APortion

OfWSGIWith

Akilesh

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

Components of a web service

Client

Internet Web Server

MiddlewareApplication

Application1Application2

Webservers

● Pythonic– Gunicorn

– Tornado

– Twisted

– Zope

● Non Pythonic– Apache

– Just too many of them

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

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.

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.

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

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

Server receives request

Server extract request headersAnd populates 'environ'

Server callsApplication(environ, start_response)

Application executes Logic

Application callsstart_respons(status, headers)

start_responsesaves status and headers

Appliation returnsbody

Server returns status,headers and body

Server validates status,headers and body

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

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()

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