.. currentmodule:: tornado.web
FriendFeed'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 or Google's webapp, but with additional tools and optimizations to take advantage of the non-blocking web server and tools.
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 or kqueue, it can handle thousands 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.)
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()
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.
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 MyFormHandler(tornado.web.RequestHandler): def get(self): self.write('<html><body><form action="/myform" 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 theGET
andPOST
argumentsfiles
- all of the uploaded files (viamultipart/form-data
POST requests)path
- the request path (everything before the?
)headers
- the request headers
See the class definition for tornado.httpserver.HTTPRequest for a complete list of attributes.
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:
- A new RequestHandler object is created on each request
initialize()
is called with keyword arguments from theApplication
configuration. (theinitialize
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 likesend_error
.prepare()
is called. This is most useful in a base class shared by all of your handler subclasses, asprepare
is called no matter which HTTP method is used.prepare
may produce output; if it callsfinish
(orsend_error
, etc), processing stops here.- 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. - When the request is finished,
on_finish()
is called. For synchronous handlers this is immediately afterget()
(etc) return; for asynchronous handlers it is after the call tofinish()
.
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:
write_error(self, status_code, exc_info=None, **kwargs)
- outputs HTML for use on error pages.get_current_user(self)
- see User Authentication belowget_user_locale(self)
- returnslocale
object to use for the current userget_login_url(self)
- returns login url to be used by the@authenticated
decorator (default is inApplication
settings)get_template_path(self)
- returns location of template files (default is inApplication
settings)set_default_headers(self)
- may be used to set additional headers on the response (such as a customServer
header)
There are three ways to return an error from a RequestHandler:
- Manually call ~tornado.web.RequestHandler.set_status and output the response body normally.
- Call ~RequestHandler.send_error. This discards any pending unflushed output and calls ~RequestHandler.write_error to generate an error page.
- Raise an exception. tornado.web.HTTPError can be used to generate a specified status code; all other exceptions return a 500 status. The exception handler uses ~RequestHandler.send_error and ~RequestHandler.write_error to generate the error page.
The default error page includes a stack trace in debug mode and a one-line
description of the error (e.g. "500: Internal Server Error") otherwise.
To produce a custom error page, override RequestHandler.write_error.
This method may produce output normally via methods such as
~RequestHandler.write and ~RequestHandler.render. If the error was
caused by an exception, an exc_info
triple will be passed as a keyword
argument (note that this exception is not guaranteed to be the current
exception in sys.exc_info
, so write_error
must use e.g.
traceback.format_exception instead of traceback.format_exc).
In Tornado 2.0 and earlier, custom error pages were implemented by overriding
RequestHandler.get_error_html
, which returned the error page as a string
instead of calling the normal output methods (and had slightly different
semantics for exceptions). This method is still supported, but it is
deprecated and applications are encouraged to switch to
RequestHandler.write_error.
There are two main ways you can redirect requests in Tornado:
self.redirect
and with the RedirectHandler
.
You can use self.redirect
within a RequestHandler
method (like
get
) to redirect users elsewhere. There is also an optional
parameter permanent
which you can use to indicate that the
redirection is considered permanent.
This triggers a 301 Moved Permanently
HTTP status, which is useful
for e.g. redirecting to a canonical URL for a page in an SEO-friendly
manner.
The default value of permanent
is False
, which is apt for things
like redirecting users on successful POST requests.
self.redirect('/some-canonical-page', permanent=True)
RedirectHandler
is available for your use when you initialize
Application
.
For example, notice how we redirect to a longer download URL on this website:
application = tornado.wsgi.WSGIApplication([ (r"/([a-z]*)", ContentHandler), (r"/static/tornado-0.2.tar.gz", tornado.web.RedirectHandler, dict(url="https://github.com/downloads/facebook/tornado/tornado-0.2.tar.gz")), ], **settings)
The default RedirectHandler
status code is
301 Moved Permanently
, but to use 302 Found
instead, set
permanent
to False
.
application = tornado.wsgi.WSGIApplication([ (r"/foo", tornado.web.RedirectHandler, {"url":"/bar", "permanent":False}), ], **settings)
Note that the default value of permanent
is different in
self.redirect
than in RedirectHandler
. This should make some
sense if you consider that self.redirect
is used in your methods and
is probably invoked by logic involving environment, authentication, or
form submission, but RedirectHandler
patterns are going to fire 100%
of the time they match the request URL.
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 tornado.template 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 tornado.template.
