自己寫一個Web伺服器（2）

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自己寫一個Web伺服器（1）

自己寫一個Web伺服器（2）

自己寫一個Web伺服器（3）

還記著第一篇的問題嗎？你怎麼在你剛建立的Web伺服器上執行一個Django應用，Flask應用和Pyramid應用，如何不做任何改變而適應不同的web架構呢？往下看，來找答案。

在以前，你選擇 Python web 架構會受制於可用的web伺服器，反之亦然。如果架構和伺服器可以協同工作，那你就走運了：

但你有可能面對（或者曾有過）下面的問題，當要把一個伺服器和一個架構結合起來是發現他們不是被設計成協同工作的：

基本上你只能用可以一起執行的而非你想要使用的。

那麼，你怎麼可以不修改伺服器和架構程式碼而確保可以在多個架構下執行web伺服器呢？答案就是 Python Web Server Gateway Interface (或簡稱 WSGI，讀作“wizgy”)。

WSGI允許開發者將選擇web框架和web伺服器分開。現在你可以混合匹配web伺服器和web框架，選擇一個適合你需要的配對。比如,你可以在Gunicorn 或者 Nginx/uWSGI 或者 Waitress上執行 Django, Flask, 或 Pyramid。真正的混合匹配，得益於WSGI同時支援伺服器和架構：

WSGI是第一篇和這篇開頭又重複問道問題的答案。你的web伺服器必須具備WSGI介面，所有的現代Python Web框架都已具備WSGI介面，它讓你不對程式碼作修改就能使伺服器和特點的web框架協同工作。

現在你知道WSGI由web伺服器支援，而web框架允許你選擇適合自己的配對，但它同樣對於伺服器和框架開發者提供便利使他們可以專注於自己偏愛的領域和專長而不至於相互牽制。其他語言也有類似介面：java有Servlet API，Ruby 有 Rack。

說這麼多了，你肯定在喊，給我看程式碼！好吧，看看這個極簡的WSGI伺服器實現：

# Tested with Python 2.7.9, Linux & Mac OS X
import socket
import StringIO
import sys

class WSGIServer(object):

    address_family = socket.AF_INET
    socket_type = socket.SOCK_STREAM
    request_queue_size = 1

    def __init__(self, server_address):
        # Create a listening socket
        self.listen_socket = listen_socket = socket.socket(
            self.address_family,
            self.socket_type
        )
        # Allow to reuse the same address
        listen_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        # Bind
        listen_socket.bind(server_address)
        # Activate
        listen_socket.listen(self.request_queue_size)
        # Get server host name and port
        host, port = self.listen_socket.getsockname()[:2]
        self.server_name = socket.getfqdn(host)
        self.server_port = port
        # Return headers set by Web framework/Web application
        self.headers_set = []

    def set_app(self, application):
        self.application = application

    def serve_forever(self):
        listen_socket = self.listen_socket
        while True:
            # New client connection
            self.client_connection, client_address = listen_socket.accept()
            # Handle one request and close the client connection. Then
            # loop over to wait for another client connection
            self.handle_one_request()

    def handle_one_request(self):
        self.request_data = request_data = self.client_connection.recv(1024)
        # Print formatted request data a la 'curl -v'
        print(''.join(
            '< {line}\n'.format(line=line)
            for line in request_data.splitlines()
        ))

        self.parse_request(request_data)

        # Construct environment dictionary using request data
        env = self.get_environ()

        # It's time to call our application callable and get
        # back a result that will become HTTP response body
        result = self.application(env, self.start_response)

        # Construct a response and send it back to the client
        self.finish_response(result)

    def parse_request(self, text):
        request_line = text.splitlines()[0]
        request_line = request_line.rstrip('\r\n')
        # Break down the request line into components
        (self.request_method,  # GET
         self.path,            # /hello
         self.request_version  # HTTP/1.1
         ) = request_line.split()

    def get_environ(self):
        env = {}
        # The following code snippet does not follow PEP8 conventions
        # but it's formatted the way it is for demonstration purposes
        # to emphasize the required variables and their values
        #
        # Required WSGI variables
        env['wsgi.version']      = (1, 0)
        env['wsgi.url_scheme']   = 'http'
        env['wsgi.input']        = StringIO.StringIO(self.request_data)
        env['wsgi.errors']       = sys.stderr
        env['wsgi.multithread']  = False
        env['wsgi.multiprocess'] = False
        env['wsgi.run_once']     = False
        # Required CGI variables
        env['REQUEST_METHOD']    = self.request_method    # GET
        env['PATH_INFO']         = self.path              # /hello
        env['SERVER_NAME']       = self.server_name       # localhost
        env['SERVER_PORT']       = str(self.server_port)  # 8888
        return env

