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Tornado is a Python web framework and asynchronous networking library, originally developed at FriendFeed. By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for long polling, WebSockets, and other applications that require a long-lived connection to each user.

Tornado can be roughly divided into four major components:

  • A web framework (including RequestHandler which is subclassed to create web applications, and various supporting classes).
  • Client- and server-side implementions of HTTP (HTTPServer and AsyncHTTPClient).
  • An asynchronous networking library including the classes IOLoop and IOStream, which serve as the building blocks for the HTTP components and can also be used to implement other protocols.
  • A coroutine library (tornado.gen) which allows asynchronous code to be written in a more straightforward way than chaining callbacks.

The Tornado web framework and HTTP server together offer a full-stack alternative to WSGI. While it is possible to use the Tornado web framework in a WSGI container (WSGIAdapter), or use the Tornado HTTP server as a container for other WSGI frameworks (WSGIContainer), each of these combinations has limitations and to take full advantage of Tornado you will need to use the Tornado’s web framework and HTTP server together.