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 fortornado.escape.xhtml_escape
xhtml_escape
: alias fortornado.escape.xhtml_escape
url_escape
: alias fortornado.escape.url_escape
json_encode
: alias fortornado.escape.json_encode
squeeze
: alias fortornado.escape.squeeze
linkify
: alias fortornado.escape.linkify
datetime
: the Pythondatetime
modulehandler
: the currentRequestHandler
objectrequest
: alias forhandler.request
current_user
: alias forhandler.current_user
locale
: alias forhandler.locale
_
: alias forhandler.locale.translate
static_url
: alias forhandler.static_url
xsrf_form_html
: alias forhandler.xsrf_form_html
reverse_url
: alias forApplication.reverse_url
- All entries from the
ui_methods
andui_modules
Application
settings - Any keyword arguments passed to
render
orrender_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 tornado.template
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.
All template output is escaped by default, using the
tornado.escape.xhtml_escape
function. This behavior can be changed
globally by passing autoescape=None
to the Application
or
TemplateLoader
constructors, for a template file with the
{% autoescape None %}
directive, or for a single expression by
replacing {{ ... }}
with {% raw ...%}
. Additionally, in each of
these places the name of an alternative escaping function may be used
instead of None
.
Note that while Tornado's automatic escaping is helpful in avoiding
XSS vulnerabilities, it is not sufficient in all cases. Expressions
that appear in certain locations, such as in Javascript or CSS, may need
additional escaping. Additionally, either care must be taken to always
use double quotes and xhtml_escape
in HTML attributes that may contain
untrusted content, or a separate escaping function must be used for
attributes (see e.g. http://wonko.com/post/html-escaping)
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 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!")
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
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 tornado.auth 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, or XSRF, is a common problem for personalized web applications. See the Wikipedia article 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 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
.
If you need to customize XSRF behavior on a per-handler basis, you can
override RequestHandler.check_xsrf_cookie()
. For example, if you
have an API whose authentication does not use cookies, you may want to
disable XSRF protection by making check_xsrf_cookie()
do nothing.
However, if you support both cookie and non-cookie-based authentication,
it is important that XSRF protection be used whenever the current
request is authenticated with a cookie.
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), (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; 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 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; } }
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 tornado.locale documentation for detailed information on the CSV format and other localization methods.
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", entry=entry, 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 %} {% module Entry(entry) %} {% end %}
Within entry.html
, you reference the Entry
module with the
show_comments
argument to show the expanded form of the entry:
{% module 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.
When additional Python code is not required, a template file itself may
be used as a module. For example, the preceding example could be
rewritten to put the following in module-entry.html
:
{{ set_resources(embedded_css=".entry { margin-bottom: 1em; }") }} <!-- more template html... -->
This revised template module would be invoked with
{% module Template("module-entry.html", show_comments=True) %}
The set_resources
function is only available in templates invoked
via {% module Template(...) %}
. Unlike the {% include ... %}
directive, template modules have a distinct namespace from their
containing template - they can only see the global template namespace
and their own keyword arguments.
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. 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).
Tornado includes two non-blocking HTTP client implementations:
SimpleAsyncHTTPClient
and CurlAsyncHTTPClient
. The simple client
has no external dependencies because it is implemented directly on top
of Tornado's IOLoop
. The Curl client requires that libcurl
and
pycurl
be installed (and a recent version of each is highly
recommended to avoid bugs in older version's asynchronous interfaces),
but is more likely to be compatible with sites that exercise little-used
parts of the HTTP specification.
Each of these clients is available in its own module
(tornado.simple_httpclient
and tornado.curl_httpclient
), as well
as via a configurable alias in tornado.httpclient
.
SimpleAsyncHTTPClient
is the default, but to use a different
implementation call the AsyncHTTPClient.configure
method at startup:
AsyncHTTPClient.configure('tornado.curl_httpclient.CurlAsyncHTTPClient')
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, 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 tornado.auth module documentation for more details.
If you pass debug=True
to the Application
constructor, the app
will be run in debug mode. In this mode, templates will not be cached
and 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.
Debug mode is not compatible with HTTPServer
's multi-process mode.
You must not give HTTPServer.start
an argument greater than 1 if you
are using debug mode.
The automatic reloading feature of debug mode is available as a
standalone module in tornado.autoreload
, and is optionally used by
the test runner in tornado.testing.main
.
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.
At FriendFeed, we use nginx 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; } } }
Tornado comes with limited support for WSGI.
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 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.