原创精读：带你从零看清Node源码createServer和负载均衡整个过程

写在开头：

作为一名曾经重度使用Node.js作为即时通讯客户端接入层的开发人员，无法避免调试V8，配合开发addon。于是对Node.js源码产生了很大的兴趣～

顺便吐槽一句，Node的内存控制，由于是自动回收，我之前做的产品是20万人超级群的IM产品，像一秒钟1000条消息，持续时间长了内存和CPU占用还是会有一些问题

之前写过cluster模块源码分析、PM2原理等,感觉兴趣的可以去公众号翻一翻

Node.js的js部分源码基本看得差不多了，今天写一个createServer过程给大家,对于不怎么熟悉Node.js源码的朋友来说，可能是一个不错的开始，源码在gitHub上有，直接克隆即可，最近一直比较忙，公司和业余的工作也是，所以原创比较少。

原生Node.js创建一个基本的服务：


var http = require('http');
http.createServer(function (request, response) {
// 发送 HTTP 头部 
// HTTP 状态值: 200 : OK
// 内容类型: text/plain
response.writeHead(200, {'Content-Type': 'text/plain'});
// 发送响应数据 "Hello World"
response.end('Hello World\n');
}).listen(8888);
// 终端打印如下信息
console.log('Server running at http://127.0.0.1:8888/');

我们目前只分析Node.js源码的js部分的

首先找到Node.js源码的lib文件夹

然后找到http.js文件

发现createServer真正返回的是new Server，而Server来自_http_server

于是找到同目录下的_http_server.js文件，发现整个文件有800行的样子，全局搜索Server找到函数


function Server(options, requestListener) {
if (!(this instanceof Server)) return new Server(options, requestListener);
if (typeof options === 'function') {
    requestListener = options;
    options = {};
  } else if (options == null || typeof options === 'object') {
    options = { ...options };
  } else {
throw new ERR_INVALID_ARG_TYPE('options', 'object', options);
  }
this[kIncomingMessage] = options.IncomingMessage || IncomingMessage;
this[kServerResponse] = options.ServerResponse || ServerResponse;
  net.Server.call(this, { allowHalfOpen: true });
if (requestListener) {
this.on('request', requestListener);
  }

createServer函数解析：

参数控制有点像redux源码里的initState和reducer，根据传入类型不同，做响应的处理
```
this.on('request', requestListener);}
```
每次有请求，就会调用requestListener这个回调函数
至于IncomingMessage和ServerResponse,请求是流，响应也是流，请求是可读流，响应是可写流，当时写那个静态资源服务器时候有提到过
那么怎么可以链式调用？有人可能会有疑惑。Node.js源码遵循commonJs规范，大都挂载在prototype上，所以函数开头有,就是确保可以链式调用

   if (!(this instanceof Server)) 
    return new Server(options, requestListener);

上面已经将onrequest事件触发回调函数讲清楚了，那么链式调用listen方法，监听端口是怎么回事呢？

传统的链式调用，像JQ源码是return this, 手动实现A+规范的Promise则是返回一个全新的Promise,然后Promise原型上有then方法，于是可以链式调用

怎么实现.listen链式调用，重点在这行代码：

 net.Server.call(this, { allowHalfOpen: true });

allowHalfOpen实验结论: 这里TCP的知识不再做过度的讲解


（1）allowHalfOpen为true，一端发送FIN报文：
进程结束了，那么肯定会发送FIN报文；
进程未结束，不会发送FIN报文
（2）allowHalfOpen为false，一端发送FIN报文：
进程结束了，肯定发送FIN报文；
进程未结束，也会发送FIN报文；

于是找到net.js文件模块中的Server函数

function Server(options, connectionListener) {
  if (!(this instanceof Server))
    return new Server(options, connectionListener);

  EventEmitter.call(this);

  if (typeof options === 'function') {
    connectionListener = options;
    options = {};
    this.on('connection', connectionListener);
  } else if (options == null || typeof options === 'object') {
    options = { ...options };

    if (typeof connectionListener === 'function') {
      this.on('connection', connectionListener);
    }
  } else {
    throw new ERR_INVALID_ARG_TYPE('options', 'Object', options);
  }

  this._connections = 0;

  Object.defineProperty(this, 'connections', {
    get: deprecate(() => {

      if (this._usingWorkers) {
        return null;
      }
      return this._connections;
    }, 'Server.connections property is deprecated. ' +
       'Use Server.getConnections method instead.', 'DEP0020'),
    set: deprecate((val) => (this._connections = val),
                   'Server.connections property is deprecated.',
                   'DEP0020'),
    configurable: true, enumerable: false
  });

  this[async_id_symbol] = -1;
  this._handle = null;
  this._usingWorkers = false;
  this._workers = [];
  this._unref = false;

  this.allowHalfOpen = options.allowHalfOpen || false;
  this.pauseOnConnect = !!options.pauseOnConnect;
}

