Since the release of Node.js v10.5.0 there’s a new worker_threads module available.
Living in a single-threaded world
Ryan Dahl, the creator of Node.js, saw this limitation as an opportunity. He wanted to implement a server-side platform based on asynchronous I/O, which means you don’t need threads (which makes things a lot easier). Concurrency can be a very hard problem to solve. Having many threads accessing the same memory can produce race conditions that are very hard to reproduce and fix.
Is Node.js single-threaded?
So, our Node.js applications are single-threaded, right? Well, kind of.
Maybe this query to the database takes a minute but the “Running query” message will be shown immediately after invoking the query. And we will see the “Hey there” message a second after invoking the query if the query is still running or not. Our Node.js application just invokes the function and does not block the execution of other pieces of code. It will get notified through the callback when the query is done and we will receive the result.
CPU intensive tasks
What happens if we need to do synchronous intense stuff? Such as doing complex calculations in memory in a large dataset? Then we might have a synchronous block of code that takes a lot of time and will block the rest of the code. Imagine that a calculation takes 10s. If we are running a web server that means that all of the other requests get blocked for at least 10s because of that calculation. That’s a disaster. Anything more than 100ms could be too much.
So, at this point, many people will think that somebody needs to add a new module in the Node.js core and allow us to create and sync threads. That should be it, right? It’s a shame we don’t have a nice way of solving this use case in a mature server-side platform as Node.js.
Well, if we add threads, then we are changing the nature of the language. We cannot just add threads as a new set of classes or functions available. We need to change the language. Languages that support multithreading have keywords such as “synchronized” in order to enable threads to cooperate. For example in Java even some numeric types are not atomic, meaning that if you don’t synchronize their access you could end up having two threads changing the value of a variable and resulting that after both threads have accessed it, the variable has a few bytes changed by one thread and a few bytes changed by the other thread and thus, not resulting in any valid value.
The naïve solution: tick, tick, tick
Node.js won’t evaluate the next code block in the event queue until the previous one has finished executing. So one simple thing we can do is split our code into smaller synchronous code blocks and call setImmediate(callback) to tell Node.js we are done and that it can continue executing pending things that are in the queue.
It can continue on the next iteration or ‘tick’ of the event loop. Let’s see how we can refactor some code to take advantage of this. Let’s imagine we have a large array that we want to process and every item on the array requires CPU-intensive processing:
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Now we process ten items each time and call setImmediate(callback) so if there’s something else the program needs to do, it will do it between those chunks of ten items. I’ve added a setInterval() for demonstrating exactly that.
As you can see the code gets more complicated. And many times the algorithm is a lot more complex than this so it’s hard to know where to put the setImmediate() to find a good balance. Besides, the code now is asynchronous and if we depend on third-party libraries we might not be able to split the execution into smaller chunks.
So setImmediate() is maybe okay for some simple use cases, but it’s far from being an ideal solution. Also, we didn’t have threads (for good reasons) and we don’t want to modify the language. Can we do parallel processing without threads? Yes, what we need is just some kind of background processing: a way of running a task with input, that could use whatever amount of CPU and time it needs, and return a result back to the main application. Something like this:
The reality is that we can already do background processing in Node.js. We can fork the process and do exactly that using message passing. The main process can communicate with the child process by sending and receiving events. No memory is shared. All the data exchanged is “cloned” meaning that changing it in one side doesn’t change it on the other side. Like an HTTP response, once you have sent it, the other side has just a copy of it. If we don’t share memory, we don’t have race conditions and we don’t need threads. Problem solved!
Well, hold on. This is a solution, but it’s not the ideal solution. Forking a process is an expensive process in terms of resources. And it is slow. It means running a new virtual machine from scratch using a lot of memory since processes don’t share memory. Can we reuse the same forked process? Sure, but sending different heavy workloads that are going to be executed synchronously inside the forked process, has two problems:
- Yes, you are not blocking the main app, but the forked process will only be able to process one task at a time. If you have two tasks, one that will take 10s and one that will take 1s (in that order), it’s not ideal to have to wait 10s to execute the second task. Since we are forking processes we want to take advantage of the scheduling of the operating system and all the cores of our machine. The same way you can listen to music and browse the internet at the same time you can fork two processes and execute all the tasks in parallel.
- Besides, if one task crashes the process, it will leave all tasks sent to the same process unfinished.
In order to fix these problems we need multiple forks, not only one, but we need to limit the number of forked processes because each one will have all the virtual machine code duplicated in memory, meaning a few Mbs per process and a non-trivial boot time. So, like database connections, we need a pool of processes ready to be used, run a task at a time in each one and reuse the process once the task has finished. This looks complex to implement, and it is! Let’s use worker-farm to help us out:
So, problem solved? Yes, we have solved the problem, but we are still using a lot more memory than a multithreaded solution. Threads are still very lightweight in terms of resources compared to forked processes. And this is the reason why worker threads were born!
Worker threads have isolated contexts. They exchange information with the main process using message passing, so we avoid the race conditions problem threads have! But they do live in the same process, so they use a lot less memory.
Well, you can share memory with worker threads. You can pass SharedArrayBuffer objects that are specifically meant for that. Only use them if you need to do CPU-intensive tasks with large amounts of data. They allow you to avoid the serialization step of the data.
Let’s start using worker threads!
You can start using worker threads today if you run Node.js v10.5.0 or higher, but keep in mind that this is an experimental API that is subject to change. In fact, it is not available by default: you need to enable it by using — experimental-worker when invoking Node.js.
Also, keep in mind that creating a Worker (like threads in any language) even though it’s a lot cheaper than forking a process, can also use too many resources depending on your needs. In that case, the docs recommend you create a pool of workers. You can probably look for a generic pool implementation or a specific one in NPM instead of creating your own pool implementation.
But let’s see a simple example. First, we are going to implement the main file where we are going to create a Worker Thread and give it some data. The API is event-driven but I’m going to wrap it into a promise that resolves in the first message received from the Worker:
As you can see this is as easy as passing the file name as an argument and the data we want the Worker to process. Remember that this data is cloned and it is not in any shared memory. Then, we wait for the Worker Thread to send us a message by listening to the “message” event.
Now, we need to implement the service.
Here we need two things: the workerData that the main app sent to us, and a way to return information to the main app. This is done with the parentPort that has a postMessage method where we will pass the result of our processing.
That’s it! This is the simplest example, but we can build more complex things, for example, we could send multiple messages from the Worker Thread indicating the execution status if we need to provide feedback. Or if we can send partial results. For example, imagine that you are processing thousands of images, maybe you want to send a message per image processed but you don’t want to wait until all of them are processed.
In order to run the example, remember to use the experimental-worker flag if you are in Node.js 10.x:
node --experimental-worker index.js
For additional information check out the worker_threads documentation.
What about web workers?
Maybe you have heard of Web workers. They are a more mature API for the web and well supported by modern browsers. The API is different because the needs and technical conditions are different, but they can solve similar problems in the browser runtime. It can be useful if you are doing crypto, compressing/decompressing, image manipulation, computer vision (e.g. face recognition), etc. in your web application.
Worker threads is a promising experimental module if you need to do CPU-intensive tasks in your Node.js application. It’s like threads without shared memory and thus, without the potential race conditions they introduce. Since it’s still experimental I would wait before using it and I would just use worker-farm (or similar modules) to do background processing. In the future, your program should be easy to migrate to worker threads once they are mature enough!
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