OpenStack点滴02-WSGI

OpenStack对外提供REST API，那么REST API是如何建立起来的呢？

OpenStack用了一种叫WSGI(Web Service Gateway Interface)的东西，WSGI是Web服务器与Web应用程序或应用框架之间的一种低级别的接口。

下面是个简单的例子：

#!/usr/bin/env python

from wsgiref.simple_server import make_server  

def hello_world_app(environ, start_response):

    status = '200 OK' # HTTP Status

    headers = [('Content-type', 'text/plain')] # HTTP Headers

    start_response(status, headers)  

    # The returned object is going to be printed

    return ["Hello World"]  

httpd = make_server('', 8088, hello_world_app)

print "Serving on port 8088..."  

# Serve until process is killed

httpd.serve_forever()

执行这段代码，然后浏览器访问http://localhost:8088就会出现Hello World页面。

大致流程如下：

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(1)Client(上例中浏览器)发送请求到Server。

(2)Server转发请求给Application(上例中hello_world_app)。注：Server和Application之间还有middleware，此处省略。

(3)Application进行操作后将相应发送给Server。

(4)Server再将相应转发给Client。

OpenStack使用WSGI的一个工具包paste来配置WSGI appliaction和server的系统，它的好处是将配置和代码分离。python代码写好后如果想要修改页面到app的映射关系，只需要修改配置文件即可。

用一个简单的例子来示范paste.deploy的工作机制：

pastedeploy.ini

[composite:test_composite]

use=egg:Paste#urlmap

/:root

[pipeline:root]

pipeline = logrequest showversion

[filter:logrequest]

username = root

password = root123

paste.filter_factory = pastedeploylab:LogFilter.factory

[app:showversion]

version = 1.0.0

paste.app_factory = pastedeploylab:ShowVersion.factory

app：表示它定义了一个wsgi的application，是一个callable对象。paste. app_factory返回值是一个application对象

filter：表示这个段定义了一个filter，filter需要完成的工作是将application包装成另一个application（“过滤”），并返回这个包装后的application。

pipeline：Pipeline 由一些列的filter组成，最后一个是应用，即将前面的fiiter应用到application。

composite：自己不处理请求，根据映射关系把请求分发到filter、app或者pipeline。/:root就是表示访问url根目录的请求全部分发到root这个pipeline处理

pastedeploy.py

import os

import webob

from webob import Request

from webob import Response

from paste.deploy import loadapp

from wsgiref.simple_server import make_server

#Filter

class LogFilter():

    def __init__(self,app):

        self.app = app

        pass

    def __call__(self,environ,start_response):

        print "filter:LogFilter is called."

        return self.app(environ,start_response)

    @classmethod

    def factory(cls, global_conf, **kwargs):

        print "in LogFilter.factory", global_conf, kwargs

        return LogFilter

class ShowVersion():

    def __init__(self):

        pass

    def __call__(self,environ,start_response):

        start_response("200 OK",[("Content-type", "text/plain")])

        return ["Paste Deploy LAB: Version = 1.0.0",]

    @classmethod

    def factory(cls,global_conf,**kwargs):

        print "in ShowVersion.factory", global_conf, kwargs

        return ShowVersion()

if __name__ == '__main__':

    configfile="pastedeploy.ini"

    appname="test_composite"

    wsgi_app = loadapp("config:%s" % os.path.abspath(configfile), appname)

    server = make_server('localhost',8080,wsgi_app)

    server.serve_forever()

    pass

执行命令python pastedeploy.py，然后在浏览器中输入http://localhost:8080/就可以在网页输出Paste Deploy LAB: Version = 1.0.0

下面讲解一下工作流程，运行pastedeploy.py文件，首先会调用loadapp函数加载运用，在配置文件pastedeploy.ini找到appname为test_composite，test_composite是一个composite，然后找到pipeline root，根据pipeline找到filter logrequest和app showversion，logrequest和showversion各自用factory生成callable对象。加载完应用后调用make_server启动服务。

在浏览器输入http://localhost:8080/就会根据urlmap将请求分发到pipeline root，调用LogFilter的__call__方法，其中app就是ShowVersion，然后调用ShowVersion的__call__方法返回消息。

