documentation.txt

Overview
--------
[FriendFeed](http://friendfeed.com/)'s web server is a relatively simple,
non-blocking web server written in Python. The FriendFeed application is
written using a web framework that looks a bit like
[web.py](http://webpy.org/) or Google's
[webapp](http://code.google.com/appengine/docs/python/tools/webapp/),
but with additional tools and optimizations to take advantage of the
non-blocking web server and tools.

[Tornado](http://github.com/facebook/tornado) is an open source
version of this web server and some of the tools we use most often at
FriendFeed. The framework is distinct from most mainstream web server
frameworks (and certainly most Python frameworks) because it is
non-blocking and reasonably fast. Because it is non-blocking
and uses [epoll](http://www.kernel.org/doc/man-pages/online/pages/man4/epoll.4.html), it can handle 1000s of simultaneous standing connections,
which means the framework is ideal for real-time web services. We built the
web server specifically to handle FriendFeed's real-time features —
every active user of FriendFeed maintains an open connection to the
FriendFeed servers. (For more information on scaling servers to support
thousands of clients, see
[The C10K problem](http://www.kegel.com/c10k.html).)

Here is the canonical "Hello, world" example app:

    import tornado.ioloop
    import tornado.web

    class MainHandler(tornado.web.RequestHandler):
        def get(self):
            self.write("Hello, world")

    application = tornado.web.Application([
        (r"/", MainHandler),
    ])

    if __name__ == "__main__":
        application.listen(8888)
        tornado.ioloop.IOLoop.instance().start()

See [Tornado walkthrough](#tornado-walkthrough) below for a detailed
walkthrough of the `tornado.web` package.

We attempted to clean up the code base to reduce interdependencies between
modules, so you should (theoretically) be able to use any of the modules
independently in your project without using the whole package.


Download and install
--------------------
  <p><b>Automatic installation:</b> Tornado is listed in <a href="http://pypi.python.org/pypi/tornado">PyPI</a> and can be installed with <code>pip</code> or <code>easy_install</code>.  If you do not already have libcurl installed you may need to install it separately; see the prerequisites section below.  Note that the source distribution includes demo applications that are not present when Tornado is installed using <code>pip</code> or <code>easy_install</code></p>

  <p><b>Manual installation:</b> Download <a href="http://github.com/downloads/facebook/tornado/tornado-1.2.1.tar.gz">tornado-1.2.1.tar.gz</a></p>
  <pre><code>tar xvzf tornado-1.2.1.tar.gz
cd tornado-1.2.1
python setup.py build
sudo python setup.py install</code></pre>
  <p>The Tornado source code is <a href="http://github.com/facebook/tornado">hosted on GitHub</a>.  On Python 2.6+, it is also possible to simply add the tornado directory to your <code>PYTHONPATH</code> instead of building with <code>setup.py</code>, since the standard library includes <code>epoll</code> support.</p>

  <h3>Prerequisites</h3>
  <p>Tornado has been tested on Python 2.5, 2.6, and 2.7. To use all of the features of Tornado, you need to have <a href="http://pycurl.sourceforge.net/">PycURL</a> (version 7.18.2 or higher) and (for Python 2.5 only) <a href="http://pypi.python.org/pypi/simplejson/">simplejson</a> installed (Python 2.6 includes JSON support in the standard library so simplejson is not needed). Complete installation instructions for Mac OS X and Ubuntu are included below for convenience.</p>
  <p style="font-weight:bold">Mac OS X 10.6 (Python 2.6+)</p>
  <pre><code>sudo easy_install setuptools pycurl</code></pre>

  <p style="font-weight:bold">Ubuntu Linux (Python 2.6+)</p>
  <pre><code>sudo apt-get install python-pycurl</code></pre>

  <p style="font-weight:bold">Ubuntu Linux (Python 2.5)</p>
  <pre><code>sudo apt-get install python-dev python-pycurl python-simplejson</code></pre>


Module index
------------
The most important module is [`web`](http://github.com/facebook/tornado/blob/master/tornado/web.py), which is the web framework
that includes most of the meat of the Tornado package. The other modules
are tools that make `web` more useful. See
[Tornado walkthrough](#tornado-walkthrough) below for a detailed
walkthrough of the `web` package.

