Multithreading in Node.js
Hey, wait, what does this “non-blocking I/O” thing means?
Other servers usually have a thread pool and get a single thread to process each request they get, after the request is fulfilled the thread is sent back to the pool.
You should also know that Node keeps a single thread for your code, this means that if you have a CPU intensive code it will be executed from top to bottom and it will block all other requests.
In a nutshell: everything runs parallel, except your code
Interesting Knowledge Tip: If you want to learn about how your operating system deals with these kinds of problems you should read about how time-sharing systems and multiprogramming systems work. You will certainly like it if you are into this whole performance and multithreading thing.
Ok cap’n, but can you tell me the pros and cons of this?
Of course I can, fellow sailor of the internet sea.
- Code gets simpler both to write and to read
- It eliminates race conditions
- I/O operations (which are expensive) do not block the thread’s execution
- CPU intensive operations block the thread’s execution
- Less control over how your code works
Of course using or not a non-blocking I/O solution depends on the problem you are trying to solve. If you are dealing with heavy loads of I/O you may bet your chips into Node.js, but you may not do it when it comes to heavy loads of CPU intensive operations.
Enough talking, let’s get to code
Disclaimer: For these examples I’m going to use Node.js’ cluster module and a module for sorting I’ve made called sugar-sorting. I also recommend that you take a look into the Node’s Process docs.
Let’s start with a simple example: we’re going to create 5 threads and get them to notify their own Process ID.
Simple isn’t it?
All you need to do is use the
fork() method to create new
process instances. The
fork() method returns a worker object and it also pushes the worker into the
cluster.workers hash, this hash stores all the worker objects mapped by their
Whoa! Is that all I’ve gotta know?
Unfortunately (or not) no. I’m sorry but there are some details I really need to tell you.
The first thing you should know when creating a new thread is that, unlike C’s
fork(), the new thread will execute the whole file that created it.
If you don’t wan’t this to happen you can use the
cluster.isWorker properties to differentiate between the two kinds of threads or you can create a new .js file an then use cluster’s
setupMaster() method to configure your workers to use it. Let’s say you’ve got a
myWorkerCode.js file and you want your worker threads to run it, you would write something like:
Now when running
threadCreator.js you will see ‘
PID XXXX says: I am running on a separate file’ printed five times on your console.
The second thing you should know is that Node.js threads cannot share memory, although this means you won’t be able to have two threads reading and writing the same variable you also won’t need to implement any lock mechanisms. As a workaround you could use Redis or Memcached.
The last but not least thing you should know is that workers cannot directly see their masters, this means you won’t be able to send direct messages from workers to their masters. How to overcome this? When creating worker threads you’ve gotta watch for the messages they send themselves, then you can use this into your current thread. Just like:
Can you give me a real world example?
Well, let’s say we’ve got a CPU intensive operation like sorting a bunch of enormous arrays (and we’re dumb enough to use bubble sort). We certainly don’t want to do this synchronously and lose tons of performance therefore we’re going to use more threads.
What we’re gonna do is simple: we’re going to create other threads and split the processing between them, when every thread has finished the master will print the elapsed time and kill itself. Do some changes to the code below and you will be able to notice the performance difference between using one or many threads.
Okay, can you give some tips, please?
Here they go:
require('os').cpus().lengthto get the number of cores you have, creating one thread for each one of them will improve performance
- Read the cluster docs to learn about every event and how they’re triggered
- To deal with the current thread (either on a worker or a master thread) you can use the process object
You can also take a look into the Child Process module if you want to learn more about multithreading in Node.js
- Recommended for Further Reading:
In this post you should’ve learned:
- What “non-blocking I/O” means
- The pros and cons of using non-blocking I/O technologies
- When to use multiple threads in Node.js
- How to create and manage multiple threads in Node.js