tornado_5_异步web服务

• 大部分Web应用（包括我们之前的例子）都是阻塞性质的，包括之前提到的案例，而tornado支持应用程序在等待第一个处理完成的过程中，让I/O循环打开以便服务于其他客户端，直到处理完成时启动一个请求并给予反馈，而不再是等待请求完成的过程中挂起进程

同步

• 例子1

import tornado.httpserverimport tornado.ioloopimport tornado.optionsimport tornado.webimport tornado.httpclientimport datetimefrom tornado.options import define, optionsdefine("port", default=8004, help="run on the given port", type=int)class IndexHandler(tornado.web.RequestHandler):    def get(self):        query = self.get_argument('q')        client = tornado.httpclient.HTTPClient()        response = client.fetch("http://www.baidu.com?wd={}".format(query))        body = response.body        result_count = len(body)        now = datetime.datetime.utcnow()        self.write("""            <div style="text-align: center">                <div style="font-size: 72px">%s</div>                <div style="font-size: 144px">%s</div>                <div style="font-size: 144px">%s</div>                <div style="font-size: 24px">tweets per second</div>            </div>""" % (query,result_count, now))if __name__ == "__main__":    tornado.options.parse_command_line()    app = tornado.web.Application(handlers=[(r"/", IndexHandler)])    http_server = tornado.httpserver.HTTPServer(app)    http_server.listen(options.port)    tornado.ioloop.IOLoop.instance().start()

• 主要使用到httpclient,让请求指定为同步请求

        client = tornado.httpclient.HTTPClient()        response = client.fetch("http://www.baidu.com?wd={}".format(query))        body = response.body

• 测试： 在10秒内执行大约100个并发请求，命令如下

siege http://localhost:8000/?q=pants -c100 -t10s

• 同步缺陷：无论每个请求自身返回多么快，API往返都会以至于产生足够大的滞后，因为进程直到请求完成并且数据被处理前都一直处于强制挂起状态

异步

异步demo

• 例子

import tornado.httpserverimport tornado.ioloopimport tornado.optionsimport tornado.webimport tornado.httpclientimport datetimefrom tornado.options import define, optionsdefine("port", default=8005, help="run on the given port", type=int)class IndexHandler(tornado.web.RequestHandler):    @tornado.web.asynchronous # 在get方法的定义之前    def get(self):        query = self.get_argument('q')        client = tornado.httpclient.AsyncHTTPClient()        client.fetch("http://www.baidu.com?wd={}".format(query), callback=self.on_response)    def on_response(self, response):        body = response.body        result_count = len(body)        now = datetime.datetime.utcnow()        self.write("""            <div style="text-align: center">                <div style="font-size: 72px">%s</div>                <div style="font-size: 144px">%s</div>                <div style="font-size: 24px">%s</div>                <div style="font-size: 24px">tweets per second</div>            </div>""" % (self.get_argument('q'), result_count, now))        self.finish() # 回调方法结尾处调用if __name__ == "__main__":    tornado.options.parse_command_line()    app = tornado.web.Application(handlers=[(r"/", IndexHandler)])    http_server = tornado.httpserver.HTTPServer(app)    http_server.listen(options.port)    tornado.ioloop.IOLoop.instance().start()

• 测试： 在10秒内执行大约100个并发请求，命令如下

siege http://localhost:8000/?q=pants -c100 -t10s

• 结论，改成异步后，速度边快、单位时间内的响应变多

tornado.gen

• 异步生成器，当我们在处理多个请求的时候，如果都是异步，那么会在回调中回调，比如下面

def get(self):    client = AsyncHTTPClient()    client.fetch("http://example.com", callback=on_response)def on_response(self, response):    client = AsyncHTTPClient()    client.fetch("http://another.example.com/", callback=on_response2)def on_response2(self, response):    client = AsyncHTTPClient()    client.fetch("http://still.another.example.com/", callback=on_response3)def on_response3(self, response):    [etc., etc.]

因此引入了tornado.gen模块，可以提供一个更整洁的方式来执行异步请求,可以解决以上的问题

import tornado.httpserverimport tornado.ioloopimport tornado.optionsimport tornado.webimport tornado.httpclientimport tornado.genimport datetimefrom tornado.options import define, optionsdefine("port", default=8006, help="run on the given port", type=int)class IndexHandler(tornado.web.RequestHandler):    @tornado.web.asynchronous    @tornado.gen.engine    def get(self):        query = self.get_argument('q')        client = tornado.httpclient.AsyncHTTPClient()        response = yield tornado.gen.Task(client.fetch, "http://www.baidu.com?wd={}".format(query))        body = response.body        result_count = len(body)        now = datetime.datetime.utcnow()        self.write("""            <div style="text-align: center">                <div style="font-size: 24px">%s</div>                <div style="font-size: 24px">%s</div>                <div style="font-size: 24px">%s</div>                <div style="font-size: 24px">tweets per second</div>            </div>""" % (query, result_count, now))        self.finish()if __name__ == "__main__":    tornado.options.parse_command_line()    app = tornado.web.Application(handlers=[(r"/", IndexHandler)])    http_server = tornado.httpserver.HTTPServer(app)    http_server.listen(options.port)    tornado.ioloop.IOLoop.instance().start()