### Main modules
  * [`web`](http://github.com/facebook/tornado/blob/master/tornado/web.py) - The web framework on which FriendFeed is built. `web` incorporates most of the important features of Tornado
  * [`escape`](http://github.com/facebook/tornado/blob/master/tornado/escape.py) - XHTML, JSON, and URL encoding/decoding methods
  * [`database`](http://github.com/facebook/tornado/blob/master/tornado/database.py) - A simple wrapper around `MySQLdb` to make MySQL easier to use
  * [`template`](http://github.com/facebook/tornado/blob/master/tornado/template.py) - A Python-based web templating language
  * [`httpclient`](http://github.com/facebook/tornado/blob/master/tornado/httpclient.py) - A non-blocking HTTP client designed to work with `web` and `httpserver`
  * [`auth`](http://github.com/facebook/tornado/blob/master/tornado/auth.py) - Implementation of third party authentication and authorization schemes (Google OpenID/OAuth, Facebook Platform, Yahoo BBAuth, FriendFeed OpenID/OAuth, Twitter OAuth)
  * [`locale`](http://github.com/facebook/tornado/blob/master/tornado/locale.py) - Localization/translation support
  * [`options`](http://github.com/facebook/tornado/blob/master/tornado/options.py) - Command line and config file parsing, optimized for server environments

### Low-level modules
  * [`httpserver`](http://github.com/facebook/tornado/blob/master/tornado/httpserver.py) - A very simple HTTP server built on which `web` is built
  * [`iostream`](http://github.com/facebook/tornado/blob/master/tornado/iostream.py) - A simple wrapper around non-blocking sockets to aide common reading and writing patterns
  * [`ioloop`](http://github.com/facebook/tornado/blob/master/tornado/ioloop.py) - Core I/O loop

### Random modules
  * [`s3server`](http://github.com/facebook/tornado/blob/master/tornado/s3server.py) - A web server that implements most of the [Amazon S3](http://aws.amazon.com/s3/) interface, backed by local file storage


Tornado walkthrough
-------------------

### Request handlers and request arguments

A Tornado web application maps URLs or URL patterns to subclasses of
`tornado.web.RequestHandler`. Those classes define `get()` or `post()`
methods to handle HTTP `GET` or `POST` requests to that URL.

This code maps the root URL `/` to `MainHandler` and the URL pattern
`/story/([0-9]+)` to `StoryHandler`. Regular expression groups are passed
as arguments to the `RequestHandler` methods:

    class MainHandler(tornado.web.RequestHandler):
        def get(self):
            self.write("You requested the main page")

    class StoryHandler(tornado.web.RequestHandler):
        def get(self, story_id):
            self.write("You requested the story " + story_id)

    application = tornado.web.Application([
        (r"/", MainHandler),
        (r"/story/([0-9]+)", StoryHandler),
    ])

You can get query string arguments and parse `POST` bodies with the
`get_argument()` method:

    class MainHandler(tornado.web.RequestHandler):
        def get(self):
            self.write('<html><body><form action="/" method="post">'
                       '<input type="text" name="message">'
                       '<input type="submit" value="Submit">'
                       '</form></body></html>')

        def post(self):
            self.set_header("Content-Type", "text/plain")
            self.write("You wrote " + self.get_argument("message"))

Uploaded files are available in `self.request.files`, which maps names
(the name of the HTML `<input type="file">` element) to a list of
files. Each file is a dictionary of the form `{"filename":...,
"content_type":..., "body":...}`.

If you want to send an error response to the client, e.g., 403 Unauthorized,
you can just raise a `tornado.web.HTTPError` exception:

    if not self.user_is_logged_in():
        raise tornado.web.HTTPError(403)

The request handler can access the object representing the current request
with `self.request`. The `HTTPRequest` object includes a number of useful
attributes, including:

 * `arguments` - all of the `GET` and `POST` arguments
 * `files` - all of the uploaded files (via `multipart/form-data` POST requests)
 * `path` - the request path (everything before the `?`)
 * `headers` - the request headers

See the class definition for `HTTPRequest` in `httpserver` for a complete list
of attributes.