这里巧妙的通过.call调用net模块Server函数，保证了this指向一致

this.\_handle = null 这里是因为Node.js考虑到多进程问题，所以会hack掉这个属性，因为.listen方法最终会调用_handle中的方法，多个进程只会启动一个真正进程监听端口，然后负责分发给不同进程，这个后面会讲

Node.js源码的几个特色：

遵循conmonjs规范，很多方法挂载到prototype上了
很多object.definepropoty数据劫持
this指向的修改，配合第一个进行链式调用
自带自定义事件模块，很多内置的函数都继承或通过Object.setPrototypeOf去封装了一些自定义事件
代码模块互相依赖比较多，一个.listen过程就很麻烦，初学代码者很容易睡着
源码学习，本就枯燥。没什么好说的了

我在net.js文件模块中发现了一个原型上.listen的方法：

Server.prototype.listen = function(...args) {
  const normalized = normalizeArgs(args);
  var options = normalized[0];
  const cb = normalized[1];

  if (this._handle) {
    throw new ERR_SERVER_ALREADY_LISTEN();
  }

  if (cb !== null) {
    this.once('listening', cb);
  }
  const backlogFromArgs =
    // (handle, backlog) or (path, backlog) or (port, backlog)
    toNumber(args.length > 1 && args[1]) ||
    toNumber(args.length > 2 && args[2]);  // (port, host, backlog)

  options = options._handle || options.handle || options;
  const flags = getFlags(options.ipv6Only);
  // (handle[, backlog][, cb]) where handle is an object with a handle
  if (options instanceof TCP) {
    this._handle = options;
    this[async_id_symbol] = this._handle.getAsyncId();
    listenInCluster(this, null, -1, -1, backlogFromArgs);
    return this;
  }
  // (handle[, backlog][, cb]) where handle is an object with a fd
  if (typeof options.fd === 'number' && options.fd >= 0) {
    listenInCluster(this, null, null, null, backlogFromArgs, options.fd);
    return this;
  }

  // ([port][, host][, backlog][, cb]) where port is omitted,
  // that is, listen(), listen(null), listen(cb), or listen(null, cb)
  // or (options[, cb]) where options.port is explicitly set as undefined or
  // null, bind to an arbitrary unused port
  if (args.length === 0 || typeof args[0] === 'function' ||
      (typeof options.port === 'undefined' && 'port' in options) ||
      options.port === null) {
    options.port = 0;
  }
  // ([port][, host][, backlog][, cb]) where port is specified
  // or (options[, cb]) where options.port is specified
  // or if options.port is normalized as 0 before
  var backlog;
  if (typeof options.port === 'number' || typeof options.port === 'string') {
    if (!isLegalPort(options.port)) {
      throw new ERR_SOCKET_BAD_PORT(options.port);
    }
    backlog = options.backlog || backlogFromArgs;
    // start TCP server listening on host:port
    if (options.host) {
      lookupAndListen(this, options.port | 0, options.host, backlog,
                      options.exclusive, flags);
    } else { // Undefined host, listens on unspecified address
      // Default addressType 4 will be used to search for master server
      listenInCluster(this, null, options.port | 0, 4,
                      backlog, undefined, options.exclusive);
    }
    return this;
  }

  // (path[, backlog][, cb]) or (options[, cb])
  // where path or options.path is a UNIX domain socket or Windows pipe
  if (options.path && isPipeName(options.path)) {
    var pipeName = this._pipeName = options.path;
    backlog = options.backlog || backlogFromArgs;
    listenInCluster(this, pipeName, -1, -1,
                    backlog, undefined, options.exclusive);

    if (!this._handle) {
      // Failed and an error shall be emitted in the next tick.
      // Therefore, we directly return.
      return this;
    }

    let mode = 0;
    if (options.readableAll === true)
      mode |= PipeConstants.UV_READABLE;
    if (options.writableAll === true)
      mode |= PipeConstants.UV_WRITABLE;
    if (mode !== 0) {
      const err = this._handle.fchmod(mode);
      if (err) {
        this._handle.close();
        this._handle = null;
        throw errnoException(err, 'uv_pipe_chmod');
      }
    }
    return this;
  }

  if (!(('port' in options) || ('path' in options))) {
    throw new ERR_INVALID_ARG_VALUE('options', options,
                                    'must have the property "port" or "path"');
  }

  throw new ERR_INVALID_OPT_VALUE('options', inspect(options));
};