以下写一个简单的OpenStack WSGI实例，参考了臭蛋的博客，臭蛋写的和OpenStack源码很一致。

其中用到的一些python库：

1. paste.deploy 配置WSGI appliaction和server

2. webob 用来对http请求和响应进行封装

3. routes 实现URL映射

4. eventlet.wsgi 或者 wsgiref.simple_server，提供wsgi server功能，后者更简单。

首先建立一个test包，然后在test包里面建立如下文件：

test-paste.ini

[composite:test_composite]

use=egg:Paste#urlmap

/v1:testapp

[app:testapp]

paste.app_factory = test.router:API.factory

server.py

import os

import logging

import sys

from paste import deploy

from wsgiref.simple_server import make_server

LOG = logging.getLogger(__name__)

module_dir = os.path.normpath(os.path.join(os.path.abspath(sys.argv[0]),

                        os.pardir,os.pardir))

sys.path.insert(0,module_dir)

bind_host = "127.0.0.1"

bind_port = 8088

def server(app_name, conf_file):

    print "server"

    app = load_paste_app(app_name,conf_file)

    serve = make_server(bind_host,bind_port,app)

    serve.serve_forever()

def load_paste_app(app_name, conf_file):

    print "load_paste_app"

    LOG.debug("Loading %(app_name) from %(conf_file)",

                {'app_name':app_name, 'conf_file':conf_file})

    try:

        app = deploy.loadapp("config:%s" % os.path.abspath(conf_file), name=app_name)

        return app

    except (LookupError, ImportError) as e:

        LOG.error(str(e))

        raise RuntimeError(str(e))

if __name__ == '__main__':

    app_name = "test_composite"

    conf_file = "test-paste.ini"

    server(app_name,conf_file)

wsgi.py

import logging

import routes.middleware

import webob.dec

import webob.exc

class Router(object):

    def __init__(self, mapper=None):

        print "Router.__init__"

        self.map =  mapper

        self._router = routes.middleware.RoutesMiddleware(self._dispatch,

                                                         self.map)

    @classmethod

    def factory(cls, global_conf, **local_conf):

        print "Router.__factory__"

        return cls()

    @webob.dec.wsgify

    def __call__(self,req):

        print "Router.__call__"

        return self._router

    @staticmethod

    @webob.dec.wsgify

    def _dispatch(req):

        print "Router._dispatch"

        # TODO

        match = req.environ['wsgiorg.routing_args'][1]

        if not match:

            return webob.exc.HTTPNotFound()

        app = match['controller']

        return app

router.py

import routes

from test import wsgi

from test import versions

class API(wsgi.Router):

    def __init__(self, mapper=None):

        print "API.__init__"

        if(mapper == None):

            mapper = routes.Mapper()

        versions_resource = versions.create_resource()

        mapper.connect("/test",controller=versions_resource,

                        action="index")

        super(API,self).__init__(mapper)

versions.py

import httplib

import json

import webob.dec

from test import wsgi

from webob import Response

class Controller(object):

    def __init__(self):

        print "Controller.__init__"

        # TODO

        self.version = "0.1"

    def index(self,req):

        print "Controller.index"

        response = Response(request=req,

                                  status=httplib.MULTIPLE_CHOICES,

                                  content_type='application/json')

        response.body = json.dumps(dict(versions=self.version))

        return response

    @webob.dec.wsgify

    def __call__(self, request):

        print "Controller.__call__"

        # TODO

        return self.index(request)

def create_resource():

    print "create_resource"

    return Controller()

@webob.dec.wsgify 装饰器将一个普通函数转变成WSGI应用程序

执行python server.py ，然后在浏览器输入http://localhost:8088/v1/test 就会出现相关页面。

由于在函数中加了打印语句，启动时会输出：

server

load_paste_app

Router.__factory__

API.__init__

create_resource

Controller.__init__

Router.__init__

访问页面会输出：

Router.__call__

Router._dispatch

Controller.__call__

Controller.index

这是一个OpenStack WSGI原型，还需完善，比如在router.py文件中，/test并没有和index方法绑定，只是在Controller.__call__方法中静态的调用了index方法。

秒客网

OpenStack点滴02-WSGI

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