### Overriding RequestHandler methods

In addition to `get()`/`post()`/etc, certain other methods in `RequestHandler`
are designed to be overridden by subclasses when necessary.  On every request,
the following sequence of calls takes place:

 1. A new RequestHandler object is created on each request
 2. `initialize()` is called with keyword arguments from the `Application`
    configuration. (the `initialize` method is new in Tornado 1.1; in older
    versions subclasses would override `__init__` instead).  `initialize`
    should typically just save the arguments passed into member variables;
    it may not produce any output or call methods like `send_error`.
 3. `prepare()` is called.  This is most useful in a base class shared
    by all of your handler subclasses, as `prepare` is called no matter
    which HTTP method is used.  `prepare` may produce output; if it calls
    `finish` (or `send_error`, etc), processing stops here.
 4. One of the HTTP methods is called: `get()`, `post()`, `put()`, etc.
    If the URL regular expression contains capturing groups, they are
    passed as arguments to this method.

Here is an example demonstrating the `initialize()` method:

    class ProfileHandler(RequestHandler):
        def initialize(self, database):
            self.database = database

        def get(self, username):
            ...

    app = Application([
        (r'/user/(.*)', ProfileHandler, dict(database=database)),
        ])

Other methods designed for overriding include:

 * `get_error_html(self, status_code, exception=None, **kwargs)` - returns
   HTML (as a string) for use on error pages.
 * `get_current_user(self)` - see
   [User Authentication](#user-authentication) below
 * `get_user_locale(self)` - returns `locale` object to use for the current
   user
 * `get_login_url(self)` - returns login url to be used by the
   `@authenticated` decorator (default is in `Application` settings)
 * `get_template_path(self)` - returns location of template files (default is
   in `Application` settings)

### Templates

You can use any template language supported by Python, but Tornado ships
with its own templating language that is a lot faster and more flexible
than many of the most popular templating systems out there. See the
[`template`](http://github.com/facebook/tornado/blob/master/tornado/template.py) module documentation for complete documentation.

A Tornado template is just HTML (or any other text-based format) with
Python control sequences and expressions embedded within the markup:

    <html>
       <head>
          <title>{{ title }}</title>
       </head>
       <body>
         <ul>
           {% for item in items %}
             <li>{{ escape(item) }}</li>
           {% end %}
         </ul>
       </body>
     </html>

If you saved this template as "template.html" and put it in the same
directory as your Python file, you could render this template with:

    class MainHandler(tornado.web.RequestHandler):
        def get(self):
            items = ["Item 1", "Item 2", "Item 3"]
            self.render("template.html", title="My title", items=items)

Tornado templates support *control statements* and *expressions*. Control
statements are surronded by `{%` and `%}`, e.g., `{% if len(items) > 2 %}`.
Expressions are surrounded by `{{` and `}}`, e.g., `{{ items[0] }}`.

Control statements more or less map exactly to Python statements. We support
`if`, `for`, `while`, and `try`, all of which are terminated with `{% end %}`.
We also support *template inheritance* using the `extends` and `block`
statements, which are described in detail in the documentation for the
[`template` module](http://github.com/facebook/tornado/blob/master/tornado/template.py).

Expressions can be any Python expression, including function calls.
Template code is executed in a namespace that includes the following objects
and functions (Note that this list applies to templates rendered using
`RequestHandler.render` and `render_string`.  If you're using the `template`
module directly outside of a `RequestHandler` many of these entries are
not present).