这个就是我们要找的listen方法，可是里面很多ipv4和ipv6的处理，最重要的方法是listenInCluster

这个函数需要好好看一下，只有几十行

function listenInCluster(server, address, port, addressType,
                         backlog, fd, exclusive, flags) {
  exclusive = !!exclusive;
  if (cluster === undefined) cluster = require('cluster');

  if (cluster.isMaster || exclusive) {
    // Will create a new handle
    // _listen2 sets up the listened handle, it is still named like this
    // to avoid breaking code that wraps this method
    server._listen2(address, port, addressType, backlog, fd, flags);
    return;
  }

  const serverQuery = {
    address: address,
    port: port,
    addressType: addressType,
    fd: fd,
    flags,
  };

  // Get the master's server handle, and listen on it
  cluster._getServer(server, serverQuery, listenOnMasterHandle);

  function listenOnMasterHandle(err, handle) {
    err = checkBindError(err, port, handle);

    if (err) {
      var ex = exceptionWithHostPort(err, 'bind', address, port);
      return server.emit('error', ex);
    }

    // Reuse master's server handle
    server._handle = handle;
    // _listen2 sets up the listened handle, it is still named like this
    // to avoid breaking code that wraps this method
    server._listen2(address, port, addressType, backlog, fd, flags);
  }
}

如果是主进程，那么就直接调用_.listen2方法了

Server.prototype._listen2 = setupListenHandle;

找到setupListenHandle函数

function setupListenHandle(address, port, addressType, backlog, fd, flags) {

里面的createServerHandle是重点


function setupListenHandle(address, port, addressType, backlog, fd, flags) {
  debug('setupListenHandle', address, port, addressType, backlog, fd);

  // If there is not yet a handle, we need to create one and bind.
  // In the case of a server sent via IPC, we don't need to do this.
  if (this._handle) {
    debug('setupListenHandle: have a handle already');
  } else {
    debug('setupListenHandle: create a handle');

    var rval = null;

    // Try to bind to the unspecified IPv6 address, see if IPv6 is available
    if (!address && typeof fd !== 'number') {
      rval = createServerHandle(DEFAULT_IPV6_ADDR, port, 6, fd, flags);

      if (typeof rval === 'number') {
        rval = null;
        address = DEFAULT_IPV4_ADDR;
        addressType = 4;
      } else {
        address = DEFAULT_IPV6_ADDR;
        addressType = 6;
      }
    }

    if (rval === null)
      rval = createServerHandle(address, port, addressType, fd, flags);

    if (typeof rval === 'number') {
      var error = uvExceptionWithHostPort(rval, 'listen', address, port);
      process.nextTick(emitErrorNT, this, error);
      return;
    }
    this._handle = rval;
  }

  this[async_id_symbol] = getNewAsyncId(this._handle);
  this._handle.onconnection = onconnection;
  this._handle[owner_symbol] = this;

  // Use a backlog of 512 entries. We pass 511 to the listen() call because
  // the kernel does: backlogsize = roundup_pow_of_two(backlogsize + 1);
  // which will thus give us a backlog of 512 entries.
  const err = this._handle.listen(backlog || 511);

  if (err) {
    var ex = uvExceptionWithHostPort(err, 'listen', address, port);
    this._handle.close();
    this._handle = null;
    defaultTriggerAsyncIdScope(this[async_id_symbol],
                               process.nextTick,
                               emitErrorNT,
                               this,
                               ex);
    return;
  }

  // Generate connection key, this should be unique to the connection
  this._connectionKey = addressType + ':' + address + ':' + port;

  // Unref the handle if the server was unref'ed prior to listening
  if (this._unref)
    this.unref();

  defaultTriggerAsyncIdScope(this[async_id_symbol],
                             process.nextTick,
                             emitListeningNT,
                             this);
}

已经可以看到TCP了，离真正的绑定监听端口，更近了一步

最终通过下面的方法绑定监听端口

 handle.bind6(address, port, flags);
 或者
 handle.bind(address, port);

首选ipv6绑定，是因为ipv6可以接受到ipv4的套接字，而ipv4不可以接受ipv6的套接字，当然也有方法可以接收，就是麻烦了一点

上面的内容，请你认真看，因为下面会更复杂，设计到Node.js的多进程负载均衡原理

如果不是主进程，就调用cluster._getServer，找到cluster源码


'use strict';

const childOrMaster = 'NODE_UNIQUE_ID' in process.env ? 'child' : 'master';
module.exports = require(`internal/cluster/${childOrMaster}`);

找到_getServer函数源码


// `obj` is a net#Server or a dgram#Socket object.
cluster._getServer = function(obj, options, cb) {
  let address = options.address;

  // Resolve unix socket paths to absolute paths
  if (options.port < 0 && typeof address === 'string' &&
      process.platform !== 'win32')
    address = path.resolve(address);

  const indexesKey = [address,
                      options.port,
                      options.addressType,
                      options.fd ].join(':');

  let index = indexes.get(indexesKey);

  if (index === undefined)
    index = 0;
  else
    index++;

  indexes.set(indexesKey, index);

  const message = {
    act: 'queryServer',
    index,
    data: null,
    ...options
  };

  message.address = address;

