Node.js Cluster with Workers

1const cluster = require('cluster');
2const os = require('os');
3console.log(`Master PID: ${process.pid}`);
4/*
5    Availability Strategy
6    kill -SIGUSR2 PID // send signal to the cluster process
7*/
8
9// Worker Processes Fork Factory
10// --------------------------------------------
11let workers = 0;
12const cpus = os.cpus().length;
13let totalUsers = 50;
14let workersArray = [];
15
16if (cluster.isMaster) {
17    console.log(`Forking for ${cpus} CPUs`);
18    for (let i = 0; i < cpus; i++) {
19        cluster.fork();
20    }
21    // Availabaility Strategy
22    cluster.on('exit', (worker, code, signal) => {
23        if (code !== 0 && !worker.exitedAfterDisconnect) { // prevent forking new processes if the cluster disconnected a worker
24            console.log(`Worker ${worker.id} crashed. ` +
25                'Starting a new worker...');
26            cluster.fork();
27        }
28    });
29} else {
30    require('./server');
31}
32
33// Master - Workers Communication Hub
34// Wait until all workers are ready (listening)
35// --------------------------------------------
36cluster.addListener('listening', (worker) => {
37    workers++;
38    if (workers == os.cpus().length) {
39        workersArray = Object.values(cluster.workers); // copy references to array (easy indexing)
40        broadCastToAllWorkers();
41        setInterval(() => {
42            updateWorkersWithCachedValue();
43        }, 5000);
44    }
45});
46
47const broadCastToAllWorkers = function () {
48    Object.values(cluster.workers).forEach(worker => {
49        worker.send(`Worker ${worker.process.pid}`);
50    });
51}
52
53const numberOfUsersInDB = function () {
54    return totalUsers++;
55}
56
57const updateWorkersWithCachedValue = () => {
58    const usersCount = numberOfUsersInDB();
59    Object.values(cluster.workers).forEach(worker => {
60        worker.send({ usersCount });
61    });
62};
63
64
65// Intentional Restart / Server Code Update with 0 Downtime
66// 1 at a time to have 100% availability)
67// --------------------------------------------
68process.on('SIGUSR2', () => {
69    console.log('SIGUSR2 RECEIVED', workersArray);
70    restartWorker(0); // start with the first indexed worker
71});
72
73// Recursive function to restart all workers one by one
74const restartWorker = (workerIndex) => {
75    const worker = workersArray[workerIndex];
76    if (!worker) return;
77
78    // Current worker disconnection successfull
79    worker.on('exit', () => {
80        if (!worker.exitedAfterDisconnect) return; // Guard: Not caused by -SIGUSR2
81        console.log(`>> Exited process ${worker.process.pid}`);
82
83        // Create a new Worker and Disconnect the next one (i++)
84        cluster.fork().on('listening', (newWorker) => {
85            restartWorker(workerIndex + 1);
86        });
87    });
88
89    worker.disconnect(); // disconnect the existing worker
90};