从koa-session中间件源码学习cookie与session

从koa-session中间件学习cookie与session

原文链接

关于cookie和session是什么网上有很多介绍,但是具体的用法自己事实上一直不是很清楚,通过koa-session中间件的源码自己也算是对cookie和session大致搞明白了。

在我了解cookie的时候,大多数教程讲的是这些:

function setCookie(name,value) 
{ 
    var Days = 30; 
    var exp = new Date(); 
    exp.setTime(exp.getTime() + Days*24*60*60*1000); 
    document.cookie = name + "="+ escape (value) + ";expires=" + exp.toGMTString(); 
} 

它给我一个错觉:cookie只能在客户端利用js设置读取删除等,但事实上很多的cookie是由服务端在response的headers里面写进去的:

const Koa = require('koa');
const app = new Koa();

app.use((ctx) => {
  ctx.cookies.set('test', 'hello', {httpOnly: false});
  ctx.body = 'hello world';
})

app.listen(3000);

访问localhost:3000,打开控制台可以看到:

img
img

那么下次浏览器再访问localhost:3000的时候就会把这些cookie信息通过request的headers带给服务器。

了解http协议的话可以经常看到这么一句话:http是无状态的协议。什么意思呢?大致这么理解一下,就是你请求一个网站的时候,服务器不知道你是谁,比如你第一次访问了www.google.com,过了三秒钟你又访问了www.google.com,虽然这两次都是你操作的但是服务器事实上是不知道的。不过根据我们的生活经验,你登录了一个网站后,过了三秒你刷新一下,你还是在登录态的,这好像与无状态的http矛盾,其实这是因为有session。

按照上面的说法,session是用来保存用户信息的,那他与cookie有什么关系,事实上按照我的理解session只是一个信息保存的解决方法,实现这个方法可以有多种途径。既然cookie可以保存信息,那么我们可以直接利用cookie来实现session。对应于koa-session中间件,当我们没有写store的时候,默认即利用cookie实现session。

看一个官方例子:

const session = require('koa-session');
const Koa = require('koa');
const app = new Koa();

app.keys = ['some secret hurr'];

const CONFIG = {
  key: 'koa:sess', /** (string) cookie key (default is koa:sess) */
  /** (number || 'session') maxAge in ms (default is 1 days) */
  /** 'session' will result in a cookie that expires when session/browser is closed */
  /** Warning: If a session cookie is stolen, this cookie will never expire */
  maxAge: 86400000,
  overwrite: true, /** (boolean) can overwrite or not (default true) */
  httpOnly: true, /** (boolean) httpOnly or not (default true) */
  signed: true, /** (boolean) signed or not (default true) */
  rolling: false, /** (boolean) Force a session identifier cookie to be set on every response. The expiration is reset to the original maxAge, resetting the expiration countdown. default is false **/
};

app.use(session(CONFIG, app));
// or if you prefer all default config, just use => app.use(session(app));

app.use(ctx => {
  // ignore favicon
  if (ctx.path === '/favicon.ico') return;

  let n = ctx.session.views || 0;
  ctx.session.views = ++n;
  ctx.body = n + ' views';
});

app.listen(3000);
console.log('listening on port 3000');

每次我们访问views都会+1。

看一下koa-session是怎么实现的:

module.exports = function(opts, app) {
  // session(app[, opts])
  if (opts && typeof opts.use === 'function') {
    [ app, opts ] = [ opts, app ];
  }
  // app required
  if (!app || typeof app.use !== 'function') {
    throw new TypeError('app instance required: `session(opts, app)`');
  }

  opts = formatOpts(opts);
  extendContext(app.context, opts);

  return async function session(ctx, next) {
    const sess = ctx[CONTEXT_SESSION];
    if (sess.store) await sess.initFromExternal();
    try {
      await next();
    } catch (err) {
      throw err;
    } finally {
      await sess.commit();
    }
  };
};

