openstack nova 基础知识——wsgi
激励自己的话：在微博上看到一句话：一件事成功的概率只有1%，但是要是做上100遍的话，成功的概率将会是63%，这句话对现在的我真是一个极大的鼓励。
正文：
在nova源码中看到了wsgi这个模块，不懂，于是谷歌去，又全是英文啊！
官方网站：http://wsgi.readthedocs.org/en/latest/index.html
1.什么是wsgi？
Web服务器网关接口（Python Web Server Gateway Interface，缩写为WSGI)是Python应用程序或框架和Web服务器之间的一种接口，已经被广泛接受, 它已基本达成它了可移植性方面的目标。WSGI 没有官方的实现, 因为WSGI更像一个协议. 只要遵照这些协议,WSGI应用(Application)都可以在任何实现(Server)上运行, 反之亦然。
2.如何实现WSGI？
看了几个例子，发现有这么几个“角色”：application、server、client，它们之间的关系是client发送请求给server，server转发这个请求给application，然后application进行一系列操作之后，将响应发送给server，server再将响应转发给client，server就起一个转发的作用。
WSGI就在application和server之间发挥作用：
首先是在server中定义了一个start_response(status,response_headers,exc_info=None)方法，返回值是一个write()函数，也就是在必须在执行过start_response()函数之后，才能执行write()方法。还定义了environ变量，即环境变量，里面存放的是server接收到的client端的环境变量。WSGI就是规定了server要将start_response和environ发送给application。
然后就是application就要有接收这两个参数的“接口”了，这个“接口”可以以各种方式实现：方法、函数、类都可以。接受到之后，就要在application端调用start_resopnse()方法，而且这个调用一定要在return之前。WSGI就是规定了application要实现接收这两个参数的“接口”，并且执行start_response()。
server在接收到application的响应之后，就开始write()数据给client，首先要判断start_response()方法执行了没有，如果没有执行，那么就报出：AssertionError("write() before start_response()") 这样的异常。如果执行了start_response()，那么write()就顺利执行。
经过这样的“接口”设计，就可以让application在任何server上运行了？Maybe吧！
3.如何使用wsgi？
这里具一个简单的例子，是从官网上看到的，觉得很简单，很具有代表性：
查看文本打印
[*] #! /usr/bin/env python
[*]
[*] # Our tutorial's WSGI server
[*] from wsgiref.simple_server import make_server
[*]
[*] def application(environ, start_response):
[*] “”“这个就是application，实现了接收这两个参数的接口，并且在下面调用了start_response()”“”
[*] # Sorting and stringifying the environment key, value pairs
[*] response_body = ['%s: %s' % (key, value)
[*] for key, value in sorted(environ.items())]
[*] response_body = '\n'.join(response_body)
[*]
[*] status = '200 OK'
[*] response_headers = [('Content-Type', 'text/plain'),
[*] ('Content-Length', str(len(response_body)))]
[*] start_response(status, response_headers)
[*]
[*] return
[*]
[*] # Instantiate the WSGI server.
[*] # It will receive the request, pass it to the application
[*] # and send the application's response to the client
[*] # 这个是server，在它内部定义了start_response()方法和environ变量，并且调用application
[*] httpd = make_server(
[*] 'localhost', # The host name.
[*] 8051, # A port number where to wait for the request.
[*] application # Our application object name, in this case a function.
[*] )
[*]
[*] # Wait for a single request, serve it and quit.
[*] httpd.handle_request()
查看文本打印
[*] #! /usr/bin/env python
[*]
[*] # Our tutorial's WSGI server
[*] from wsgiref.simple_server import make_server
[*]
[*] def application(environ, start_response):
[*] “”“这个就是application，实现了接收这两个参数的接口，并且在下面调用了start_response()”“”
[*] # Sorting and stringifying the environment key, value pairs
[*] response_body = ['%s: %s' % (key, value)
[*] for key, value in sorted(environ.items())]
[*] response_body = '\n'.join(response_body)
[*]
[*] status = '200 OK'
[*] response_headers = [('Content-Type', 'text/plain'),
[*] ('Content-Length', str(len(response_body)))]
[*] start_response(status, response_headers)
[*]
[*] return
[*]
[*] # Instantiate the WSGI server.
[*] # It will receive the request, pass it to the application
[*] # and send the application's response to the client
[*] # 这个是server，在它内部定义了start_response()方法和environ变量，并且调用application
[*] httpd = make_server(
[*] 'localhost', # The host name.
[*] 8051, # A port number where to wait for the request.
[*] application # Our application object name, in this case a function.
[*] )
[*]
[*] # Wait for a single request, serve it and quit.