• gen的优点：在请求处理程序中返回HTTP响应，而不是回调函数中。因此，代码更易理解：所有请求相关的逻辑位于同一个位置。而HTTP请求依然是异步执行的，所以我们使用tornado.gen可以达到和使用回调函数的异步请求版本相同的性能

长轮询

• 所谓的”服务器推送”技术允许Web应用实时发布更新，同时保持合理的资源使用以及确保可预知的扩展。对于一个可行的服务器推送技术而言，它必须在现有的浏览器上表现良好。最流行的技术是让浏览器发起连接来模拟服务器推送更新。这种方式的HTTP连接被称为长轮询或Comet请求，长轮询意味着浏览器只需启动一个HTTP请求，其连接的服务器会有意保持开启。浏览器只需要等待更新可用时服务器”推送”响应。当服务器发送响应并关闭连接后，（或者浏览器端客户请求超时），客户端只需打开一个新的连接并等待下一个更新
• 例子：

# coding=utf-8import tornado.webimport tornado.httpserverimport tornado.ioloopimport tornado.optionsfrom uuid import uuid4class ShoppingCart(object):    """    ShoppingCart维护我们的库存中商品的数量    """    totalInventory = 10    callbacks = []    carts = {}    def register(self, callback):        self.callbacks.append(callback)    def moveItemToCart(self, session):        if session in self.carts:            return        self.carts[session] = True        self.notifyCallbacks()    def removeItemFromCart(self, session):        if session not in self.carts:            return        del(self.carts[session])        self.notifyCallbacks()    def notifyCallbacks(self):        for c in self.callbacks:            self.callbackHelper(c)        self.callbacks = []    def callbackHelper(self, callback):        callback(self.getInventoryCount())    def getInventoryCount(self):        return self.totalInventory - len(self.carts)class DetailHandler(tornado.web.RequestHandler):    """    为每个页面请求产生一个唯一标识符，在每次请求时提供库存数量，并向浏览器渲染index.html模板    """    def get(self):        session = uuid4()        count = self.application.shoppingCart.getInventoryCount()        self.render("index_poll.html", session=session, count=count)class CartHandler(tornado.web.RequestHandler):    """    CartHandler用于提供操作购物车的接口    """    def post(self):        action = self.get_argument('action')        session = self.get_argument('session')        if not session:            self.set_status(400)            return        if action == 'add':            self.application.shoppingCart.moveItemToCart(session)        elif action == 'remove':            self.application.shoppingCart.removeItemFromCart(session)        else:            self.set_status(400)class StatusHandler(tornado.web.RequestHandler):    """    StatusHandler用于查询全局库存变化的通知    """    @tornado.web.asynchronous  # 使得Tornado在get方法返回时不会关闭连接    def get(self):        # 我们使用self.async_callback包住回调函数以确保回调函数中引发的异常不会使RequestHandler关闭连接        self.application.shoppingCart.register(self.on_message)    def on_message(self, count):        self.write('{"inventoryCount":"%d"}' % count)        # 长轮询连接已经关闭，购物车控制器必须删除已注册的回调函数列表中的回调函数        self.finish()class Application(tornado.web.Application):    def __init__(self):        self.shoppingCart = ShoppingCart()        handlers = [            (r'/', DetailHandler),            (r'/cart', CartHandler),            (r'/cart/status', StatusHandler)        ]        settings = {            'template_path': 'templates',            'static_path': 'static'        }        tornado.web.Application.__init__(self, handlers, **settings)if __name__ == '__main__':    tornado.options.parse_command_line()    app = Application()    server = tornado.httpserver.HTTPServer(app)    server.listen(8007)    tornado.ioloop.IOLoop.instance().start()

• inventory.js

$(document).ready(function() {    document.session = $('#session').val();    setTimeout(requestInventory, 100);    $('#add-button').click(function(event) {        jQuery.ajax({            url: '//localhost:8007/cart',            type: 'POST',            data: {                session: document.session,                action: 'add'            },            dataType: 'json',            beforeSend: function(xhr, settings) {                $(event.target).attr('disabled', 'disabled');            },            success: function(data, status, xhr) {                $('#add-to-cart').hide();                $('#remove-from-cart').show();                $(event.target).removeAttr('disabled');            }        });    });    $('#remove-button').click(function(event) {        jQuery.ajax({            url: '//localhost:8007/cart',            type: 'POST',            data: {                session: document.session,                action: 'remove'            },            dataType: 'json',            beforeSend: function(xhr, settings) {                $(event.target).attr('disabled', 'disabled');            },            success: function(data, status, xhr) {                $('#remove-from-cart').hide();                $('#add-to-cart').show();                $(event.target).removeAttr('disabled');            }        });    });});function requestInventory() {    jQuery.getJSON('//localhost:8007/cart/status', {session: document.session},        function(data, status, xhr) {            $('#count').html(data['inventoryCount']);            //setTimeout(requestInventory, 0);        }    );}