 * `escape`: alias for `tornado.escape.xhtml_escape`
 * `xhtml_escape`: alias for `tornado.escape.xhtml_escape`
 * `url_escape`: alias for `tornado.escape.url_escape`
 * `json_encode`: alias for `tornado.escape.json_encode`
 * `squeeze`: alias for `tornado.escape.squeeze`
 * `linkify`: alias for `tornado.escape.linkify`
 * `datetime`: the Python `datetime` module
 * `handler`: the current `RequestHandler` object
 * `request`: alias for `handler.request`
 * `current_user`: alias for `handler.current_user`
 * `locale`: alias for `handler.locale`
 * `_`: alias for `handler.locale.translate`
 * `static_url`: alias for `handler.static_url`
 * `xsrf_form_html`: alias for `handler.xsrf_form_html`
 * `reverse_url`: alias for `Application.reverse_url`
 * All entries from the `ui_methods` and `ui_modules` `Application` settings
 * Any keyword arguments passed to `render` or `render_string`

When you are building a real application, you are going to want to use
all of the features of Tornado templates, especially template inheritance.
Read all about those features in the [`template` module](http://github.com/facebook/tornado/blob/master/tornado/template.py)
section (some features, including `UIModules` are implemented in the
`web` module)

Under the hood, Tornado templates are translated directly to Python.
The expressions you include in your template are copied verbatim into
a Python function representing your template. We don't try to prevent
anything in the template language; we created it explicitly to provide
the flexibility that other, stricter templating systems prevent.
Consequently, if you write random stuff inside of your template expressions,
you will get random Python errors when you execute the template.


### Cookies and secure cookies

You can set cookies in the user's browser with the `set_cookie` method:

    class MainHandler(tornado.web.RequestHandler):
        def get(self):
            if not self.get_cookie("mycookie"):
                self.set_cookie("mycookie", "myvalue")
                self.write("Your cookie was not set yet!")
            else:
                self.write("Your cookie was set!")

Cookies are easily forged by malicious clients. If you need to set cookies
to, e.g., save the user ID of the currently logged in user, you need to
sign your cookies to prevent forgery. Tornado supports this out of the
box with the `set_secure_cookie` and `get_secure_cookie` methods. To use
these methods, you need to specify a secret key named `cookie_secret` when
you create your application. You can pass in application settings as keyword
arguments to your application:

    application = tornado.web.Application([
        (r"/", MainHandler),
    ], cookie_secret="61oETzKXQAGaYdkL5gEmGeJJFuYh7EQnp2XdTP1o/Vo=")

Signed cookies contain the encoded value of the cookie in addition to a
timestamp and an [HMAC](http://en.wikipedia.org/wiki/HMAC) signature. If the
cookie is old or if the signature doesn't match, `get_secure_cookie` will
return `None` just as if the cookie isn't set. The secure version of the
example above:

    class MainHandler(tornado.web.RequestHandler):
        def get(self):
            if not self.get_secure_cookie("mycookie"):
                self.set_secure_cookie("mycookie", "myvalue")
                self.write("Your cookie was not set yet!")
            else:
                self.write("Your cookie was set!")


### User authentication

The currently authenticated user is available in every request handler
as `self.current_user`, and in every template as `current_user`. By
default, `current_user` is `None`.

To implement user authentication in your application, you need to
override the `get_current_user()` method in your request handlers to
determine the current user based on, e.g., the value of a cookie.
Here is an example that lets users log into the application simply
by specifying a nickname, which is then saved in a cookie:

    class BaseHandler(tornado.web.RequestHandler):
        def get_current_user(self):
            return self.get_secure_cookie("user")

    class MainHandler(BaseHandler):
        def get(self):
            if not self.current_user:
                self.redirect("/login")
                return
            name = tornado.escape.xhtml_escape(self.current_user)
            self.write("Hello, " + name)

    class LoginHandler(BaseHandler):
        def get(self):
            self.write('<html><body><form action="/login" method="post">'
                       'Name: <input type="text" name="name">'
                       '<input type="submit" value="Sign in">'
                       '</form></body></html>')

        def post(self):
            self.set_secure_cookie("user", self.get_argument("name"))
            self.redirect("/")

    application = tornado.web.Application([
        (r"/", MainHandler),
        (r"/login", LoginHandler),
    ], cookie_secret="61oETzKXQAGaYdkL5gEmGeJJFuYh7EQnp2XdTP1o/Vo=")