[*] httpd.handle_request()
如果想看server内部是如何实现的话，可以看一下PEP333上的这个例子：
查看文本打印
[*] import os, sys
[*]
[*] def run_with_cgi(application):
[*]
[*] environ = dict(os.environ.items())
[*] environ['wsgi.input'] = sys.stdin
[*] environ['wsgi.errors'] = sys.stderr
[*] environ['wsgi.version'] = (1, 0)
[*] environ['wsgi.multithread']= False
[*] environ['wsgi.multiprocess'] = True
[*] environ['wsgi.run_once'] = True
[*]
[*] if environ.get('HTTPS', 'off') in ('on', '1'):
[*] environ['wsgi.url_scheme'] = 'https'
[*] else:
[*] environ['wsgi.url_scheme'] = 'http'
[*]
[*] headers_set = []
[*] headers_sent = []
[*]
[*] def write(data):
[*] if not headers_set:
[*] raise AssertionError("write() before start_response()")
[*]
[*] elif not headers_sent:
[*] # Before the first output, send the stored headers
[*] status, response_headers = headers_sent[:] = headers_set
[*] sys.stdout.write('Status: %s\r\n' % status)
[*] for header in response_headers:
[*] sys.stdout.write('%s: %s\r\n' % header)
[*] sys.stdout.write('\r\n')
[*]
[*] sys.stdout.write(data)
[*] sys.stdout.flush()
[*]
[*] def start_response(status, response_headers, exc_info=None):
[*] if exc_info:
[*] try:
[*] if headers_sent:
[*] # Re-raise original exception if headers sent
[*] raise exc_info[0], exc_info[1], exc_info[2]
[*] finally:
[*] exc_info = None # avoid dangling circular ref
[*] elif headers_set:
[*] raise AssertionError("Headers already set!")
[*]
[*] headers_set[:] =
[*] return write
[*]
[*] result = application(environ, start_response)
[*] try:
[*] for data in result:
[*] if data: # don't send headers until body appears
[*] write(data)
[*] if not headers_sent:
[*] write('') # send headers now if body was empty
[*] finally:
[*] if hasattr(result, 'close'):
[*] result.close()
查看文本打印
[*] import os, sys
[*]
[*] def run_with_cgi(application):
[*]
[*] environ = dict(os.environ.items())
[*] environ['wsgi.input'] = sys.stdin
[*] environ['wsgi.errors'] = sys.stderr
[*] environ['wsgi.version'] = (1, 0)
[*] environ['wsgi.multithread']= False
[*] environ['wsgi.multiprocess'] = True
[*] environ['wsgi.run_once'] = True
[*]
[*] if environ.get('HTTPS', 'off') in ('on', '1'):
[*] environ['wsgi.url_scheme'] = 'https'
[*] else:
[*] environ['wsgi.url_scheme'] = 'http'
[*]
[*] headers_set = []
[*] headers_sent = []
[*]
[*] def write(data):
[*] if not headers_set:
[*] raise AssertionError("write() before start_response()")
[*]
[*] elif not headers_sent:
[*] # Before the first output, send the stored headers
[*] status, response_headers = headers_sent[:] = headers_set
[*] sys.stdout.write('Status: %s\r\n' % status)
[*] for header in response_headers:
[*] sys.stdout.write('%s: %s\r\n' % header)
[*] sys.stdout.write('\r\n')
[*]
[*] sys.stdout.write(data)
[*] sys.stdout.flush()
[*]
[*] def start_response(status, response_headers, exc_info=None):
[*] if exc_info:
[*] try:
[*] if headers_sent:
[*] # Re-raise original exception if headers sent
[*] raise exc_info[0], exc_info[1], exc_info[2]
[*] finally:
[*] exc_info = None # avoid dangling circular ref
[*] elif headers_set:
[*] raise AssertionError("Headers already set!")
[*]
[*] headers_set[:] =
[*] return write
[*]
[*] result = application(environ, start_response)
[*] try:
[*] for data in result:
[*] if data: # don't send headers until body appears
[*] write(data)
[*] if not headers_sent:
[*] write('') # send headers now if body was empty
[*] finally:
[*] if hasattr(result, 'close'):
[*] result.close()
4.eventlet中的wsgi
因为nova中用的是eventlet中的wsgi，所以再来看一下wsgi在eventlet中是怎么实现的。其实不用看源码也可以知道evenlet中，将和服务器建立的连接放在了绿色线程中，socket用的也是绿化过的socket。简单的使用，如下示例：
查看文本打印
[*] from eventlet import wsgi
[*] import eventlet
[*]
[*] def hello_world(env, start_response):
[*] start_response('200 OK', [('Content-Type', 'text/plain')])
[*] return ['Hello, World!\r\n']
[*]
[*] wsgi.server(eventlet.listen(('', 8090)), hello_world)
查看文本打印
[*] from eventlet import wsgi
[*] import eventlet
[*]
[*] def hello_world(env, start_response):
[*] start_response('200 OK', [('Content-Type', 'text/plain')])
[*] return ['Hello, World!\r\n']
[*]
[*] wsgi.server(eventlet.listen(('', 8090)), hello_world)
参考资料：
http://webpython.codepoint.net/wsgi_tutorial
http://www.python.org/dev/peps/pep-0333/
http://eventlet.net/doc/modules/wsgi.html
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