一步一步的来看,formatOpts是用来做一些默认参数处理,extendContext的主要任务是对ctx做一个拦截器,如下:

function extendContext(context, opts) {
  Object.defineProperties(context, {
    [CONTEXT_SESSION]: {
      get() {
        if (this[_CONTEXT_SESSION]) return this[_CONTEXT_SESSION];
        this[_CONTEXT_SESSION] = new ContextSession(this, opts);
        return this[_CONTEXT_SESSION];
      },
    },
    session: {
      get() {
        return this[CONTEXT_SESSION].get();
      },
      set(val) {
        this[CONTEXT_SESSION].set(val);
      },
      configurable: true,
    },
    sessionOptions: {
      get() {
        return this[CONTEXT_SESSION].opts;
      },
    },
  });
}

所以走到下面这个代码时,事实上是新建了一个ContextSession对象sess。这个对象有个属性为session(要保存的session对象),有一些方法用来初始化session(如initFromExternal、initFromCookie),具体是什么下面用到再看。

const sess = ctx[CONTEXT_SESSION]

接着看是执行了如下代码,也即执行我们的业务逻辑

await next();

然后就是下面这个了,看样子应该是类似保存cookie的操作。

await sess.commit();

至此全部流程结束,好像并没有看到有什么初始化session的操作。其实在执行我们的业务逻辑时,假入我们操作了session,如例子:

let n = ctx.session.views || 0;

就会触发ctx的session属性拦截器,ctx.session实际上是sess的get方法返回值(返回值其实是一个Session对象),代码如下:

  get() {
    const session = this.session;
    // already retrieved
    if (session) return session;
    // unset
    if (session === false) return null;

    // cookie session store
    if (!this.store) this.initFromCookie();
    return this.session;
  }

在get里面执行了session的初始化操作,我们考虑没有store的情况即执行initFromCookie();

  initFromCookie() {
    debug('init from cookie');
    const ctx = this.ctx;
    const opts = this.opts;
    const cookie = ctx.cookies.get(opts.key, opts);
    if (!cookie) {
      this.create();
      return;
    }

    let json;
    debug('parse %s', cookie);
    try {
      json = opts.decode(cookie);
    } catch (err) {
      // backwards compatibility:
      // create a new session if parsing fails.
      // new Buffer(string, 'base64') does not seem to crash
      // when `string` is not base64-encoded.
      // but `JSON.parse(string)` will crash.
      debug('decode %j error: %s', cookie, err);
      if (!(err instanceof SyntaxError)) {
        // clean this cookie to ensure next request won't throw again
        ctx.cookies.set(opts.key, '', opts);
        // ctx.onerror will unset all headers, and set those specified in err
        err.headers = {
          'set-cookie': ctx.response.get('set-cookie'),
        };
        throw err;
      }
      this.create();
      return;
    }

    debug('parsed %j', json);

    if (!this.valid(json)) {
      this.create();
      return;
    }

    // support access `ctx.session` before session middleware
    this.create(json);
    this.prevHash = util.hash(this.session.toJSON());
  }
class Session {
  /**
   * Session constructor
   * @param {Context} ctx
   * @param {Object} obj
   * @api private
   */

  constructor(ctx, obj) {
    this._ctx = ctx;
    if (!obj) {
      this.isNew = true;
    } else {
      for (const k in obj) {
        // restore maxAge from store
        if (k === '_maxAge') this._ctx.sessionOptions.maxAge = obj._maxAge;
        else this[k] = obj[k];
      }
    }
  }

很明了的可以看出来其主要逻辑就是新建一个session,第一次访问服务器时session.isNew为true。

当我们执行完业务逻辑时,最后执行sess.commit()

  async commit() {
    const session = this.session;
    const prevHash = this.prevHash;
    const opts = this.opts;
    const ctx = this.ctx;
    // not accessed
    if (undefined === session) return;

    // removed
    if (session === false) {
      await this.remove();
      return;
    }

