Node.js 将http.createServer与集群一起使用时，为什么多次调用提供给http.createServer的回调？

我试图理解集群的工作人员是如何处理http请求的。我从的代码开始，并添加了一些日志记录

我将以下代码放在server.js中，并使用执行该文件： NODE_DEBUG=cluster NODE server.js

var cluster = require('cluster');
var http = require('http');
var numCPUs = require('os').cpus().length;
if (cluster.isMaster) {
    console.log('number of CPUs: ' + numCPUs);

    // Fork workers.
    for (var i = 0; i < numCPUs; i++) {
        console.log('forking worker ' + i);
        cluster.fork();
    }

    cluster.on('exit', function(worker, code, signal) {
        console.log('worker ' + worker.process.pid + ' died');
    });

} else {
    console.log('worker spawned: ' + cluster.worker.process.pid);

    var someVar = {'b': 1};
    // Workers can share any TCP connection
    // In this case its a HTTP server
    http.createServer(function(req, res) {
        console.log('someVar.b current value: ' + someVar.b + ". someVar.b will be incremented");
        someVar.b = someVar.b + 1;
        console.log('Request being processed by worker: ' + cluster.worker.process.pid);
        res.writeHead(200);
        res.end("hello world\n");
        console.log('response ended by worker: ' + cluster.worker.process.pid);
    }).listen(8000);
}

具体来说，我希望对于我的服务器的一个请求，以下行只打印一次：

someVar.b current value: X. someVar.b will be incremented
Request being processed by worker: XXX
response ended by worker: XXX

相反，我的函数被同一个工作者执行了两次。有人能解释一下吗？

你查过req.url了吗？我打赌这两个请求中有一个是for/favicon.ico

someVar.b current value: X. someVar.b will be incremented
Request being processed by worker: XXX
response ended by worker: XXX