually dealing with callbacks that should have been performed in the last event loop due to operating system errors. For example, some TCP errors. Therefore, this part, developers can not take the initiative to operate, is some of the fault-tolerant mechanisms of NodeJs.

Check

Similarly, setImmediate is a nodejs-specific api that can immediately create an asynchronous macro task. Not only that, nodejs also sets a special check period in the event loop, during which the callback of setImmediate is specifically performed. Even if you keep generating setImmediate callbacks during this period, eventloop will give priority.

Close callbacks

This period handles shutdown events, such as socket.on ('close',...). This ensures that all tasks are completed before some communications are completed.

Micro tasks in eventloop

Let's first review the differences between browsers and nodejs:

Macro task: task browser NodeI/O ✅✅ setTimeout ✅✅ setInterval ✅✅ setImmediate ❌✅ requestAnimationFrame ✅❌ micro task: task browser Nodeprocess.nextTick ❌✅ MutationObserver ✅❌ Promise.then catch finally ✅✅

You can see that process.nextTick is a microtask unique to nodejs. Not only that, process.nextTick () takes precedence over all microtasks. Every time you clear the list of microtasks, you execute process.nextTick () first.

Execution difference

There are differences not only in the types of tasks, but also in the execution of the two environments. When executing a task on a browser, you need to make sure that the micro-task queue is finished before each macro task is executed. On nodejs, before each period, make sure that the micro-task queue is finished. That is to say, during the timer period, all setTimeout,setInterval macro tasks will be executed first. After performing the micro task, move on to the next period.

Note: the above execution rules are rules prior to the v11 version of nodejs. After version 11, the execution output of nodejs is the same as that of the browser.

SetImmediate () vs setTimeout ()

The order in which setImmediate () and setTimeout () are executed is not necessarily the same, which means that if you keep executing the following code, you may get different results each time.

SetTimeout (() = > {console.log ('timeout');}, 0); setImmediate (() = > {console.log (' immediate');})

The reason is that there is an error in the program's processing of time. The time set in the setTimeout method is not necessarily accurate. At the same time, when the callback is triggered, it is not possible to determine which period the event loop is in, which may be timer or check. All will have different results.

Thank you for your reading, the above is the content of "how to use eventloop in Node.js". After the study of this article, I believe you have a deeper understanding of how to use eventloop in Node.js, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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