Twisted Web serves Python objects that implement the interface IResource.
Site Objects are responsible for
creating HTTPChannel instances to parse the HTTP request,
and begin the object lookup process. They contain the root Resource,
the resource which represents the URL / on the site.
Resource objects represent a single URL segment. The IResource interface describes the methods a Resource object must implement in order to participate in the object publishing process.
Resource trees are arrangements of Resource objects into a Resource tree. Starting at the root Resource object, the tree of Resource objects defines the URLs which will be valid.
.rpy scripts are python scripts which the twisted.web static file server will execute, much like a CGI. However, unlike CGI they must create a Resource object which will be rendered when the URL is visited.
Resource rendering occurs when Twisted Web locates a leaf Resource object. A Resource can either return an html string or write to the request object.
Session objects allow you to store information across multiple requests. Each individual browser using the system has a unique Session instance.
The Twisted Web server is started through the Twisted Daemonizer, as in:
% twistd web
Site objects serve as the glue between a port to listen for HTTP requests on, and a root Resource object.
When using twistd -n web --path /foo/bar/baz , a Site object is created with a root Resource that serves files out of the given path.
You can also create a Site instance by hand, passing
it a Resource object which will serve as the root of the
site:
from twisted.web import server, resource
from twisted.internet import reactor, endpoints
class Simple(resource.Resource):
isLeaf = True
def render_GET(self, request):
return "<html>Hello, world!</html>"
site = server.Site(Simple())
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8080)
endpoint.listen(site)
reactor.run()
Resource objects represent a single URL segment of a site. During URL parsing, getChild is called on the current Resource to produce the next Resource object.
When the leaf Resource is reached, either because there were no more URL segments or a Resource had isLeaf set to True, the leaf Resource is rendered by calling render(request) . See “Resource Rendering” below for more about this.
During the Resource location process, the URL segments which have already been processed and those which have not yet been processed are available in request.prepath and request.postpath .
A Resource can know where it is in the URL tree by looking at request.prepath , a list of URL segment strings.
A Resource can know which path segments will be processed after it by looking at request.postpath .
If the URL ends in a slash, for example http://example.com/foo/bar/ , the final URL segment will be an empty string. Resources can thus know if they were requested with or without a final slash.
Here is a simple Resource object:
from twisted.web.resource import Resource
class Hello(Resource):
isLeaf = True
def getChild(self, name, request):
if name == '':
return self
return Resource.getChild(self, name, request)
def render_GET(self, request):
return "Hello, world! I am located at %r." % (request.prepath,)
resource = Hello()
Resources can be arranged in trees using putChild . putChild puts a Resource instance into another Resource instance, making it available at the given path segment name:
root = Hello()
root.putChild('fred', Hello())
root.putChild('bob', Hello())
If this root resource is served as the root of a Site instance, the following URLs will all be valid:
http://example.com/
http://example.com/fred
http://example.com/bob
http://example.com/fred/
http://example.com/bob/
Files with the extension .rpy are python scripts which, when placed in a directory served by Twisted Web, will be executed when visited through the web.
An .rpy script must define a variable, resource , which is the Resource object that will render the request.
.rpy files are very convenient for rapid development and prototyping. Since they are executed on every web request, defining a Resource subclass in an .rpy will make viewing the results of changes to your class visible simply by refreshing the page:
from twisted.web.resource import Resource
class MyResource(Resource):
def render_GET(self, request):
return "<html>Hello, world!</html>"
resource = MyResource()
However, it is often a better idea to define Resource subclasses in Python modules. In order for changes in modules to be visible, you must either restart the Python process, or reload the module:
import myresource
## Comment out this line when finished debugging
reload(myresource)
resource = myresource.MyResource()
Creating a Twisted Web server which serves a directory is easy:
% twistd -n web --path /Users/dsp/Sites
Resource rendering occurs when Twisted Web locates a leaf Resource object to handle a web request. A Resource’s render method may do various things to produce output which will be sent back to the browser:
Return a string
Call request.write("stuff") as many times as desired, then call request.finish() and return server.NOT_DONE_YET (This is deceptive, since you are in fact done with the request, but is the correct way to do this)
Request a Deferred , return server.NOT_DONE_YET , and call request.write("stuff") and request.finish() later, in a callback on the Deferred .
