# Running and deploying

Since Tornado supplies its own HTTPServer, running and deploying it is a little different from other Python web frameworks. Instead of configuring a WSGI container to find your application, you write a `main()` function that starts the server:

    def main():
        app = make_app()
        app.listen(8888)
        IOLoop.current().start()

    if __name__ == '__main__':
        main()

Configure your operating system or process manager to run this program to start the server. Please note that it may be necessary to increase the number of open files per process (to avoid “Too many open files”-Error). To raise this limit (setting it to 50000 for example) you can use the `ulimit` command, modify `/etc/security/limits.conf` or set `minfds` in your supervisord config.

## Processes and ports

Due to the Python GIL (Global Interpreter Lock), it is necessary to run multiple Python processes to take full advantage of multi-CPU machines. Typically it is best to run one process per CPU.

Tornado includes a built-in multi-process mode to start several processes at once (note that multi-process mode does not work on Windows). This requires a slight alteration to the standard main function:

    def main():
        app = make_app()
        server = tornado.httpserver.HTTPServer(app)
        server.bind(8888)
        server.start(0)  # forks one process per cpu
        IOLoop.current().start()

This is the easiest way to start multiple processes and have them all share the same port, although it has some limitations. First, each child process will have its own `IOLoop`, so it is important that nothing touches the global `IOLoop` instance (even indirectly) before the fork. Second, it is difficult to do zero-downtime updates in this model. Finally, since all the processes share the same port it is more difficult to monitor them individually.

For more sophisticated deployments, it is recommended to start the processes independently, and have each one listen on a different port. The “process groups” feature of supervisord is one good way to arrange this. When each process uses a different port, an external load balancer such as HAProxy or nginx is usually needed to present a single address to outside visitors.

## Running behind a load balancer

When running behind a load balancer like nginx, it is recommended to pass `xheaders=True` to the `HTTPServer` constructor. This will tell Tornado to use headers like `X-Real-IP` to get the user’s IP address instead of attributing all traffic to the balancer’s IP address.

This is a barebones nginx config file that is structurally similar to the one we use at FriendFeed. It assumes nginx and the Tornado servers are running on the same machine, and the four Tornado servers are running on ports 8000 - 8003:

    user nginx;
    worker_processes 1;

    error_log /var/log/nginx/error.log;
    pid /var/run/nginx.pid;

    events {
        worker_connections 1024;
        use epoll;
    }

    http {
        # Enumerate all the Tornado servers here
        upstream frontends {
            server 127.0.0.1:8000;
            server 127.0.0.1:8001;
            server 127.0.0.1:8002;
            server 127.0.0.1:8003;
        }

        include /etc/nginx/mime.types;
        default_type application/octet-stream;

        access_log /var/log/nginx/access.log;

        keepalive_timeout 65;
        proxy_read_timeout 200;
        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        gzip on;
        gzip_min_length 1000;
        gzip_proxied any;
        gzip_types text/plain text/html text/css text/xml
                   application/x-javascript application/xml
                   application/atom+xml text/javascript;

        # Only retry if there was a communication error, not a timeout
        # on the Tornado server (to avoid propagating "queries of death"
        # to all frontends)
        proxy_next_upstream error;

        server {
            listen 80;

            # Allow file uploads
            client_max_body_size 50M;

            location ^~ /static/ {
                root /var/www;
                if ($query_string) {
                    expires max;
                }
            }
            location = /favicon.ico {
                rewrite (.*) /static/favicon.ico;
            }
            location = /robots.txt {
                rewrite (.*) /static/robots.txt;
            }

            location / {
                proxy_pass_header Server;
                proxy_set_header Host $http_host;
                proxy_redirect off;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Scheme $scheme;
                proxy_pass http://frontends;
            }
        }
    }

## Static files and aggressive file caching

You can serve static files from Tornado by specifying the `static_path` setting in your application:

    settings = {
        "static_path": os.path.join(os.path.dirname(__file__), "static"),
        "cookie_secret": "__TODO:_GENERATE_YOUR_OWN_RANDOM_VALUE_HERE__",
        "login_url": "/login",
        "xsrf_cookies": True,
    }
    application = tornado.web.Application([
        (r"/", MainHandler),
        (r"/login", LoginHandler),
        (r"/(apple-touch-icon\.png)", tornado.web.StaticFileHandler,
         dict(path=settings['static_path'])),
    ], **settings)

This setting will automatically make all requests that start with `/static/` serve from that static directory, e.g. `http://localhost:8888/static/foo.png` will serve the file `foo.png` from the specified static directory. We also automatically serve `/robots.txt` and `/favicon.ico` from the static directory (even though they don’t start with the `/static/` prefix).

