Wait, React Is Multi-Threaded?

Hey, everyone. Thanks for the amazing introduction. Super glad and pumped at the same time to be here virtually at React Berlin and be able to share my thoughts with all of you. As you might have got the introduction again, I'm Nikhil, and I work as an engineer at Postman. I mostly handle stuff around design systems, Postman on the web, and Postman's desktop platform. So if you want to talk to me about performance design systems at scale in general, I love to talk about those, by the way. So come say hi. I would love to connect on Twitter, on GitHub. I think you would be able to see the relevant tags, how you can connect with me. So we'd love to have a chat.

Okay. So before diving into the presentation, I would like to just give a brief overview of what I'm going to talk about here. So in this session, I'll be trying to answer like a very simple question, which is, can React be multi-threaded or can it not? So does it have those multi-threading capabilities and if it does, how it can help us with improving Okay. I'm going to start with the talk with one of the very noble statements, which is user experience is important, right? So what good user experience actually means is it is very delightful for users to actually use different parts of their applications, like different features with a very seamless experience. They don't have to hunt for like how to do something. They don't have to wait for stuff, stuff like that. Right? So user experience is always beneficial for your product and for generating business for it, right? Because users are always happy if your product is all seamless.

And to also talk about it in the in the extension to it, right, there was a survey that was conducted and it showed various reasons of why users leave, like they quit using an application. So if you see in these various reasons, one of the top reasons that was, was slow loading, which is like 88% users actually felt like they don't want to use your product if it is loading very slow, but we don't want to talk about it in our session. What do you want to focus more on in this is this yellow circle that you see that is 73% users not using an application or leaving that because of it being non-responsive or being a lot more janky. And we know that user experience is always beneficial to your users, as I mentioned, right? So you know, in this case, if you fall into any of these categories and user experience is not that good, right, your users might be like doing rage quits or like they might not want to use that application itself. So you don't want to use those to have such an experience, right? You always want them to be happy. And this is what like you want to talk about in our talk.

So let's try to understand the problem real quick and show you what exactly is a type of problem that I'm talking about. So if I go to the demo, you'll see this very nice small application, which shows you a nice spinner of React that is spinning in just to just to like show you when like give a glimpse of like when our application becomes unresponsive. So there's this large list of items that I have, and it's nothing fancy in it. There's just some items, I want to sort them. And I've initially like kept the logic of sorting to be like super slow, which is like bubble sort in this case, and which is intended to show you the experience of like a bad and janky UX, of doing like a task which is very big. So if I click on this button, that is the old way of doing it, I'm going to perform a sorting on this very large list of items. And since that item is going to take too much time, let's see what happens to the user experience into that app, right? So I click on this button. Now my app is all frozen up. I can't do anything. I click on I can't click on the buttons. And this is very bad. So after some time and this is done, now the spinner again gets to its spinning state, which is like for some point of time, my application was all stuck. So this is actually the problem.

Now let's try to do a root cause analysis of what went wrong, and what could have been improved when you were building this type of an experience. So let's look at a very simple diagram, which shows the current working of our event loop side. What our loop consists of is your JavaScript code, your event loop is like a stack, or not exactly a stack, it just takes in certain amount of operations, be it like some JavaScript functions, be it some other events, like mouse events, click events, and it just starts catering to them one by one, right? And if there is some event that is super big, in that case our event loop is all jacked up. And your users can't do anything else because of this very long running task, right? And since your event loop is all busy, and your JavaScript is taking too much time to free itself, your UI is going to appear to be all frozen up, and your users can't do anything until the time this big event is all done or not.

So that is the exact problem, is this very big event that is coming in. Now, to back my statement up, let's do a quick math of how all of this aligns in a central place, right? So if you see that spinner in our demo that we showed, right? If you want to get it done to be 60 frames per second, that means we would have a thousand milliseconds to render 60 frames, right? That's the math. And it means that you have 16 milliseconds for your JavaScript to run per frame.

Now, to talk more realistically, browsers usually take up four or six milliseconds out of this 16 millisecond time, which is their internal tasks, composition, painting, understanding how to parse HTML, JavaScript and stuff like that. So roughly if you talk about it, your JavaScript actually has only 10 to 12 milliseconds or less to run and render that frame in a constant time and avoid lag.

Now, let's try to see this small example of how your code actually goes through a browser's pipeline, what exactly is going to happen when your JavaScript is run, when your CSS is run, and when everything is compiled. So all of this, if you see it in one frame, all of this has to be done in 16 milliseconds or less amount of time. So your browser has to run JavaScript, it has to compute what are the styles, what is the CSS for it, it has to render all of the CSS, prepare the DOM, it has to compose everything and parse everything down and show the final result onto your webpage. Now as you keep on adding more JavaScript, be it like, we are not just using vanilla JavaScript right now. Maybe we are using CSS and JS engine like style components, we are using maybe read us or a library called React. Not to mention that React, adding React can add time to your JavaScript running. But the intention behind this example is the more time your JavaScript is going to take, the more time is going to get extended for your browser to catch up with this. Because all of this has to be done in 16 milliseconds. If your JavaScript is taking more time, you are going to surpass that 16 millisecond time limit. And because your subsequent frames are going to also have to catch up, your browser needs to skip some of the frames in between to catch up to the speed of getting this JavaScript done and also catering to the other frames that are coming in. And because of that, as I mentioned, it needs to skip the frames. And that is exactly what a jank is, because you just see a not moving spinner, because it had to skip all the frames to catch up to the speed, right. And that exactly was the problem there.