    def start_response(self, status, response_headers, exc_info=None):
        # Add necessary server headers
        server_headers = [
            ('Date', 'Tue, 31 Mar 2015 12:54:48 GMT'),
            ('Server', 'WSGIServer 0.2'),
        ]
        self.headers_set = [status, response_headers + server_headers]
        # To adhere to WSGI specification the start_response must return
        # a 'write' callable. We simplicity's sake we'll ignore that detail
        # for now.
        # return self.finish_response

    def finish_response(self, result):
        try:
            status, response_headers = self.headers_set
            response = 'HTTP/1.1 {status}\r\n'.format(status=status)
            for header in response_headers:
                response += '{0}: {1}\r\n'.format(*header)
            response += '\r\n'
            for data in result:
                response += data
            # Print formatted response data a la 'curl -v'
            print(''.join(
                '> {line}\n'.format(line=line)
                for line in response.splitlines()
            ))
            self.client_connection.sendall(response)
        finally:
            self.client_connection.close()

SERVER_ADDRESS = (HOST, PORT) = '', 8888

def make_server(server_address, application):
    server = WSGIServer(server_address)
    server.set_app(application)
    return server

if __name__ == '__main__':
    if len(sys.argv) < 2:
        sys.exit('Provide a WSGI application object as module:callable')
    app_path = sys.argv[1]
    module, application = app_path.split(':')
    module = __import__(module)
    application = getattr(module, application)
    httpd = make_server(SERVER_ADDRESS, application)
    print('WSGIServer: Serving HTTP on port {port} ...\n'.format(port=PORT))
    httpd.serve_forever()

$ python webserver2.py
Provide a WSGI application object as module:callable

$ [sudo] pip install virtualenv
$ mkdir ~/envs
$ virtualenv ~/envs/lsbaws/
$ cd ~/envs/lsbaws/
$ ls
bin  include  lib
$ source bin/activate
(lsbaws) $ pip install pyramid
(lsbaws) $ pip install flask
(lsbaws) $ pip install django

from pyramid.config import Configurator
from pyramid.response import Response

def hello_world(request):
    return Response(
        'Hello world from Pyramid!\n',
        content_type='text/plain',
    )

config = Configurator()
config.add_route('hello', '/hello')
config.add_view(hello_world, route_name='hello')
app = config.make_wsgi_app()

(lsbaws) $ python webserver2.py pyramidapp:app
WSGIServer: Serving HTTP on port 8888 ...

$ curl -v http://localhost:8888/hello
...

from flask import Flask
from flask import Response
flask_app = Flask('flaskapp')

@flask_app.route('/hello')
def hello_world():
    return Response(
        'Hello world from Flask!\n',
        mimetype='text/plain'
    )

app = flask_app.wsgi_app

(lsbaws) $ python webserver2.py flaskapp:app
WSGIServer: Serving HTTP on port 8888 ...

$ curl -v http://localhost:8888/hello
...

import sys
sys.path.insert(0, './helloworld')
from helloworld import wsgi

app = wsgi.application

(lsbaws) $ python webserver2.py djangoapp:app
WSGIServer: Serving HTTP on port 8888 ...

$ curl -v http://localhost:8888/hello
...

def run_application(application):
    """Server code."""
    # This is where an application/framework stores
    # an HTTP status and HTTP response headers for the server
    # to transmit to the client
    headers_set = []
    # Environment dictionary with WSGI/CGI variables
    environ = {}

    def start_response(status, response_headers, exc_info=None):
        headers_set[:] = [status, response_headers]

    # Server invokes the ‘application' callable and gets back the
    # response body
    result = application(environ, start_response)
    # Server builds an HTTP response and transmits it to the client
    …

def app(environ, start_response):
    """A barebones WSGI app."""
    start_response('200 OK', [('Content-Type', 'text/plain')])
    return ['Hello world!']

run_application(app)

def app(environ, start_response):
    """A barebones WSGI application.

    This is a starting point for your own Web framework  
    """
    status = '200 OK'
    response_headers = [('Content-Type', 'text/plain')]
    start_response(status, response_headers)
    return ['Hello world from a simple WSGI application!\n']

(lsbaws) $ python webserver2.py wsgiapp:app
WSGIServer: Serving HTTP on port 8888 ...