You can require that the user be logged in using the
[Python decorator](http://www.python.org/dev/peps/pep-0318/)
`tornado.web.authenticated`. If a request goes to a method with this
decorator, and the user is not logged in, they will be redirected to
`login_url` (another application setting). The example above could
be rewritten:

    class MainHandler(BaseHandler):
        @tornado.web.authenticated
        def get(self):
            name = tornado.escape.xhtml_escape(self.current_user)
            self.write("Hello, " + name)

    settings = {
        "cookie_secret": "61oETzKXQAGaYdkL5gEmGeJJFuYh7EQnp2XdTP1o/Vo=",
        "login_url": "/login",
    }
    application = tornado.web.Application([
        (r"/", MainHandler),
        (r"/login", LoginHandler),
    ], **settings)

If you decorate `post()` methods with the `authenticated` decorator, and
the user is not logged in, the server will send a `403` response.

Tornado comes with built-in support for third-party authentication schemes
like Google OAuth. See the [`auth` module](http://github.com/facebook/tornado/blob/master/tornado/auth.py) for more details. Check
out the Tornado Blog example application for a complete example that
uses authentication (and stores user data in a MySQL database).


### Cross-site request forgery protection

[Cross-site request forgery](http://en.wikipedia.org/wiki/Cross-site_request_forgery), or XSRF, is a common problem for personalized web applications. See the
[Wikipedia article](http://en.wikipedia.org/wiki/Cross-site_request_forgery)
for more information on how XSRF works.

The generally accepted solution to prevent XSRF is to cookie every user
with an unpredictable value and include that value as an additional
argument with every form submission on your site. If the cookie and the
value in the form submission do not match, then the request is likely
forged.

Tornado comes with built-in XSRF protection. To include it in your site,
include the application setting `xsrf_cookies`:

    settings = {
        "cookie_secret": "61oETzKXQAGaYdkL5gEmGeJJFuYh7EQnp2XdTP1o/Vo=",
        "login_url": "/login",
        "xsrf_cookies": True,
    }
    application = tornado.web.Application([
        (r"/", MainHandler),
        (r"/login", LoginHandler),
    ], **settings)

If `xsrf_cookies` is set, the Tornado web application will set the
`_xsrf` cookie for all users and reject all `POST`, `PUT`, and
`DELETE` requests that do not contain a correct `_xsrf` value. If you
turn this setting on, you need to instrument all forms that submit via
`POST` to contain this field. You can do this with the special
function `xsrf_form_html()`, available in all templates:

    <form action="/new_message" method="post">
      {{ xsrf_form_html() }}
      <input type="text" name="message"/>
      <input type="submit" value="Post"/>
    </form>

If you submit AJAX `POST` requests, you will also need to instrument your
JavaScript to include the `_xsrf` value with each request. This is the
[jQuery](http://jquery.com/) function we use at FriendFeed for AJAX `POST`
requests that automatically adds the `_xsrf` value to all requests:

    function getCookie(name) {
        var r = document.cookie.match("\\b" + name + "=([^;]*)\\b");
        return r ? r[1] : undefined;
    }

    jQuery.postJSON = function(url, args, callback) {
        args._xsrf = getCookie("_xsrf");
        $.ajax({url: url, data: $.param(args), dataType: "text", type: "POST",
	        success: function(response) {
            callback(eval("(" + response + ")"));
        }});
    };

For `PUT` and `DELETE` requests (as well as `POST` requests that do not
use form-encoded arguments), the XSRF token may also be passed via
an HTTP header named `X-XSRFToken`.


### 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": "61oETzKXQAGaYdkL5gEmGeJJFuYh7EQnp2XdTP1o/Vo=",
        "login_url": "/login",
        "xsrf_cookies": True,
    }
    application = tornado.web.Application([
        (r"/", MainHandler),
        (r"/login", LoginHandler),
    ], **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](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).

