#async_hooks

[ajilalma]

async_hooks: behind the scenes of Node.js http request context handling

Despite Node.js being single threaded, some libraries do help us to keep track of http request context. Do you know how do they do it? I tried to explain it with the help of a thought experiment here. I hope you find it informative. Happy coding :)

In a previous post, we discussed how Node.js is a single-threaded environment and still manages to serve multiple requests simultaneously: Node.js event loop: concurrent processing in a single-threaded environment. This is a good way to avoid the problems that come along with multi-threading like race conditions. But this extra peace of mind comes at a price. Now that you do not have a…

[chaosmenu]

jesse. jesse what do you mean you cant resolve async_hooks. i installed the module jesse. look its sitting there in packages. its resolved. jesse i dont know what you want from me. Could not resolve 'async_hooks' from '' jesse what the hell are you talking about

[obscurejavascript]

Node.js Async Hooks

This allows you to directly track all async calls and where they are going. Unlike alot of other tactics, this does not require modifying the original code that needs to be tracked. Firstly, here is a simple HTTP server:

const http = require('http'); http.createServer((req, res) => { res.end('hello\n'); }).listen(5000); console.log('The HTTP server was started on port 5000'); // Result of: curl localhost:5000 // hello

If an async hook is created and enabled before the script, all the events can be tracked:

const async_hooks = require('async_hooks'); const fs = require('fs'); const asyncHook = async_hooks.createHook({ init: (asyncId, type, triggerAsyncId, resource) => { // Using console would trigger hooks which would trigger infinite recursion // console.info(asyncId, type, triggerAsyncId, resource); fs.writeSync(1, `${type} ${resource} \n`); } }); asyncHook.enable(); // When the server is started with the above first: // Event: TCPSERVERWRAP // Event: TickObject // The HTTP server was started on port 5000 // Event: TickObject

The TickObject represents each time the JavaScript even loop is incremented and new processing is done. That code can be put into a separate file. So to track parts of code a file can just be imported (asyncHook.disable can be used to stop tracking). For example, this is how to track when timeout callbacks are called and what those callbacks are:

const asyncHook = async_hooks.createHook({ init: (asyncId, type, triggerAsyncId, resource) => { if (type === 'Timeout') { const stack = new Error().stack; fs.writeSync(1, `Callback: ${type} ${resource._onTimeout.toString()} \n`); fs.writeSync(1, ` Location: ${stack} \n`); } } }); // asyncHook being defined in code snippet above asyncHook.enable(); http.createServer((req, res) => { setTimeout(() => { console.log('Running an async event'); }); res.end('hello\n'); }).listen(5000); // Run this in another CLI tab: // curl localhost:5000

There will be a few other timeout callbacks (called via libraries), but near this middle should be the one defined above and it will say exactly where it is called from.

Anyways, although this API is only for Node 8+ and is experimental, it is still useful for analyzing code performance and other characteristics. Especially when a lot of libraries and async calls are used.

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2018/asyncHooks.js

[vuejs2]

What Use Cases Exist for `async_hooks`? https://t.co/0Nc6zNlPJB

[javascriptw3schools]

If you use async_hooks with Node.js, please see our recent blog post and comment on the GitHub issue (linked in blog post) so that we can account for your use case as we work on new APIs: https://t.co/zhbjYaChJZ

[prevajconsultants]

RT @stephenbelanger: https://t.co/UrJzMI1vIp landed, making async_hooks and AsyncLocalStorage much faster in promise-heavy environments! Huge thanks to @matteocollina @addaleax @AndreyPechkurov @legendecas and Gerhard Stöbich for reviews, and a huge thanks to @datadoghq for sponsoring my time!

https://t.co/UrJzMI1vIp landed, making async_hooks and AsyncLocalStorage much faster in promise-heavy environments! Huge thanks to @matteocollina @addaleax @AndreyPechkurov @legendecas and Gerhard Stöbich for reviews, and a huge thanks to @datadoghq for sponsoring my time!

— Stephen Belanger (@stephenbelanger) May 11, 2020

nodejs

[holytheoristtastemaker]

Node.js 14 is out now, and with that release, it brings in Async Local Storage support. Now that might sound like "meh, okay, local storage has been around for some time", but this time, it's different.

For starters, this is the Node runtime we're talking about and not the browser. Thus, having a "localStorage" browser-like concept doesn't really make sense for Node. And the fact is, it is not the localStorage you're probably thinking of either. So what is it then?

Introducing Async Local Storage - Storage for asynchronous tasks

Consider a web server like Apache running PHP for hosting a website. When PHP receives a request from a client - your web server makes sure to spin off a new thread, and let that thread manage all the resources, local variables, function call stack, etc. for that particular request. Simple and easy. But the problem arises with JavaScript.

JavaScript is single threaded - that means you cannot have multiple threads of JS running together under a same parent process. But don't let it fool you, JS is as fast (even faster) as mature solutions like Java backend in handling web server requests. How does that happen? Well, that's something for another article, but the important thing here is that Node is single threaded, hence you do not have the benefits of thread local storage. Thread local storage is nothing but variables and functions local to a particular thread - in our case, for handling a particular user on the webpage.