Now, coming back to the problem, which was there was a very big task that was blocking your event loop, right? Now, if I had to actually get this out of my event loop, right, and prevented from not blocking, my problem would get solved, right? So the other bigger tasks can keep running in some separate context, while my event loop is all free. So any bigger tasks are not blocking my webpages, UX or like it's not blocking or not jarring up the experience. And this is exactly the ideology of a web worker, right? So simple things, simple put, a web workers actually allow us to do work in parallel using separate threads, right? That is like the very simplest analogy. So you can do more amount of stuff in like some parallel context or like parallel threads to avoid your main thread to get blocked, right? And you can give all those bigger tasks to your worker threads.

Now, if I want to show you how this all works up, right? Let's understand the small analogy. There is your React app that is running in a totally different thread. And there is a worker thread that is again, a totally separate context. Now, what happens is we create a worker instance using the new workers, new worker API. And the worker that we get, we attach, we send a message to our worker thread, which is done by worker.postMessage API. So we send it a message that, hey, there is this one big task you need to do. And the worker receives it because we attach an event listener on the worker side, which is self.eventlistener. So make sure that workers do not have access to your window.

It only has an access to a global object called this, right? So in the event listener, it listens that, oh, React is telling me to do something. Let me do that work. And in the meantime, React's main thread is not blocked because the whole of the big task is being done by the workers. And if it is done by the time, the worker then posts a message again back to your React app. And your React app again listens back that, oh, now, worker has completed something, and to do something on my UI to show it is maybe done or not. And then the result is shown on the React app. And finally, when everything is done, we also want to free up all users' resources because a worker is also using the user's resources itself to run it as a separate thread, so we do worker.terminate, which is a good practice to do.

Now, the million dollar question, isn't it all done by concurrent mode? Because it also handles stuff like doing stuff, which is taking longer time or a similar context like web workers. What does web workers do that is different from this? So if I give you a quick recap of what concurrent mode was, if you imagine there is a product search application where you type something and the list of the product updates based on your search query. It shows that there is a user event, like a type, and there is a render phase where React app has to update your UI based on the search that you did. And that render phase, which is this very big yellow strip, is uninterruptable. Because React did not have the capability to jump to a priority-specific task, which was to handle that user event before doing the rendering part. In the newer concurrent mode, this yellow strip can be broken down, which means React can switch back, pause its rendering, and go to a different priority task, which was a user event. So cater to that, show that the type is working fine, and then resume its rendering back. If you understand this analogy of concurrent mode, the difference that lies in is the paradigm itself, which is concurrent mode on the first hand being context-switching. It is doing the same task, but it's creating sub-tasks out of those, and it's switching between the contexts. So it feels like it is parallelism, but it is actually just context-switching and it's doing the task in a synchronous type fashion.

Whereas, parallelism in general is not context-switching itself, it is like utilizing different resources of your CPU to do a different amount of tasks. So in the first example, I'm just doing one task. But actually, parallelism can do any number of tasks in parallel. Another question that now arises is, how do you do multi-threaded even if it's possible? Because JavaScript itself is single-threaded, right? How can you do this? So we need to understand thread and CPU as two also different entities to understand that answer better. A thread is a totally different entity. A CPU core, which can open a thread, is a totally different ballgame altogether. So you can see earlier, our machines used to be single core. But right now, in the new era of computers, you have multi-core machines, right? So what we want to do is our React app is running in a totally separate CPU core, right? Our web worker, which is again a single-threaded thing, but it is running inside a totally different CPU core itself. Which means a totally different execution context, and all are working in terms of passing messages to one another to communicate when the work is done. So it is single-threaded, but kind of multi-threaded, right? So you get the gist why it is functioning like a multi-threaded thing. So we are utilizing users' resources of different cores and we are running these two different worlds altogether out of these cores. So since these bring in so much to the table, why don't we use it, is the next question. So I personally found out that there are a lot of blockers which make us not use web workers in an easier way. So if you see, one example is you have a web worker, but you have to create a message-passing instance onto it, right? You have to create an, you have to set up event listeners and your workers and your main React app have to listen onto passing event listeners onto it, which is like some additional code that I don't like. Instead, wouldn't it have been better that you just need to create a function inside your worker and when you create an instance of the worker in the main thread, you just do worker.that function that you created, which is a lot simpler, right? Another class of problem that I saw is how to know when the worker thread is complete or not, like how to know the status, is it still running? Has it started or not? Is it done or not? So that is like totally, it is like very difficult to do, I would say, because it is, these messages are like just fired and forget. So there's no direct way to get how to update your UI based on whenever this worker is going to get complete. And an extension to this problem is actually one of the other key things that I discovered, which is how to coordinate between the web workers, right? Because if you see in this example, there is one worker that is doing one task, there is another worker doing some other task. And then there's a third worker that is actually waiting for the first and the second task to get completed, right? So it is like, it like the complexity brings, like comes in when you are using this sort of architecture and like handling too many web workers around that, right? So what could be the solution obviously promises, right? So because asynchronous JavaScript is so easy to detect like when something is going to get completed, right? So it would be like slightly easier if you could do a POST message to a worker and just await the result, like instead of returning a value, your worker actually gives you a promise. And you can now know that, okay, I know when it is going to get completed. And then I can update my UI based on that, right?