  // Set custom data on handle (i.e. tls tickets key)
  if (obj._getServerData)
    message.data = obj._getServerData();

  send(message, (reply, handle) => {
    if (typeof obj._setServerData === 'function')
      obj._setServerData(reply.data);

    if (handle)
      shared(reply, handle, indexesKey, cb);  // Shared listen socket.
    else
      rr(reply, indexesKey, cb);              // Round-robin.
  });

  obj.once('listening', () => {
    cluster.worker.state = 'listening';
    const address = obj.address();
    message.act = 'listening';
    message.port = (address && address.port) || options.port;
    send(message);
  });
};

我们之前传入了三个参数给它,分别是

server,serverQuery,listenOnMasterHandle

这里是比较复杂的，曾经我也在这里迷茫过一段时间，但是想着还是看下去吧。坚持下，大家如果看到这里看不下去了，先休息下，保存着。后面等心情平复了再静下来接下去看

首先我们传入了Server、serverQuery和cb（回调函数listenOnMasterHandle），整个cluster模块的_getServer中最重要的就是：


if (obj._getServerData)
    message.data = obj._getServerData();

  send(message, (reply, handle) => {
    if (typeof obj._setServerData === 'function')
      obj._setServerData(reply.data);

    if (handle)
      shared(reply, handle, indexesKey, cb);  // Shared listen socket.
    else
      rr(reply, indexesKey, cb);              // Round-robin.
  });

首先我们会先获取server上的data数据，然后调用send函数


function send(message, cb) {
  return sendHelper(process, message, null, cb);
}

send函数调用的是cluster模块的utills文件内的函数，传入了一个默认值process



function sendHelper(proc, message, handle, cb) {
  if (!proc.connected)
    return false;

  // Mark message as internal. See INTERNAL_PREFIX in lib/child_process.js
  message = { cmd: 'NODE_CLUSTER', ...message, seq };

  if (typeof cb === 'function')
    callbacks.set(seq, cb);

  seq += 1;
  return proc.send(message, handle);
}

这里要看清楚，我们调用sendHelper传入的第三个参数是null ！！！

那么主进程返回也是null



send(message, (reply, handle) => {
    if (typeof obj._setServerData === 'function')
      obj._setServerData(reply.data);

    if (handle)
      shared(reply, handle, indexesKey, cb);  // Shared listen socket.
    else
      rr(reply, indexesKey, cb);              // Round-robin.
  });

所以我们会进入rr函数调用的这个判断,这里调用rr传入的cb就是在net.js模块定义的listenOnMasterHandle函数

Node.js的负载均衡算法是轮询，官方给出的解释是简单粗暴效率高

上面的sendHelper函数就是做到了这点，每次+1


 if (typeof cb === 'function')
    callbacks.set(seq, cb);

  seq += 1;

function rr(message, indexesKey, cb) {
  if (message.errno)
    return cb(message.errno, null);

  var key = message.key;

  function listen(backlog) {
    // TODO(bnoordhuis) Send a message to the master that tells it to
    // update the backlog size. The actual backlog should probably be
    // the largest requested size by any worker.
    return 0;
  }

  function close() {
    // lib/net.js treats server._handle.close() as effectively synchronous.
    // That means there is a time window between the call to close() and
    // the ack by the master process in which we can still receive handles.
    // onconnection() below handles that by sending those handles back to
    // the master.
    if (key === undefined)
      return;

    send({ act: 'close', key });
    handles.delete(key);
    indexes.delete(indexesKey);
    key = undefined;
  }

  function getsockname(out) {
    if (key)
      Object.assign(out, message.sockname);

    return 0;
  }

  // Faux handle. Mimics a TCPWrap with just enough fidelity to get away
  // with it. Fools net.Server into thinking that it's backed by a real
  // handle. Use a noop function for ref() and unref() because the control
  // channel is going to keep the worker alive anyway.
  const handle = { close, listen, ref: noop, unref: noop };

  if (message.sockname) {
    handle.getsockname = getsockname;  // TCP handles only.
  }

  assert(handles.has(key) === false);
  handles.set(key, handle);
  cb(0, handle);
}

此时的handle已经被重写，listen方法调用会返回0，不会再占用端口了。所以这样Node.js多个进程也只是一个进程监听端口而已

此时的cb还是net.js模块的setupListenHandle即 - _listen2方法。

官方的注释：

Faux handle. Mimics a TCPWrap with just enough fidelity to get away

花了一晚上整理，之前还有一些像cluster模块源码、pm2负载均衡原理等，有兴趣的可以翻一翻。觉得写得不错的可以点个在看，谢谢。时间匆忙，如果有写得不对的地方可以指出。

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原创精读：带你从零看清Node源码createServer和负载均衡整个过程

PeterTan

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