The Road to Async Context

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The AsyncLocalStorage API is arguably one of the most important relatively recent additions to Node.js. Today we are seeing implementations being added to other runs such as workerd, deno, and bun. And there is an effort underway in TC-39 to introduce a new AsyncContext API to the language. This talk will introduce async context tracking with AsyncLocalStorage and AsyncContext and discuss how the model is evolving as it is being implemented across multiple platforms.

James Snell
James Snell
26 min
14 Apr, 2023

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Video Summary and Transcription

This Talk discusses the implementation and performance improvements of the async local storage API in Node and Cloudflare Workers. It explores the concept of continuations and execution contexts and how async local storage allows for passing contextual data through async flows. The talk also highlights the challenges in implementing async local storage in Node and the need for a standardized API. The introduction of async context as a replacement for async local storage is discussed, along with its benefits and the ongoing development of the async context API.

Available in Español

1. Introduction to Async Local Storage

Short description:

I will talk about code in Node, Cloudflare Workers, and the async local storage API. We have been working on improving its implementation and performance in Workers. Additionally, I will discuss the differences between Node and Workers and the upcoming Async Context API. Async local storage creates a store that stays coherent through async operations by using execution contexts and continuations.

Good morning, how are you all doing? I was, when they invited me to come up and come out and speak, I was kind of thinking about, you know, what do I want to talk about? You know, there was, you know, we could talk about Node in general, kind of what's happening in the project. And I just wanted to talk about code.

So you know, I want to talk about some of the code that we've been writing recently, kind of been working on not just in Node, but also in Cloudflare Workers and the async local storage API. If you're not familiar with it, it's one of the, it's been on Node for a couple of years, but it's one of them that's not really well known kind of what it does, how it works. And one of the things that, you know, as we started to work on this in Workers here recently as part of our Node compatibility story, we tried to figure out like, you know, how is this implemented, how, you know, is the performance of it and how it's written? Is it what it should be? All right.

So in Workers, we decided to take a different design approach to how it works under the covers and I kind of want to talk about the differences between the way it works in Node, the way it works in Workers, and kind of where things are going. Now, you know, that original title of the talk, the Road to Async Context. Async context is actually a new proposed standard API that covers the exact same use case as async local storage but it's actually going to be added to the language itself. And I'll talk a little bit about that here at the end. So a little bit, you know, more for me, like, you know, for those of you that don't know me, I have been involved with Node for a number of years. I'm also contributing to CloudFlare Workers. I'm actually on the Workers run time team at CloudFlare. I'm not going to talk too much about Workers itself. My colleague Matt Alonzo is here. He's going to be showing off some bits and some more specific details of Workers. If you're interested in that, I highly recommend you go over to his talk later on. Alright, so let's get things going. What is async local storage? The node documentation gives us this very helpful definition. Creates a store that stays coherent through async operations. That's basically it. And then it gives you an example. It's an extremely unhelpful explanation of what it is. So we're going to break it down a little bit further. Alright, so we have this notion of what's called an execution context. This is whatever code is running right now. That execution context can schedule a continuation. Continuation is any code that's scheduled to run later. So think of a timer, or a promise. Anytime you attach a then, a catch, a finally to it. Callbacks.

2. Understanding Continuations and Execution Context

Short description:

When calling the FS API and the async callback is run later, the continuation is the generic term for things scheduled to run later. The execution context can schedule any number of these things, such as QMicroTask, promises, callbacks, or process next take.

So when you're calling the FS API and that async callback is run later. You know, the continuation is kind of the generic term for those things that are scheduled to run later. Alright? Alright. So, as the execution context is running, it can schedule any number of these things, right? You know, if you're using QMicroTask, or promises, or callbacks, or process next take. All of these things are things that the current code, the current running code can schedule to run, you know, either just a few moments from now, later, you know, sometime later in the application. Whatever. But these things can get stacked up.

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