A few libraries that I want to mention, which is first is Commlink.

That is amazing. There's another amazing library created by Google Chrome Labs themself. But like another library that I like personally, which is like super helpful when you're creating stuff with React, is UseWebWorkerHook, which is just simple steps, or like two simple steps that you can use to create the same, right? So you just create a worker by using a UseWebWorkerHook. Pass it the function like that big function that's going to take a lot of time in case here that I added as bubble sort, right? And it gives you the instance of the worker. It gives you an instance of like a function to kill the worker and all the stuff. And whenever you want to perform it, you just do sort worker, which is the function here that you got. So you do the sort worker and you just pass in the relevant data. And that is it. So you just define the worker and you just use it. And that is all that you need to know.

All right. So enough of the knowledge, and let's get back to the same old demo that you were talking about. So in here, now what I'll do is I will use this newer button which says new wave. And in here, what I'm doing is the same exact thing, which is creating a worker and pass and allowing that worker to do the same bubble sort in its own context. So now let's try to see what happens to the spinner when I try to do that. So I click on this. Now my spinner is not like jank and all frozen up. So my app is again usable and my users are like all happy. In the meantime, I was also able to do this very big task itself. And if you want to also check this by performance, I, like, let's try to do this again and see the performance track of it, right? So if I open this performance tab, I click on the folder where we are doing things, which is when my app gets jammed up because of doing things synchronously. And if I close it down, you'll see that the main thread is actually spending the time, like if I open it here. So you'll see that there is like a very big 5.69 seconds even that was blocking the main thread similarly that we talked about, right? So that is why the spinner gets like got all frozen up. But in the other case, if I do the same with again, starting the profiling and clicking on the web, the use worker way of doing things, we should actually see that our main thread should be all unblocked, right? So if I close this down again, and we see the profiling now. We would see that now that very big bar is not is gone in the main thread, right? And if I close it down, you would see that now the worker thread actually has that very big bar. So that's why a spinner was not all janked up, right? Because the worker thread is catering to that bigger task. All right. So with this, I think we seem to have achieved something onto this thing and like improve the user experience in these scenarios. Right? So yeah, kudos, great job on to that. So a few more things to just mention it down. So there are a few instances that where you can use web workers, which is exactly where you need to do some CPU intensive tasks.

And you don't want to block your UI onto that, as I mentioned, right. Which is, for example, virtual DOM diffing that React does, right? You can assign a worker to do all the computation stuff of like what Node has been updated or not, what is the updated tree. So all those calculations can be given down to the web worker. Similar this maybe for image manipulation and processing and for canvas drawing. So the gist is the same. Anytime you see something that is taking too much time and is going to block your event loop onto this we should revert to the web worker strategy.

And which is again, a word of caution, which is we need to be wise in terms of choosing between those two things, right? So if you see tasks like which are IO bound, right, which are like maybe network calls and other stuff, you don't need to use web workers in those cases, right? Because those are already async and it just adds onto the complexity. Similarly, like if you go to DOM manipulation and you want to add it to a web worker, a web worker does not have access to your document object, right? Like again, the reason being different CPU, different contexts, like different CPU cores, so it does not know what the window is, what's the other world doing? It does not know about that. And similarly for local storage, because again, it does not have access to that API. And last but not the least, it's not like super, you would need to be like super cautious if you want to use web workers, right? Because when you grow and like you keep on adding web workers, it just adds on to the complexity and it can become super hard to maintain them down, right? So you need to be super cautious if you want to go with this approach or not. So that should be the decision to make.

All right, so with this, I would like to end my talk and I hope you guys were able to get some insights out of it. Last but not the least, we are doing some amazing stuff at Postman itself. So we are hiring and if you would want to be a part of our amazing journey, please visit out the careers website and I think we'd be glad to have you on board. And as I mentioned about interesting, we had been working super hard to release the WeTen feature of Postman. I been having many people asking me about what the features are going to be. And I know the people are super excited about this WeTen release. So if you'd want to know more about it, please check out the Postman's blog and take a look at the amazing features coming in. Alright. So with this I finally now end my talk. So I would like to thank everybody, the organizers, the community for organizing this amazing event and having me here. It was super amazing to be here. And last, but not the least, the amazing audience. So a kudos to everybody for making this event a success.