cluster

当我们去了解Cluster的时候，我们会在node的官方文档的一开始就会看到一个简单的例子

// 这个例子是根据cpu的核数来生成对应的子进程数量，
// 然后每个子进程都运行一个绑定到8000端口的http server
const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    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) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

我们可以看到创建子进程的方式就是很简单的一句cluster.fork(),接下来我们就来看一下这个函数做了什么样的事情。

首先我们打印了一下这个函数，可以看到函数如下

function (env) {
    cluster.setupMaster();
    const id = ++ids;
    const workerProcess = createWorkerProcess(id, env);
    const worker = new Worker({
      id: id,
      process: workerProcess
    });

    worker.on('message', function(message, handle) {
      cluster.emit('message', this, message, handle);
    });

    worker.process.once('exit', function(exitCode, signalCode) {
      /*
       * Remove the worker from the workers list only
       * if it has disconnected, otherwise we might
       * still want to access it.
       */
      if (!worker.isConnected()) {
        removeHandlesForWorker(worker);
        removeWorker(worker);
      }

      worker.exitedAfterDisconnect = !!worker.exitedAfterDisconnect;
      worker.state = 'dead';
      worker.emit('exit', exitCode, signalCode);
      cluster.emit('exit', worker, exitCode, signalCode);
    });

    worker.process.once('disconnect', function() {
      /*
       * Now is a good time to remove the handles
       * associated with this worker because it is
       * not connected to the master anymore.
       */
      removeHandlesForWorker(worker);

      /*
       * Remove the worker from the workers list only
       * if its process has exited. Otherwise, we might
       * still want to access it.
       */
      if (worker.isDead()) removeWorker(worker);

      worker.exitedAfterDisconnect = !!worker.exitedAfterDisconnect;
      worker.state = 'disconnected';
      worker.emit('disconnect');
      cluster.emit('disconnect', worker);
    });

    worker.process.on('internalMessage', internal(worker, onmessage));
    process.nextTick(emitForkNT, worker);
    cluster.workers[worker.id] = worker;
    return worker;
  }

我们看到一开始就调用了一个函数cluster.setupMaster()

我们在官方文档中可以查到这个函数, 可以看到这个函数的第一个参数exec默认为process.argv[1],在主线程中打印process.argv[1]可以看到process.argv[1]指向了入口文件，所以cluster.fork()的时候，是重新运行了入口文件。这也对应了文档中的下面这句话。

Unlike the fork(2) POSIX system call, child_process.fork() does not clone the current process.

同样我们通过查看源码可以了解到下面这行代码，是根据当前的环境变量和cluster配置，传递给child_process.fork()，workerProcess就是fork后的返回值，通过查看文档，我们知道child_process.fork()返回了一个<ChildProcess>,关于这里面的细节，可以看child_process.fork()的细节

const workerProcess = createWorkerProcess(id, env);

再下面，Worker对象对ChildProcess包裹了一层，提供了一些API,具体可以看文档

其他的就是一个信息的传输