In the above settings, we have explicitly configured Tornado to serve `apple-touch-icon.png` from the root with the `StaticFileHandler`, though it is physically in the static file directory. (The capturing group in that regular expression is necessary to tell `StaticFileHandler` the requested filename; recall that capturing groups are passed to handlers as method arguments.) You could do the same thing to serve e.g. `sitemap.xml` from the site root. Of course, you can also avoid faking a root `apple-touch-icon.png` by using the appropriate `<link />` tag in your HTML.

To improve performance, it is generally a good idea for browsers to cache static resources aggressively so browsers won’t send unnecessary `If-Modified-Since` or `Etag` requests that might block the rendering of the page. Tornado supports this out of the box with static content versioning.

To use this feature, use the `static_url` method in your templates rather than typing the URL of the static file directly in your HTML:

    <html>
       <head>
          <title>FriendFeed - {{ _("Home") }}</title>
       </head>
       <body>
         <div><img src="{{ static_url("images/logo.png") }}"/></div>
       </body>
     </html>

The `static_url()` function will translate that relative path to a URI that looks like `/static/images/logo.png?v=aae54`. The `v` argument is a hash of the content in `logo.png`, and its presence makes the Tornado server send cache headers to the user’s browser that will make the browser cache the content indefinitely.

Since the `v` argument is based on the content of the file, if you update a file and restart your server, it will start sending a new `v` value, so the user’s browser will automatically fetch the new file. If the file’s contents don’t change, the browser will continue to use a locally cached copy without ever checking for updates on the server, significantly improving rendering performance.

In production, you probably want to serve static files from a more optimized static file server like nginx. You can configure almost any web server to recognize the version tags used by `static_url()` and set caching headers accordingly. Here is the relevant portion of the nginx configuration we use at FriendFeed:

    location /static/ {
        root /var/friendfeed/static;
        if ($query_string) {
            expires max;
        }
     }

## Debug mode and automatic reloading

If you pass `debug=True` to the `Application` constructor, the app will be run in debug/development mode. In this mode, several features intended for convenience while developing will be enabled (each of which is also available as an individual flag; if both are specified the individual flag takes precedence):

- `autoreload=True`: The app will watch for changes to its source files and reload itself when anything changes. This reduces the need to manually restart the server during development. However, certain failures (such as syntax errors at import time) can still take the server down in a way that debug mode cannot currently recover from.

- `compiled_template_cache=False`: Templates will not be cached.

- `static_hash_cache=False`: Static file hashes (used by the `static_url` function) will not be cached.

- `serve_traceback=True`: When an exception in a `RequestHandler` is not caught, an error page including a stack trace will be generated.

Autoreload mode is not compatible with the multi-process mode of `HTTPServer`. You must not give `HTTPServer.start` an argument other than 1 (or call `tornado.process.fork_processes`) if you are using autoreload mode.

The automatic reloading feature of debug mode is available as a standalone module in `tornado.autoreload`. The two can be used in combination to provide extra robustness against syntax errors: set `autoreload=True` within the app to detect changes while it is running, and start it with `python -m tornado.autoreload myserver.py` to catch any syntax errors or other errors at startup.

Reloading loses any Python interpreter command-line arguments (e.g. `-u`) because it re-executes Python using `sys.executable` and `sys.argv`. Additionally, modifying these variables will cause reloading to behave incorrectly.

On some platforms (including Windows and Mac OSX prior to 10.6), the process cannot be updated “in-place”, so when a code change is detected the old server exits and a new one starts. This has been known to confuse some IDEs.