How to fix Error: HTTP 502 Bad Gateway (upstream server error) in Node.js

pm2 status

sudo systemctl status your-app-name

pm2 restart app-name
# or
sudo systemctl restart your-app-name

pm2 logs app-name --lines 100

sudo journalctl -u your-app-name -n 100 --no-pager

location / {
    proxy_pass http://localhost:3000;  # Must match your Node.js port
}

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server listening on port ${PORT}`);
});

sudo netstat -tlnp | grep node

curl http://localhost:3000

location / {
    proxy_pass http://localhost:3000;
    proxy_read_timeout 300s;
    proxy_connect_timeout 75s;
    proxy_send_timeout 300s;
}

sudo nginx -t
sudo systemctl reload nginx

pm2 monit
# or
top -p $(pgrep node)

node --max-old-space-size=4096 app.js

// ecosystem.config.js
module.exports = {
  apps: [{
    name: "app",
    script: "./app.js",
    node_args: "--max-old-space-size=4096"
  }]
};

// Catch uncaught exceptions
process.on('uncaughtException', (error) => {
  console.error('Uncaught Exception:', error);
  // Log to monitoring service
  process.exit(1); // Let PM2/systemd restart
});

// Catch unhandled promise rejections
process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection at:', promise, 'reason:', reason);
  process.exit(1);
});

// Graceful shutdown
process.on('SIGTERM', async () => {
  console.log('SIGTERM received, closing server...');
  await server.close();
  process.exit(0);
});

pm2 start app.js --name my-app --max-restarts 10

# /etc/systemd/system/your-app.service
[Service]
Restart=always
RestartSec=10

sudo systemctl daemon-reload
sudo systemctl restart your-app