Multiprocessing in nodejs using clusters

Bharat Kalluri / 2020-12-22

There are differences between threads and processes. The simplistic explanation would be that threads share a memory context and processes do not. Similarly, threads are usually light weight. Whereas processes are heavier. But this is not the complete story. Different operating systems work with processes and threads differently. For example, here is what microsoft says and here is what linus says. But for now, understand that you opening word is a process and you typing and it auto saving could be explained to be two threads inside the word process. This topic most probably warrants its own post/note.

Now coming to nodejs, Javascript is said to be single threaded. It is true up to some extent. But there is more to this. Some operations are offloaded to separate threads so that it does not block the main thread but everything pretty much runs on the main thread. To really understand how JS works, watch this great talk by Phillip Roberts.

But one serious limitation is that nodejs will not (by default) make use of those multiple cores you have on your modern processor. But we can accomplish this using clusters. Cluster is a nodejs library which allows us to fork our master process into multiple process. Which in turn makes use of the multiple cores.

So how does cluster work?

Each nodejs process has a memory limit of 512 mb. This can be pushed further if required. To mitigate a possible bottleneck of memory, cluster is used. Instead of scaling up on a single process, the master process is forked into multiple child processes.

Communication to and from the child processes happens using IPC (Inter process communication). So if your express server is forked into 4 processes for 4 cores. The master process later uses a round robin method to delegate requests to different processes. There is also another way calls are distributed, where the master just opens up a socket and the child process can pickup. The workers can accept the connections directly here. Read more here

Using Cluster in NodeJS and NestJS

if (cluster.isMaster) {
const cpuCount = os.cpus().length;
for (let i = 0; i < cpuCount; i += 1) {
cluster.on('online', (worker) => {
console.log('Worker ' + + ' is online.');
cluster.on('exit', ({ process }) => {
console.log('worker ' + + ' died.');
} else {
async function startServer() {
const app = await NestFactory.create(AppModule);
console.log('Started Listening for Server Port');
await app.listen(8080);

The code should be mostly straightforward. We first check if the current cluster is master. If it is, then we use it as an orchestrator to spawn and control other processes. In the master process, we have a look at the CPU count and start forking the process and log if they are online. For each of the child process, the server is run.

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