Python WSGI introduction

Learn what is wsgi, why it exists and how to use it

1. A
Portion
Of
WSGI
With
Akilesh

2. What ? Why ? And How ?
•A specification for an interface between a web
server and web application
•Allows servers, frameworks, applications to
evolve independent of each other
•Allows easy intergration of middlewares into
existing applications and servers
•Outlines the Operations and Responses a server
expects from an application and vice versa

3. Components of a web service
Client
Internet Web Server
Middleware
Application
Application1
Application2

4. Webservers
●Pythonic
–Gunicorn
–Tornado
–Twisted
–Zope
●Non Pythonic
–Apache
–Just too many of them

5. Web Application
●Frameworks
–Allow quicker application development cycle
–Abstract all mundane jobs and allows user to concentrate on
logic/business
–Often use several tools like
●Templating engine – For generating page layout from datastructure
●PasteDeploy – For loading and chaining multiple web applications
●Routers – For routing a http request to a correct application
●Applications themselves may be chained to implement a
complex logic
●Some of the middleware may be third party applications

6. Examples
●Frameworks
–Django
–Flask
●Theming
–Bootstrap
●Templating engine
–Jinja
–Mako
●Middleware
–Ldap authentication using django
●Some of these may perform all other the above jobs. But the user
has the power to mix and match, thanks to wsgi.

7. Play by the rules
●Application has to be a callable. Anything with a
__call__ method with do. Methods, Classes,
Objects all are welcome.
●Application should handle any number of calls.
Server invokes application once for every http
request.
●Application returns and Iterable that yields
bytestrings.
●Server iterates over the Iterable and sends the
data without buffering.

8. More rules
●Application accepts two objects, name not a
contraint
–environ – A dictionary of request , os and wsgi params
–start_response – A callable that
●accepts two params
–Status – HTTP Status String object
–Headers -- HTTP Response headers List of tuples of header-value
–exec_info (optional) – typically the result of 'sys.exc_info()'
●And returns one
–Write – A callable that inturn accepts one argument
●Body – A bytestring to be sent to the client
–Write – Sends the bytestring to the client
–This is for legacy application only. New applications must not use this
interface

9. More rules
●Application must call start_response with appropriate
arguments before returning
●Server stores the Status and Headers, but will wait untill
Applications returns an Iterable and the Iterable has
yielded its first bytestring
●If Application sends an 'Content-Length' header to
start_response, Server should not send any more data
to than specified by the length
●If Application sends exec_info optional argument server
should erase the

10. Server receives request
Server extract request headers
And populates 'environ'
Server calls
Application(environ, start_response)
Application executes Logic
Application calls
start_respons(status, headers)
start_response
saves status and headers
Appliation returns
body
Server returns status,
headers and body
Server validates status,
headers and body

11. Middlewares
●Plays both server and application game
●Secretly slips between server, application and
other middlewares
●Absolutely Transparent
●Used to integrate apps built using different
frameworks, third party modules, routing, load
balancing, anything else you wish for

12. Getting Dirty
from wsgiref.simple_server import make_server
def my_application(environ, start_response):
response_headers = [('Content-Type', 'text/html'),
('Akilesh', 'Cool head')]
status_code = '200 OK'
writer = start_response(status_code, response_headers)
# ignore the writer
# it exists only for legacy applications
ret = {'request method': environ['REQUEST_METHOD'],
'path': environ['PATH_INFO'],
'query string': environ['QUERY_STRING']}
return str(ret)
httpd = make_server('localhost', 9090, my_application)
httpd.serve_forever()

13. Thank you
[email protected]
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