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](http://nginx.net/). You can
configure most any web server to support these caching semantics. Here
is the nginx configuration we use at FriendFeed:

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


### Localization

The locale of the current user (whether they are logged in or not) is
always available as `self.locale` in the request handler and as `locale`
in templates. The name of the locale (e.g., `en_US`) is available as
`locale.name`, and you can translate strings with the `locale.translate`
method. Templates also have the global function call `_()` available
for string translation. The translate function has two forms:

    _("Translate this string")

which translates the string directly based on the current locale, and

    _("A person liked this", "%(num)d people liked this", len(people)) % {"num": len(people)}

which translates a string that can be singular or plural based on the value
of the third argument. In the example above, a translation of the first
string will be returned if `len(people)` is `1`, or a translation of the
second string will be returned otherwise.

The most common pattern for translations is to use Python named placeholders
for variables (the `%(num)d` in the example above) since placeholders can
move around on translation.

Here is a properly localized template:

    <html>
       <head>
          <title>FriendFeed - {{ _("Sign in") }}</title>
       </head>
       <body>
         <form action="{{ request.path }}" method="post">
           <div>{{ _("Username") }} <input type="text" name="username"/></div>
           <div>{{ _("Password") }} <input type="password" name="password"/></div>
           <div><input type="submit" value="{{ _("Sign in") }}"/></div>
           {{ xsrf_form_html() }}
         </form>
       </body>
     </html>

By default, we detect the user's locale using the `Accept-Language` header
sent by the user's browser. We choose `en_US` if we can't find an appropriate
`Accept-Language` value. If you let user's set their locale as a preference,
you can override this default locale selection by overriding `get_user_locale`
in your request handler:

    class BaseHandler(tornado.web.RequestHandler):
        def get_current_user(self):
            user_id = self.get_secure_cookie("user")
            if not user_id: return None
            return self.backend.get_user_by_id(user_id)

        def get_user_locale(self):
            if "locale" not in self.current_user.prefs:
                # Use the Accept-Language header
                return None
            return self.current_user.prefs["locale"]

If `get_user_locale` returns `None`, we fall back on the `Accept-Language`
header.

You can load all the translations for your application using the
`tornado.locale.load_translations` method. It takes in the name of the
directory which should contain CSV files named after the locales whose
translations they contain, e.g., `es_GT.csv` or `fr_CA.csv`. The method
loads all the translations from those CSV files and infers the list of
supported locales based on the presence of each CSV file. You typically
call this method once in the `main()` method of your server:

    def main():
        tornado.locale.load_translations(
            os.path.join(os.path.dirname(__file__), "translations"))
        start_server()

You can get the list of supported locales in your application with
`tornado.locale.get_supported_locales()`. The user's locale is chosen to
be the closest match based on the supported locales. For example, if the
user's locale is `es_GT`, and the `es` locale is supported, `self.locale`
will be `es` for that request. We fall back on `en_US` if no close match
can be found.

See the [`locale` module](http://github.com/facebook/tornado/blob/master/tornado/locale.py) documentation for detailed information
on the CSV format and other localization methods.


### UI modules

Tornado supports *UI modules* to make it easy to support standard, reusable
UI widgets across your application. UI modules are like special functional
calls to render components of your page, and they can come packaged with
their own CSS and JavaScript.

For example, if you are implementing a blog, and you want to have
blog entries appear on both the blog home page and on each blog entry page,
you can make an `Entry` module to render them on both pages. First, create
a Python module for your UI modules, e.g., `uimodules.py`:

    class Entry(tornado.web.UIModule):
        def render(self, entry, show_comments=False):
            return self.render_string(
                "module-entry.html", show_comments=show_comments)

Tell Tornado to use `uimodules.py` using the `ui_modules` setting in your
application:

    class HomeHandler(tornado.web.RequestHandler):
        def get(self):
            entries = self.db.query("SELECT * FROM entries ORDER BY date DESC")
            self.render("home.html", entries=entries)

    class EntryHandler(tornado.web.RequestHandler):
        def get(self, entry_id):
            entry = self.db.get("SELECT * FROM entries WHERE id = %s", entry_id)
            if not entry: raise tornado.web.HTTPError(404)
            self.render("entry.html", entry=entry)

    settings = {
        "ui_modules": uimodules,
    }
    application = tornado.web.Application([
        (r"/", HomeHandler),
        (r"/entry/([0-9]+)", EntryHandler),
    ], **settings)