Why is single thread a problem?

Single thread is a problem in this case as Node would keep executing synchronous code in one go as long as it doesn't exhaust all synchronous operations in the event loop. Then it'll keep a check on events and callbacks and execute that code whenever necessary. In Node, a simple HTTP request is nothing but an event fired by the http library to the node to handle the request - hence it is asynchronous. Now let's say you want to associate some data with this asynchronous operation. How would you do that?

Well, you can create some sort of "global" variable and assign your special data in it. Then, when another request comes from the same user, you can use the global variable to read whatever you had stored earlier. But it would spectacularly fail when you have more than 1 request on hand as now Node would not execute asynchronous code serially (of course, that's the definition of asynchronous!). Let's consider this dummy code (assume Node runtime):

server.listen(1337).on('request', (req) => { // some synchronous operation (save state) // some asynchronous operation // some asynchronous operation })

Consider this sequence of events:

User 1 hits the server on port 1337

Node starts running the synchronous operation code

While node was running that synchronous code, another User 2 hits the server.

Node would keep on executing the synchronous code, meanwhile the request to process the second HTTP request is waiting in the task queue.

When Node finishes synchronous operation part, and comes to asynchronous operation part, it throws it off in the task queue and then starts processing the first task sitting in the taskqueue - the second HTTP request.

This time it's running that synchronous piece of code, but on the behalf of User 2 request. When that synchronous code for User 2 is done, it resumes the asynchronous execution of User 1, and so on.

Now what if you want to persist specific data with that specific user whenever the asynchronous code specific to them is being called? Here's when you use AsyncStorage - storage for asynchronous flows in Node.

Consider this example straight from the official docs:

const http = require('http'); const { AsyncLocalStorage } = require('async_hooks'); const asyncLocalStorage = new AsyncLocalStorage(); function logWithId(msg) { const id = asyncLocalStorage.getStore(); console.log(`${id !== undefined ? id : '-'}:`, msg); } let idSeq = 0; http.createServer((req, res) => { asyncLocalStorage.run(idSeq++, () => { logWithId('start'); // Imagine any chain of async operations here setImmediate(() => { logWithId('finish'); res.end(); }); }); }).listen(8080); http.get('http://localhost:8080'); http.get('http://localhost:8080'); // Prints: // 0: start // 1: start // 0: finish // 1: finish

This example is showing nothing but the "stickyness" of the idSeq with the respective request. You can imagine how express populates "req.session" object with correct user everytime, in a similar fasion, this is a low level API using a lower level construct in Node called async_hooks which is still experimental, but is pretty cool to know!

Performance

Before you jump on to roll this out in production, beware that this is not something which I would really recommend anybody out there if not absolutely crazily needed. This is because it comes with a non-negligible performance hit on your application. This is primarily because the underlying API of async_hooks is still a WIP, and the situation should improve gradually.

Conclusion

That's basically it! A very simple brief introduction to what AsyncStorage is in Node 14 and what's the high level overall idea for it. If you want to learn all about new features in Node.js, check out this video for that:

Also, if you're an early adapter, try out codedamn - a platform for developers to learn and connect. I've been rolling out some sweet features there for you to try! Stay tuned.

Peace!

[topicprinter]

Article URL: https://github.com/cztomsik/minizone

Comments URL: https://news.ycombinator.com/item?id=16896003

Points: 13

# Comments: 8

[technteacher]

Show HN: Zones for Node.js using async_hooks

Show HN: Zones for Node.js using async_hooks 4 by cztomsik | from Blogger https://ift.tt/2FaVrf2

[captainsafia]

Discovering async hooks

So the other day, I was poking around the documentation for Node.JS when I accidentally clicked on something in the navigation bar titled "Async Hooks". I was intrigued. I scrolled quickly through the documentation to see if I could get anything from a cursory glance at it and made a mental note to learn more about it later.

And that's what brings me here! I figured I would dive into this async hooks business and see what was up.

As it turns out, async hooks are currently an experimental feature within the Node. That means that the interface for their API might be changed dramatically OR that they might be removed from Node altogether. So yeah, this is cutting edge stuff. Don't trust your production code with it just yet!

The documentation states the following.

The async_hooks module provides an API to register callbacks tracking the lifetime of asynchronous resources created inside a Node.js application.

Interesting! So, if I were to put the statement above in simple terms. Async hooks allow you to hook into events that occur within asynchronous resources. The documentation describes examples of an HTTP server resource or a file reader resource. For example, an event that might correlate with an asynchronous HTTP server is a new connection. Similarly, an event that might occur multiple times with a file I/O resource is a write to a file. So with async hooks, you can do something like send a notification every time someone connects to your HTTP server or send an email every time someone writes to a file. It's basically a way to plugin to the asynchronous event lifecycle in Node.

