《九浅一深node.js》Book Note

更新中。。。

模块机制

  • CommonJS希望JS可以在任何地方运行。
  • exports是唯一导出的接口，在模块中有一个module对象，代表自己，而exports就是module的属性。
  • require允许不包含扩展名，会按照.js, .node, .json尝试。
  • 同步配合缓存，可以大幅度缓解Node单线程阻塞调用的缺陷。
  • 18 ~

异步I/O

  • I/O disctributed I/O is pretty expensive
  • The cost of multi-thread when you create thread, and switch between them is expensive.
  • Asynchronous I/O
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App             OS
I/O call------> process request
               /||
other call      ||
                ||
                ||
                ||/
callback        process done
  • repeatly to call I/O is 轮询
  • 55 ~

Asynchronous Programming

Continuation Passing Style

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function foo(x, bar) {
  return bar(x);
}

业务重点由返回值转移到了毁掉函数中。

  • 偏函数
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var toString = Object.prototype.toString;
var isString = function(obj) {
  return toString.call(obj) == '[object string]';
};
var isType = function(type) {
  return function(obj) {
    return toString.call(obj) == '[object' + type + ']';
  };
};
  • 雪崩就是高访问并发的时候大量请求同时涌入数据库。
    用事件队列解决
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var proxy = new events.EventEmitter();
var status = "ready";
var select = function(callback) {
  proxy.once("selected", callback);
  if (status == "ready") {
    status = "pending";
    db.select("SQL", function(results) {
      proxy.emits("selected", results);
      status = "ready";
    });
  }
};
// 保证相同的SQL语句从开始到结束永远只有一次，Node单线程执行所以无需担心状态同步问题。
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var count = 0;
var results = {};
var done = unction(key, value) {
  results[key] = value;
  count++;
  if (count === 3) {
    render(results);
  }
};
fs.readFile(template_path, "utf8", function(err, template) {
  done("template", template);
});
db.query(sql, function(err, data) {
  done("data", data);
});
/* L10n.get
 Synchronous method for retrieving localization strings from app's resources. This function should 
 be used only when direct manipulation on the strings is needed (etc. date/time formatting) and in 
 all other UI related cases should be replaced with DOM attributes setting or L10n.setAttributes().
This method has also potential to generate race conditions so a developer has to make sure that any
 code that uses this method is not fired before l10n resources are loaded using L10n.once() or L10n.
 ready().*/
l10.get(function(err, resources) {
  done("resources", resources);
});
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// 烧饼（哨兵）变量和偏函数结合
var after = function(times, callback) {
  var count = 0, resutls = {};
  return function(key, value) {
    results[key] = value;
    counts++;
    if (count === times) {
      callback(results);
    }
  };
};
var done = after(times, render);
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// 多对一的收敛和事件订阅/发布
var emitter = new events.Emitter();
var done = after(times, render);
emitter.on("done", done);
emitter.on("done", other);
fs.readFile(template_path, "utf8", function(err, template) {
  emitter.emit("done", "template", template);
})
  • 74 ~

构建Web应用