Within `home.html`, you reference the `Entry` module rather than printing
the HTML directly:

    {% for entry in entries %}
      {{ modules.Entry(entry) }}
    {% end %}

Within `entry.html`, you reference the `Entry` module with the
`show_comments` argument to show the expanded form of the entry:

    {{ modules.Entry(entry, show_comments=True) }}

Modules can include custom CSS and JavaScript functions by overriding
the `embedded_css`, `embedded_javascript`, `javascript_files`, or
`css_files` methods:

    class Entry(tornado.web.UIModule):
        def embedded_css(self):
            return ".entry { margin-bottom: 1em; }"

        def render(self, entry, show_comments=False):
            return self.render_string(
                "module-entry.html", show_comments=show_comments)

Module CSS and JavaScript will be included once no matter how many times
a module is used on a page. CSS is always included in the `<head>` of the
page, and JavaScript is always included just before the `</body>` tag
at the end of the page.


### Non-blocking, asynchronous requests

When a request handler is executed, the request is automatically finished.
Since Tornado uses a non-blocking I/O style, you can override this default
behavior if you want a request to remain open after the main request handler
method returns using the `tornado.web.asynchronous` decorator.

When you use this decorator, it is your responsibility to call
`self.finish()` to finish the HTTP request, or the user's browser
will simply hang:

    class MainHandler(tornado.web.RequestHandler):
        @tornado.web.asynchronous
        def get(self):
            self.write("Hello, world")
            self.finish()

Here is a real example that makes a call to the FriendFeed API using
Tornado's built-in asynchronous HTTP client:

    class MainHandler(tornado.web.RequestHandler):
        @tornado.web.asynchronous
        def get(self):
            http = tornado.httpclient.AsyncHTTPClient()
            http.fetch("http://friendfeed-api.com/v2/feed/bret",
                       callback=self.on_response)

        def on_response(self, response):
            if response.error: raise tornado.web.HTTPError(500)
            json = tornado.escape.json_decode(response.body)
            self.write("Fetched " + str(len(json["entries"])) + " entries "
                       "from the FriendFeed API")
            self.finish()

When `get()` returns, the request has not finished. When the HTTP client
eventually calls `on_response()`, the request is still open, and the response
is finally flushed to the client with the call to `self.finish()`.

For a more advanced asynchronous example, take a look at the `chat` example
application, which implements an AJAX chat room using
[long polling](http://en.wikipedia.org/wiki/Push_technology#Long_polling).
Users of long polling may want to override `on_connection_close()` to clean
up after the client closes the connection (but see that method's docstring
for caveats).


### Third party authentication

Tornado's `auth` module implements the authentication and authorization
protocols for a number of the most popular sites on the web, including
Google/Gmail, Facebook, Twitter, Yahoo, and FriendFeed. The module includes
methods to log users in via these sites and, where applicable, methods to
authorize access to the service so you can, e.g., download a user's address
book or publish a Twitter message on their behalf.

Here is an example handler that uses Google for authentication, saving
the Google credentials in a cookie for later access:

    class GoogleHandler(tornado.web.RequestHandler, tornado.auth.GoogleMixin):
        @tornado.web.asynchronous
        def get(self):
            if self.get_argument("openid.mode", None):
                self.get_authenticated_user(self._on_auth)
                return
            self.authenticate_redirect()

        def _on_auth(self, user):
            if not user:
                self.authenticate_redirect()
                return
            # Save the user with, e.g., set_secure_cookie()

See the `auth` module documentation for more details.


Performance
-----------
Web application performance is generally bound by architecture, not frontend
performance. That said, Tornado is pretty fast relative to most popular
Python web frameworks.

