Python and Web --- (2) Python as WSGI scripts

0 WSGI cheat-sheet

In the last blog,  I demonstrated python as a CGI application. Even CGI is so pervasive, it has drawbacks. Every CGI app has its own process, which means every time the client accesses your python script, the server has to spawn a new process in which a python virtual machine is running. As a result, the performance of CGI applications is not tolerant.  Welcome to WSGI, Web Server Gateway Interface, an interface specially designed for Python. The official document can be found at PEP 3333.

WSGI requires a compatible Web Server App call a python callable object named "application" with special-meaning arguments. The most common format is a function like:

application(environ, start_response)->string

Here, 

• "environ" is a dictionary that contains everything the Web Server App collects, including what the client puts in via HTTP
• "start_response" is a function reference for sending HTTP headers.
• Whatever this function returns will be collected by the Web Server app and sent to the client.

Simply put, CGI uses stdout and environment, while WSGI uses function arguments and function returns, as communication media between Web Server Apps and Web applications.

CGI is for every application written in any language, while WSGI is for Python only.

1 Apache mod_wsgi configuration

Apache supports WSGI through the mod_wsgi module. So first it needs to be installed.

$ yum install mod_wsgi

During the installing process, /etc/httpd/conf.modules.d/10-wsgi.conf was created automatically.

$ cat /etc/httpd/conf.modules.d/10-wsgi.conf

LoadModule wsgi_module modules/mod_wsgi.so

Now, let's configure a WSGI application the simplest way.

$ grep WSGI /etc/httpd/conf/httpd.conf

WSGIScriptAlias /wsgi /var/www/wsgi/app.wsgi

mod_wsgi has a similar syntax to CGI. Instead of "ScriptAlias", mod_wsgi uses "WSGIScriptAlias".

2 An example WSGI application

The following example WSGI app prints out the dictionary Apache provided to it.

$ cat /var/www/wsgi/app.wsgi

def application(environ, start_response):

        response_body = 'I am a WSGI script\n'

        for k,v in environ.items():

                response_body += '%s: %s\n' % (k,v)

        status = '200 OK'

        response_headers = [

                        ('Content-Type', 'text/plain'),

                        ('Content-Length', str(len(response_body)))

                        ]

        start_response(status, response_headers)

        return [response_body]

3 Test from a client

$ curl http://127.0.0.1/wsgi

I am a WSGI script

mod_wsgi.listener_port: 80

CONTEXT_DOCUMENT_ROOT: /var/www/html

SERVER_SOFTWARE: Apache/2.4.6 (CentOS) mod_wsgi/3.4 Python/2.7.5

SCRIPT_NAME: /wsgi

mod_wsgi.enable_sendfile: 0

mod_wsgi.handler_script:

SERVER_SIGNATURE:

REQUEST_METHOD: GET

PATH_INFO:

SERVER_PROTOCOL: HTTP/1.1

QUERY_STRING:

HTTP_USER_AGENT: curl/7.29.0

SERVER_NAME: 127.0.0.1

REMOTE_ADDR: 127.0.0.1

mod_wsgi.queue_start: 1578670748114076

mod_wsgi.request_handler: wsgi-script

wsgi.url_scheme: http

mod_wsgi.callable_object: application

SERVER_PORT: 80

wsgi.multiprocess: True

mod_wsgi.input_chunked: 0

SERVER_ADDR: 127.0.0.1

DOCUMENT_ROOT: /var/www/html

mod_wsgi.process_group:

SCRIPT_FILENAME: /var/www/wsgi/app.wsgi

SERVER_ADMIN: root@localhost

wsgi.input: <mod_wsgi.Input object at 0x7fd1922b2bf0>

HTTP_HOST: 127.0.0.1

CONTEXT_PREFIX:

wsgi.multithread: False

REQUEST_URI: /wsgi

HTTP_ACCEPT: */*

wsgi.version: (1, 0)

GATEWAY_INTERFACE: CGI/1.1

wsgi.run_once: False

wsgi.errors: <mod_wsgi.Log object at 0x7fd1922b2d30>

REMOTE_PORT: 32806

mod_wsgi.listener_host:

REQUEST_SCHEME: http

mod_wsgi.version: (3, 4)

mod_wsgi.application_group: fe80::f2d:cd77:6930:5a08|/wsgi

mod_wsgi.script_reloading: 1

wsgi.file_wrapper: <built-in method file_wrapper of mod_wsgi.Adapter object at 0x7fd1922b8be8>

UNIQUE_ID: XhianDNo9BBQWx5vOyLpegAAAAE

4 "application" can be more

As said before, besides functions, WSGI works for any callable object. e.g.

$ cat app.wsgi

class MyWSGI():

        def __call__(self, environ, start_response):

                start_response('200 OK', [('Content-Type', 'text/plain')])

                return ['Hello World!']

application = MyWSGI()