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Understanding WSGI

Introduction to WSGI

WSGI, or Web Server Gateway Interface, is a specification that describes how a web server communicates with web applications. It is a common interface between web servers and web applications.

The Python community created WSGI as a standard for web servers to interface with web applications. It's been used as a standard for Python web application development since it was introduced in PEP 333.

The Basics of WSGI​

WSGI has two sides: the "server" or "gateway" side, and the "application" or "framework" side. The server side invokes a callable object that is provided by the application side. The server side and application side are composed of two distinct entities:

  1. The Server/Gateway side: The server side is responsible for sending the client's HTTP requests to the application side. It accomplishes this by invoking a callable object provided by the application.

  2. The Application/Framework side: The application side provides a callable object that can be invoked by the server. This callable object processes the HTTP requests and returns the HTTP responses.

This setup allows for a lot of flexibility, as you can mix and match servers and applications as required.

How WSGI Works​

When a user makes a request to a web server that is running a WSGI server application, the server creates a dictionary with all the CGI-like environmental variables. The server then calls the application, passing the dictionary and a callback function as parameters.

Here's a basic example of a WSGI application:

def application(environ, start_response):
    start_response('200 OK', [('Content-Type', 'text/html')])
    return ['Hello World!']

In this example, environ is a dictionary that contains all the HTTP request information. The start_response is a callback function supplied by the server to begin the HTTP response process.

WSGI in Flask​

Flask is a WSGI application and has a WSGI server built into it, which makes it easy for you to use Flask with any WSGI web server.

When you run your Flask application with the built-in server (using app.run()), Flask uses Werkzeug's WSGI server to run your application. However, while Flask's built-in server is sufficient for development, it's not efficient enough for production use. For deploying Flask applications in production, you should use a production-grade WSGI server such as Gunicorn or uWSGI.

Conclusion​

WSGI is a fundamental part of web applications in Python, and understanding it is key to understanding how your Flask application works. With WSGI, you can take your Flask application and run it on any WSGI-compatible server, which gives you a lot of flexibility and options for deployment.

In the next section, we will look at how to deploy a Flask application with a WSGI server. Happy coding!