We ran a few remedial load tests on a simple "Hello, world" application
in each of the most popular Python web frameworks
([Django](http://www.djangoproject.com/), [web.py](http://webpy.org/), and
[CherryPy](http://www.cherrypy.org/)) to get the baseline performance of
each relative to Tornado. We used Apache/mod_wsgi for Django and web.py
and ran CherryPy as a standalone server, which was our impression of how
each framework is typically run in production environments. We ran 4
single-threaded Tornado frontends behind an [nginx](http://nginx.net/)
reverse proxy, which is how we recommend running Tornado in production
(our load test machine had four cores, and we recommend 1 frontend per
core).

We load tested each with Apache Benchmark (`ab`) on the a separate machine
with the command

    ab -n 100000 -c 25 http://10.0.1.x/

The results (requests per second) on a 2.4GHz AMD Opteron processor with
4 cores:

<div style="text-align:center;margin-top:2em;margin-bottom:2em"><img src="http://chart.apis.google.com/chart?chxt=y&chd=t%3A100%2C40%2C27%2C25%2C9&chco=609bcc&chm=t+8213%2C000000%2C0%2C0%2C11%7Ct+3353%2C000000%2C0%2C1%2C11%7Ct+2223%2C000000%2C0%2C2%2C11%7Ct+2066%2C000000%2C0%2C3%2C11%7Ct+785%2C000000%2C0%2C4%2C11&chs=600x175&cht=bhs&chtt=Web+server+requests%2Fsec+%28AMD+Opteron%2C+2.4GHz%2C+4+cores%29&chxl=0%3A%7CCherryPy+%28standalone%29%7Cweb.py+%28Apache%2Fmod_wsgi%29%7CDjango+%28Apache%2Fmod_wsgi%29%7CTornado+%281+single-threaded+frontend%29%7CTornado+%28nginx%3B+4+frontends%29%7C"/></div>

In our tests, Tornado consistently had 4X the throughput of the next fastest
framework, and even a single standalone Tornado frontend got 33% more
throughput even though it only used one of the four cores.

Not very scientific, but at a high level, it should give you a sense that we
have cared about performance as we built Tornado, and it shouldn't add too
much latency to your apps relative to most Python web development frameworks.


Running Tornado in production
-----------------------------
At FriendFeed, we use [nginx](http://nginx.net/) as a load balancer
and static file server. We run multiple instances of the Tornado web
server on multiple frontend machines. We typically run one Tornado frontend
per core on the machine (sometimes more depending on utilization).

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 false;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Scheme $scheme;
                proxy_pass http://frontends;
            }
        }
    }


WSGI and Google AppEngine
-------------------------
Tornado comes with limited support for [WSGI](http://wsgi.org/). However,
since WSGI does not support non-blocking requests, you cannot use any
of the asynchronous/non-blocking features of Tornado in your application
if you choose to use WSGI instead of Tornado's HTTP server. Some of the
features that are not available in WSGI applications:
`@tornado.web.asynchronous`, the `httpclient` module, and the `auth` module.

You can create a valid WSGI application from your Tornado request handlers
by using `WSGIApplication` in the `wsgi` module instead of using
`tornado.web.Application`. Here is an example that uses the built-in WSGI
`CGIHandler` to make a valid
[Google AppEngine](http://code.google.com/appengine/) application:

    import tornado.web
    import tornado.wsgi
    import wsgiref.handlers

    class MainHandler(tornado.web.RequestHandler):
        def get(self):
            self.write("Hello, world")

    if __name__ == "__main__":
        application = tornado.wsgi.WSGIApplication([
            (r"/", MainHandler),
        ])
        wsgiref.handlers.CGIHandler().run(application)

See the `appengine` example application for a full-featured AppEngine
app built on Tornado.


Caveats and support
-------------------
Because FriendFeed and other large users of Tornado run [behind
nginx](#running-tornado-in-production) or Apache proxies, Tornado's HTTP
server currently does not attempt to handle multi-line headers and some
types of malformed input.

You can discuss Tornado and report bugs on [the Tornado developer mailing list](http://groups.google.com/group/python-tornado).