Unable to get all workers in worker block while using cluster in NodeJS

Question

I need to get all workers id list in worker block. Here is my example which is not working

const cluster = require('cluster');
var a = [];
if (cluster.isMaster) {
  console.log('I am master');
  cluster.fork();
  cluster.fork();
} else if (cluster.isWorker) {
  a.push(cluster.worker.id);
  console.log(`I am worker #${cluster.worker.id}`);
  console.log(a);
}

Output:

I am master
I am worker #1
[ 1 ]
I am worker #2
[ 2 ]

While expected output

I am master
I am worker #1
[ 1 ]
I am worker #2
[ 1, 2 ]

Answer 1

That because your code is executed three times from the very beginning, so every variable is unique for each worker. Think it about like 3 different application are being run, they don't share variable with eachother.

The variable a on worker 1 is a different variable from a in worker 2 .

If you want to share info you can either use a DB, persistent storage, or try to create a communication channel between the master and the slaves.

Here's an example to communicate between master, and slaves:

const cluster = require('cluster');
var a = [];

function onNewWorker(workers) {
    a = workers;
    console.log(`I am worker #${cluster.worker.id}, all workers = ${a}`);
}

if (cluster.isMaster) {
    cluster.on("fork", (worker) => {
        a.push(worker);
        a.forEach(w => w.send(a.map(wr => wr.id)));
    });
    console.log('I am master');
    const proc1 = cluster.fork();
    const proc2 = cluster.fork();
} else if (cluster.isWorker) {
    a.push(cluster.worker.id);
    process.on("message", onNewWorker);
    console.log(`I am worker #${cluster.worker.id}`);
    console.log(a);
}

Output

I am master
I am worker #1
I am worker #2
[ 1 ]
[ 2 ]
I am worker #1, all workers = 1
I am worker #1, all workers = 1,2
I am worker #2, all workers = 1,2

Of course you can improve this code, it's just for example.

Answer 2

Documentation about cluster

The worker processes are spawned using the child_process.fork() method

---

When you fork a processus, a new processus get created which is a strict copy of the actual processus (variables ...) . They are no relation between each process variables.

The new processus will start at the line (code) where the fork() function got called.

Definition of fork