    // force save session when `session._requireSave` set
    let changed = true;
    if (!session._requireSave) {
      const json = session.toJSON();
      // do nothing if new and not populated
      if (!prevHash && !Object.keys(json).length) return;
      changed = prevHash !== util.hash(json);
      // do nothing if not changed and not in rolling mode
      if (!this.opts.rolling && !changed) return;
    }

    if (typeof opts.beforeSave === 'function') {
      debug('before save');
      opts.beforeSave(ctx, session);
    }
    await this.save(changed);
  }

commit事保存session前的准备工作,比如在我们没有强制保存session的时候它会判断时候保存session

    let changed = true;
    if (!session._requireSave) {
      const json = session.toJSON();
      // do nothing if new and not populated
      if (!prevHash && !Object.keys(json).length) return;
      changed = prevHash !== util.hash(json);
      // do nothing if not changed and not in rolling mode
      if (!this.opts.rolling && !changed) return;
    }

还提供了hook给我们使用

    if (typeof opts.beforeSave === 'function') {
      debug('before save');
      opts.beforeSave(ctx, session);
    }

到此开始真正的save session

  async save(changed) {
    const opts = this.opts;
    const key = opts.key;
    const externalKey = this.externalKey;
    let json = this.session.toJSON();
    // set expire for check
    const maxAge = opts.maxAge ? opts.maxAge : ONE_DAY;
    if (maxAge === 'session') {
      // do not set _expire in json if maxAge is set to 'session'
      // also delete maxAge from options
      opts.maxAge = undefined;
    } else {
      // set expire for check
      json._expire = maxAge + Date.now();
      json._maxAge = maxAge;
    }

    // save to external store
    if (externalKey) {
      debug('save %j to external key %s', json, externalKey);
      await this.store.set(externalKey, json, maxAge, {
        changed,
        rolling: opts.rolling,
      });
      this.ctx.cookies.set(key, externalKey, opts);
      return;
    }

    // save to cookie
    debug('save %j to cookie', json);
    json = opts.encode(json);
    
    debug('save %s', json);

    this.ctx.cookies.set(key, json, opts);
  }

对于我们讨论的这种情况,可以看到就是将信息encode之后写入了cookie,并且包含了两个字段_expire和_maxAge。

简单验证一下,CONFIG添加encode和decode

const CONFIG = {
  key: 'koa:sess', /** (string) cookie key (default is koa:sess) */
  /** (number || 'session') maxAge in ms (default is 1 days) */
  /** 'session' will result in a cookie that expires when session/browser is closed */
  /** Warning: If a session cookie is stolen, this cookie will never expire */
  maxAge: 86400000,
  overwrite: true, /** (boolean) can overwrite or not (default true) */
  httpOnly: true, /** (boolean) httpOnly or not (default true) */
  signed: true, /** (boolean) signed or not (default true) */
  rolling: false, /** (boolean) Force a session identifier cookie to be set on every response. The expiration is reset to the original maxAge, resetting the expiration countdown. default is false **/
  encode: json => JSON.stringify(json),
  decode: str => JSON.parse(str)
};

第一次访问时
img

再次访问

img

_expire用来下次访问服务器时判断session是否已过期

  valid(json) {
    if (!json) return false;

    if (json._expire && json._expire < Date.now()) {
      debug('expired session');
      return false;
    }

    const valid = this.opts.valid;
    if (typeof valid === 'function' && !valid(this.ctx, json)) {
      // valid session value fail, ignore this session
      debug('invalid session');
      return false;
    }
    return true;
  }

_maxAge用来保存过期时间,ctx.sessionOptions经过拦截器指向的其实是sess.opts

class Session {
  /**
   * Session constructor
   * @param {Context} ctx
   * @param {Object} obj
   * @api private
   */

  constructor(ctx, obj) {
    this._ctx = ctx;
    if (!obj) {
      this.isNew = true;
    } else {
      for (const k in obj) {
        // restore maxAge from store
        if (k === '_maxAge') this._ctx.sessionOptions.maxAge = obj._maxAge;
        else this[k] = obj[k];
      }
    }
  }

