# Tornado 

[Tornado](http://www.tornadoweb.org/) is a Python web framework and asynchronous networking library, originally developed at [FriendFeed](https://en.wikipedia.org/wiki/FriendFeed). By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for [long polling](http://en.wikipedia.org/wiki/Push_technology#Long_polling), [WebSockets](http://en.wikipedia.org/wiki/WebSocket), and other applications that require a long-lived connection to each user.

Tornado can be roughly divided into three major components:

-   A web framework (including [`RequestHandler`](https://www.tornadoweb.org/en/stable/web.html#tornado.web.RequestHandler "tornado.web.RequestHandler") which is subclassed to create web applications, and various supporting classes).
    
-   Client- and server-side implementions of HTTP ([`HTTPServer`](https://www.tornadoweb.org/en/stable/httpserver.html#tornado.httpserver.HTTPServer "tornado.httpserver.HTTPServer") and [`AsyncHTTPClient`](https://www.tornadoweb.org/en/stable/httpclient.html#tornado.httpclient.AsyncHTTPClient "tornado.httpclient.AsyncHTTPClient")).
    
-   An asynchronous networking library including the classes [`IOLoop`](https://www.tornadoweb.org/en/stable/ioloop.html#tornado.ioloop.IOLoop "tornado.ioloop.IOLoop") and [`IOStream`](https://www.tornadoweb.org/en/stable/iostream.html#tornado.iostream.IOStream "tornado.iostream.IOStream"), which serve as the building blocks for the HTTP components and can also be used to implement other protocols.
    

The Tornado web framework and HTTP server together offer a full-stack alternative to [WSGI](http://www.python.org/dev/peps/pep-3333/). While it is possible to use the Tornado HTTP server as a container for other WSGI frameworks ([`WSGIContainer`](https://www.tornadoweb.org/en/stable/wsgi.html#tornado.wsgi.WSGIContainer "tornado.wsgi.WSGIContainer")), this combination has limitations and to take full advantage of Tornado you will need to use Tornado’s web framework and HTTP server together.

----
Example of concurrent web spider with ```tornado.queues```

```
#!/usr/bin/env python3

import asyncio
import time
from datetime import timedelta

from html.parser import HTMLParser
from urllib.parse import urljoin, urldefrag

from tornado import gen, httpclient, queues

base_url = "http://www.tornadoweb.org/en/stable/"
concurrency = 10

async def get_links_from_url(url):
    """Download the page at `url` and parse it for links.

    Returned links have had the fragment after `#` removed, and have been made
    absolute so, e.g. the URL 'gen.html#tornado.gen.coroutine' becomes
    'http://www.tornadoweb.org/en/stable/gen.html'.
    """
    response = await httpclient.AsyncHTTPClient().fetch(url)
    print("fetched %s" % url)

    html = response.body.decode(errors="ignore")
    return [urljoin(url, remove_fragment(new_url)) for new_url in get_links(html)]

def remove_fragment(url):
    pure_url, frag = urldefrag(url)
    return pure_url

def get_links(html):
    class URLSeeker(HTMLParser):
        def __init__(self):
            HTMLParser.__init__(self)
            self.urls = []

        def handle_starttag(self, tag, attrs):
            href = dict(attrs).get("href")
            if href and tag == "a":
                self.urls.append(href)

    url_seeker = URLSeeker()
    url_seeker.feed(html)
    return url_seeker.urls

async def main():
    q = queues.Queue()
    start = time.time()
    fetching, fetched, dead = set(), set(), set()

    async def fetch_url(current_url):
        if current_url in fetching:
            return

        print("fetching %s" % current_url)
        fetching.add(current_url)
        urls = await get_links_from_url(current_url)
        fetched.add(current_url)

        for new_url in urls:
            # Only follow links beneath the base URL
            if new_url.startswith(base_url):
                await q.put(new_url)

    async def worker():
        async for url in q:
            if url is None:
                return
            try:
                await fetch_url(url)
            except Exception as e:
                print("Exception: %s %s" % (e, url))
                dead.add(url)
            finally:
                q.task_done()

    await q.put(base_url)

    # Start workers, then wait for the work queue to be empty.
    workers = gen.multi([worker() for _ in range(concurrency)])
    await q.join(timeout=timedelta(seconds=300))
    assert fetching == (fetched | dead)
    print("Done in %d seconds, fetched %s URLs." % (time.time() - start, len(fetched)))
    print("Unable to fetch %s URLS." % len(dead))

    # Signal all the workers to exit.
    for _ in range(concurrency):
        await q.put(None)
    await workers

if __name__ == "__main__":
    asyncio.run(main())
```