At this point, I was curious to see how the async_hooks module works, so I headed over to my trusty old friend, the NodeJS codebase. The core logic for the async_hooks module is defined here. If you look at the commit history for this file, you'll notice that the first commit was pushed out in May 2017. That's pretty new with respect to a lot of things that you see in the Node codebase.

The public API for the async_hooks module consists of three different functions, so I figured I would start diving into those.

module.exports = { // Public API createHook, executionAsyncId, triggerAsyncId, // Embedder API AsyncResource, };

The first function createHook is responsible for creating a new AsyncHooks object from the parameters that the user provides.

function createHook(fns) { return new AsyncHook(fns); }

The parameters that the user provides are a set of functions, fns, that define callbacks to be executed at different stages of the asynchronous resources lifespan. For example, there is an init callback that is invoked when a new asynchronous resource (like an HTTP server) is about to be created and a before callback that is invoked when a new asynchronous event (like a connection to an HTTP server) occurs.

The executionAsyncId seems to return some data from a lookup in an associative array.

function executionAsyncId() { return async_id_fields[kExecutionAsyncId]; }

Some snooping in the docs reveals that it returns the asyncId from the current execution context. So basically, this value gives insights into where the asynchronous resource is currently. For example, the asyncId will differ if the resource has just been initialized or if the resource has just had an asynchronous event occur.

There's another function related to asyncIds that is exported by the public API. The triggerAsyncId function which also returns data from a table lookup.

function triggerAsyncId() { return async_id_fields[kTriggerAsyncId]; }

From the description of the function in the documentation, I figured that this function returns the ID of the function that invoked the callback that is currently being executed. Basically, it gives you a handy way of figuring out the "parent" of the child callback. I'm guessing that you can probably use executionAsyncId and triggerAsyncId as debugging tools to track the state of callback invocations in your application at a given time.

There's another dimension to this async_hooks module. It's the C++ dimension. Like many important modules in the Node ecosystem, there is a lot of C++ code doing the heavy lifting and making the magic happen. I'll forgo diving into what the C++ implementation of async_hooks hold but I'll look into it in another blog post.

Stay tuned!

[kjunichi]

Async HooksとAsync Resourcesの導入 - Melchior [はてなブックマーク]

Async HooksとAsync Resourcesの導入 - Melchior

2017 - 12 - 15 Async HooksとAsync Resourcesの導入 Node.js JavaScript async_hooks Promise 概要 この記事は Node.js Advent Calendar 2017 15日目 の記事です。 カナダから非同期で失礼します。 Async hooksとはNode.jsの非同期イベントをトレースすることができるネイティブライブ...

kjw_junichi node.js

from kjw_junichiのブックマーク http://ift.tt/2B3GrPH

[t-baba]

#337: An Introduction to Commonly Used ES6 Features

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Issue 337 — June 2, 2017

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A new major release of Node is here with many significant updates including V8 5.8, npm 5, async_hooks, N-API.. See this week’s Node Weekly for a comprehensive roundup.

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Into The Great Unknown: Migrating from Mocha to Jest

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Patrick Hund

Production Progressive Web Apps with JS Frameworks

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An Introduction to Commonly Used ES6 Features

If you’re not yet using ES6 on a frequent basis, this is a thorough introduction to seven of the most useful concepts.

Zell Liew

Microjs: The List of Micro-Frameworks and Micro-Libraries

It’s been around a while but has had a lot of updates recently.

Meteor 1.5: A Full Stack JS App Platform

1.5 brings dynamic imports, code splitting, and bundle size analysis. It’s still based around Node v4, but will move up to Node v6 in Meteor 1.6.

Ben Newman

The Modern JavaScript Tutorial

From the basics to advanced topics with simple, but detailed explanations.

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Announcing TypeScript Support in Electron news Zeke Sikelianos

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The Most Important Features and Fixes of Node 8 tutorial Gergely Nemeth

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WebAssembly: Mozilla Won opinion “I think WebAssembly is a big victory for asm.js and Mozilla’s vision.” Robert O'Callahan

Redux vs MobX: Which Is Best for Your Project? opinion Michael Wanyoike

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Storybook 3.0: An Interact Dev Environment for React Components tools Michael Shilman

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taxi-rank: A JSDom-Based Selenium Webdriver API code Uses Zombie under the hood. Forbes Lindesay

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[webpacknews]

What Use Cases Exist for `async_hooks`? https://t.co/0Nc6zNlPJB

[jamesmcphee30]

Node.js 8: Big Improvements for the Debugging and Native Module Ecosystem

We are excited to announce Node.js 8.0.0 today. The new improvements and features of this release create the best workflow for Node.js developers to date. Highlighted updates and features include adding Node.js API for native module developers, async_hooks, JS bindings for the inspector, zero-filling Buffers, util.promisify, and more.

from martinos https://www.linux.com/news/nodejs-8-big-improvements-debugging-and-native-module-ecosystem

[napszemuvegbe]

jóságok, végre van async_hooks, npm@5, v8 5.8