• index_poll.html

<html>    <head>        <title>Burt's Books – Book Detail</title>        <script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js"></script>        <script src="{{ static_url('scripts/inventory.js') }}"            type="application/javascript"></script>    </head>    <body>        <div>            <h1>Burt's Books</h1>            <hr/>            <p><h2>The Definitive Guide to the Internet</h2>            <em>Anonymous</em></p>        </div>        <!--<img src="static/images/internet.jpg" alt="The Definitive Guide to the Internet" />-->        <hr />        <input type="hidden" id="session" value="{{ session }}" />        <div id="add-to-cart">            <p><span style="color: red;">Only <span id="count">{{ count }}</span>                left in stock! Order now!</span></p>            <p>$20.00 <input type="submit" value="Add to Cart" id="add-button" /></p>        </div>        <div id="remove-from-cart" style="display: none;">            <p><span style="color: green;">One copy is in your cart.</span></p>            <p><input type="submit" value="Remove from Cart" id="remove-button" /></p>        </div>    </body></html>

• tornado.web.asynchronous 命中这个方法的 HTTP 请求就成为长连接，直到你调用 self.finish 发送响应之前，连接都在等待状态（self.render的实现最后调用的就是self.finish）
  
  1. 优点：长轮询在站点或特定用户状态的高度交互反馈通信中非常有用
  2. 缺点： 长轮询开发应用时，记住对于浏览器请求超时间隔无法控制是非常重要的。许多浏览器限制了对于打开的特定主机的并发请求数量，当有一个连接保持空闲时，剩下的用来下载网站内容的请求数量就会有限制。

WebSockets

• websockets是一个新标准，目前只支持特定版本浏览器
• 例子：

#coding=utf-8import tornado.webimport tornado.websocketimport tornado.httpserverimport tornado.ioloopimport tornado.optionsfrom uuid import uuid4class ShoppingCart(object):    totalInventory = 10    callbacks = []    carts = {}    def register(self, callback):        self.callbacks.append(callback)    def unregister(self, callback):        """yichu"""        self.callbacks.remove(callback)    def moveItemToCart(self, session):        if session in self.carts:            return        self.carts[session] = True        self.notifyCallbacks()    def removeItemFromCart(self, session):        if session not in self.carts:            return        del (self.carts[session])        self.notifyCallbacks()    def notifyCallbacks(self):        """        我们不需要在它们被通知后移除内部的回调函数列表。我们只需要迭代列表并调用带有当前库存量的回调函数        :return:        """        for callback in self.callbacks:            callback(self.getInventoryCount())    def getInventoryCount(self):        return self.totalInventory - len(self.carts)class DetailHandler(tornado.web.RequestHandler):    def get(self):        session = uuid4()        count = self.application.shoppingCart.getInventoryCount()        self.render("index_poll.html", session=session, count=count)class CartHandler(tornado.web.RequestHandler):    def post(self):        action = self.get_argument('action')        session = self.get_argument('session')        if not session:            self.set_status(400)            return        if action == 'add':            self.application.shoppingCart.moveItemToCart(session)        elif action == 'remove':            self.application.shoppingCart.removeItemFromCart(session)        else:            self.set_status(400)class StatusHandler(tornado.websocket.WebSocketHandler):    """    open, close分别在连接打开和接收到消息时被调用    """    def open(self):        self.application.shoppingCart.register(self.callback)    def on_close(self):        self.application.shoppingCart.unregister(self.callback)    def on_message(self, message):        """        在实现中，我们在一个新连接打开时使用ShoppingCart类注册了callback方法，并在连接关闭时注销了这个回调函数。        我们依然使用了CartHandler类的HTTP API调用，因此不需要监听WebSocket连接中的新消息，所以on_message实现是空的        :param message:        :return:        """        pass    def callback(self, count):        self.write_message('{"inventoryCount":"%d"}' % count)class Application(tornado.web.Application):    def __init__(self):        self.shoppingCart = ShoppingCart()        handlers = [            (r'/', DetailHandler),            (r'/cart', CartHandler),            (r'/cart/status', StatusHandler)        ]        settings = {            'template_path': 'templates',            'static_path': 'static'        }        tornado.web.Application.__init__(self, handlers, **settings)if __name__ == '__main__':    tornado.options.parse_command_line()    app = Application()    server = tornado.httpserver.HTTPServer(app)    server.listen(8007)    tornado.ioloop.IOLoop.instance().start()

• inventory.js修改方法requestInventory

function requestInventory() {    var host = 'ws://localhost:8007/cart/status';    var websocket = new WebSocket(host);    websocket.onopen = function (evt) { };    websocket.onmessage = function(evt) {        $('#count').html($.parseJSON(evt.data)['inventoryCount']);    };    websocket.onerror = function (evt) { };}