1. Putting What We Learned Into Context – WSGI and Web Frameworks A290/A590, Fall 2014 10/16/2014

2. Announcement You can take until Saturday @ noon for Lab 7

3. CGI Limitations CGI the way it was originally designed is not practical for large applications – Remember: Every time a request for a CGI script is received, the web server will spawn a separate process (the Python interpreter) to execute the script. – This is very inefficient and doesn't scale to large numbers of requests

4. CGI Despite this limitation, the model introduced by CGI is still followed in large scale web applications Core Python Application Programming, Chun, 2012

5. CGI Alternatives Integrating the execution of script with the web server 1.The server provides an API. 2.A module implementing the API is written for a given language. 3.The module is loaded onto the server by the system administrator/web developer. 4.The script developer makes sure their code is written in a way that respects the module's conventions. Example: mod_python for Apache.mod_python Apache

6. CGI Alternatives Server integration – some caveats – the module needs to be thread safe – a buggy module can interfere with the performance of the server – to develop a module you usually have to code in the language the web server was written in (C for Apache) – the module has to keep up with changes to the language and the server API (happens rarely)

7. CGI Alternatives Running a CGI "server" outside of the web server – a single dedicated process responsible for managing requests for CGI scripts – Example: FastCGIFastCGI – Independent of the web server

8. CGI Alternatives Drawbacks to CGI servers as external processes 1.It has to provide support for the web server you want to run it with 2.It has to provide support for your language Both methods can be problematic for web framework which need to run on a wide variety of server configurations

9. WSGI WSGI = Web Server Gateway Interface – an interface between the web server and the web application – very general interface a WSGI application is a function that takes two parameters: a dictionary of environmental variables and a function to initialize the HTTP response def simple_wsgi_app(environ, start_response): status = '200 OK' headers = [('Content-type', 'text/plain')] start_response(status, headers) return ['Hello world!']

10. WSGI A WSGI server is also necessary – can be implemented as a stand-alone server – or it can be a third-party server module, e.g. Apache's mod_wsgi Advantages – web apps need to implement a single, very general and very simple interface – web apps are independent of server – a WSGI server doesn't do a lot, hence it is easy to implement

11. WSGI Conclusion – WSGI is a higher level abstraction than the server integration or stand-alone CGI server methods for executing CGI applications efficiently – it separates the application from having to worry about server-level details – is widely used in industry – a lot like the stand-alone approach, but more general (WSGI defines an interface, it is not an application)

12. Web Frameworks A web framework is a software framework that helps you with many low-level details inherent in web programming, such as: – reading and writing cookies – user authentication – session management – database access through ORM (object-relational mapping) – separation between appearance, business logic and data model with MVC That is, a lot of the tasks you implemented in the class – in fact you may have the start of your own framework in your solution to the last lab

13. Future Reading Django - Popular Python web framework Tutorial