画一个简单的流程图看一下这整个逻辑时怎样的

img

通常情况下,把session保存在cookie有下面两个缺点:

  • Session is stored on client side unencrypted
  • Browser cookies always have length limits

所以可以把session保存在数据库中等,在koa-session中,可以设置store并提供三个方法:get、set、destroy。

当设置了store的时候,初始化操作是在initFromExternal完成的

  async initFromExternal() {
    debug('init from external');
    const ctx = this.ctx;
    const opts = this.opts;

    const externalKey = ctx.cookies.get(opts.key, opts);
    debug('get external key from cookie %s', externalKey);

    if (!externalKey) {
      // create a new `externalKey`
      this.create();
      return;
    }

    const json = await this.store.get(externalKey, opts.maxAge, { rolling: opts.rolling });
    if (!this.valid(json)) {
      // create a new `externalKey`
      this.create();
      return;
    }

    // create with original `externalKey`
    this.create(json, externalKey);
    this.prevHash = util.hash(this.session.toJSON());
  }

externalKey事实上是session数据的索引,此时相比于直接把session存在cookie来说多了一层,cookie里面存的不是session而是找到session的钥匙。当然我们保存的时候就要做两个工作,一是将session存入数据库,另一个是将session对应的key即(externalKey)写入到cookie,如下:

    // save to external store
    if (externalKey) {
      debug('save %j to external key %s', json, externalKey);
      await this.store.set(externalKey, json, maxAge, {
        changed,
        rolling: opts.rolling,
      });
      this.ctx.cookies.set(key, externalKey, opts);
      return;
    }

我们可以测试一下,事实上我们可以把session存在任意的媒介,不一定非要是数据库(主要是电脑没装数据库),只要store提供了三个接口即可:

const session = require('koa-session');
const Koa = require('koa');
const app = new Koa();
const path = require('path');
const fs = require('fs');

app.keys = ['some secret hurr'];

const store = {
  get(key) {
    const sessionDir = path.resolve(__dirname, './session');
    const files = fs.readdirSync(sessionDir);

    for (let i = 0; i < files.length; i++) {
      if (files[i].startsWith(key)) {
        const filepath = path.resolve(sessionDir, files[i]);
        delete require.cache[require.resolve(filepath)];
        const result = require(filepath);
        return result;
      }
    }
  },
  set(key, session) {
    const filePath = path.resolve(__dirname, './session', `${key}.js`);
    const content = `module.exports = ${JSON.stringify(session)};`;
    
    fs.writeFileSync(filePath, content);
  },

  destroy(key){
    const filePath = path.resolve(__dirname, './session', `${key}.js`);
    fs.unlinkSync(filePath);
  }
}

const CONFIG = {
  key: 'koa:sess', /** (string) cookie key (default is koa:sess) */
  /** (number || 'session') maxAge in ms (default is 1 days) */
  /** 'session' will result in a cookie that expires when session/browser is closed */
  /** Warning: If a session cookie is stolen, this cookie will never expire */
  maxAge: 86400000,
  overwrite: true, /** (boolean) can overwrite or not (default true) */
  httpOnly: true, /** (boolean) httpOnly or not (default true) */
  signed: true, /** (boolean) signed or not (default true) */
  rolling: false, /** (boolean) Force a session identifier cookie to be set on every response. The expiration is reset to the original maxAge, resetting the expiration countdown. default is false **/
  store
};

app.use(session(CONFIG, app));
// or if you prefer all default config, just use => app.use(session(app));

app.use(ctx => {
  // ignore favicon
  if (ctx.path === '/favicon.ico') return;
  let n = ctx.session.views || 0;
  ctx.session.views = ++n;
  if (n >=5 ) ctx.session = null;
  ctx.body = n + ' views';
});

app.listen(3000);
console.log('listening on port 3000');

浏览器输入localhost:3000,刷新五次则views重新开始计数。

全文完。

阅读 9.5k发布于 2017-12-13
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