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。