title: started learning node cluster getting error
body: <p><strong>I tried this as my first code and i am getting error</strong> </p>
  
  <pre><code>   const cluster = require('cluster');
      const http = require('http');
      const numCPUs = 4;
  
      if (cluster.isMaster) {
        // Fork workers.
        for (var i = 0; i &lt; numCPUs; i++) {
          cluster.fork();
        }
         cluster.on('online', function(worker) {
              console.log('Worker ' + worker.process.pid + ' is online');
          });
  
        cluster.on('exit', (worker, code, signal) =&gt; {
          console.log(`worker ${worker.process.pid} died`);
        });
  
      } else {
        // Workers can share any TCP connection
        // In this case it is an HTTP server
        http.createServer((req, res) =&gt; {
          res.writeHead(200);
          res.end('hello world\n');
        }).listen(8000);
      }
  </code></pre>
  
  <p>This is the Error i am getting :-</p>
  
  <p>Worker 11056 is online
  Worker 11057 is online
  Worker 11058 is online
  Worker 11059 is online
  events.js:141
        throw er; // Unhandled 'error' event
        ^</p>
  
  <p>Error: bind EADDRINUSE null:8000
      at Object.exports._errnoException (util.js:870:11)
      at exports._exceptionWithHostPort (util.js:893:20)
      at cb (net.js:1302:16)
      at rr (cluster.js:594:14)
      at Worker. (cluster.js:564:9)
      at process. (cluster.js:714:8)
      at emitTwo (events.js:92:20)
      at process.emit (events.js:172:7)
      at handleMessage (internal/child_process.js:689:10)
      at Pipe.channel.onread (internal/child_process.js:440:11)
  events.js:141
        throw er; // Unhandled 'error' event
        ^</p>
  
  <p>Error: bind EADDRINUSE null:8000
      at Object.exports._errnoException (util.js:870:11)
      at exports._exceptionWithHostPort (util.js:893:20)
      at cb (net.js:1302:16)
      at rr (cluster.js:594:14)
      at Worker. (cluster.js:564:9)
      at process. (cluster.js:714:8)
      at emitTwo (events.js:92:20)
      at process.emit (events.js:172:7)
      at handleMessage (internal/child_process.js:689:10)
      at Pipe.channel.onread (internal/child_process.js:440:11)
  events.js:141
        throw er; // Unhandled 'error' event
        ^</p>
  
  <p>Error: bind EADDRINUSE null:8000
      at Object.exports._errnoException (util.js:870:11)
      at exports._exceptionWithHostPort (util.js:893:20)
      at cb (net.js:1302:16)
      at rr (cluster.js:594:14)
      at Worker. (cluster.js:564:9)
      at process. (cluster.js:714:8)
      at emitTwo (events.js:92:20)
      at process.emit (events.js:172:7)
      at handleMessage (internal/child_process.js:689:10)
      at Pipe.channel.onread (internal/child_process.js:440:11)
  worker 11056 died
  worker 11057 died
  worker 11058 died
  events.js:141
        throw er; // Unhandled 'error' event
        ^</p>
  
  <p>Error: bind EADDRINUSE null:8000
      at Object.exports._errnoException (util.js:870:11)
      at exports._exceptionWithHostPort (util.js:893:20)
      at cb (net.js:1302:16)
      at rr (cluster.js:594:14)
      at Worker. (cluster.js:564:9)
      at process. (cluster.js:714:8)
      at emitTwo (events.js:92:20)
      at process.emit (events.js:172:7)
      at handleMessage (internal/child_process.js:689:10)
      at Pipe.channel.onread (internal/child_process.js:440:11)
  worker 11059 died</p>