The Resource
class, which is usually what one’s Resource classes subclass, has a
convenient default implementation
of render . It will call a method
named self.render_METHOD
where “METHOD” is whatever HTTP method was used to request this
resource. Examples: request_GET, request_POST, request_HEAD, and so
on. It is recommended that you have your resource classes
subclass Resource
and implement render_METHOD methods as
opposed to render itself. Note that for
certain resources, request_POST = request_GET may be desirable in case one wants to process
arguments passed to the resource regardless of whether they used GET
(?foo=bar&baz=quux , and so forth) or POST.
When using a Resource , one can specify wrap it using a EncodingResourceWrapper and passing a list of encoder factories. The encoder factories are called when a request is processed and potentially return an encoder. By default twisted provides GzipEncoderFactory which manages standard gzip compression. You can use it this way:
from twisted.web.server import Site, GzipEncoderFactory
from twisted.web.resource import Resource, EncodingResourceWrapper
from twisted.internet import reactor, endpoints
class Simple(Resource):
isLeaf = True
def render_GET(self, request):
return "<html>Hello, world!</html>"
resource = Simple()
wrapped = EncodingResourceWrapper(resource, [GzipEncoderFactory()])
site = Site(wrapped)
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8080)
endpoint.listen(site)
reactor.run()
Using compression on SSL served resources where the user can influence the content can lead to information leak, so be careful which resources use request encoders.
Note that only encoder can be used per request: the first encoder factory returning an object will be used, so the order in which they are specified matters.
HTTP is a stateless protocol; every request-response is treated as an individual unit, distinguishable from any other request only by the URL requested. With the advent of Cookies in the mid nineties, dynamic web servers gained the ability to distinguish between requests coming from different browser sessions by sending a Cookie to a browser. The browser then sends this cookie whenever it makes a request to a web server, allowing the server to track which requests come from which browser session.
Twisted Web provides an abstraction of this browser-tracking behavior called the Session object . Calling request.getSession() checks to see if a session cookie has been set; if not, it creates a unique session id, creates a Session object, stores it in the Site, and returns it. If a session object already exists, the same session object is returned. In this way, you can store data specific to the session in the session object.
A proxy is a general term for a server that functions as an intermediary between clients and other servers.
Twisted supports two main proxy variants: a Proxy and a ReverseProxy .
A proxy forwards requests made by a client to a destination server. Proxies typically sit on the internal network for a client or out on the internet, and have many uses, including caching, packet filtering, auditing, and circumventing local access restrictions to web content.
Here is an example of a simple but complete web proxy:
from twisted.web import proxy, http
from twisted.internet import reactor, endpoints
class ProxyFactory(http.HTTPFactory):
def buildProtocol(self, addr):
return proxy.Proxy()
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8080)
endpoint.listen(ProxyFactory())
reactor.run()
With this proxy running, you can configure your web browser to use localhost:8080 as a proxy. After doing so, when browsing the web
all requests will go through this proxy.
Proxy inherits
from http.HTTPChannel . Each client
request to the proxy generates a ProxyRequest from the proxy to the destination
server on behalf of the client. ProxyRequest uses
a ProxyClientFactory to create
an instance of the ProxyClient
protocol for the connection. ProxyClient inherits
from http.HTTPClient . Subclass ProxyRequest to
customize the way requests are processed or logged.
A reverse proxy retrieves resources from other servers on behalf of a client. Reverse proxies typically sit inside the server’s internal network and are used for caching, application firewalls, and load balancing.
Here is an example of a basic reverse proxy:
from twisted.internet import reactor, endpoints
from twisted.web import proxy, server
site = server.Site(proxy.ReverseProxyResource('www.yahoo.com', 80, ''))
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8080)
endpoint.listen(site)
reactor.run()
With this reverse proxy running locally, you can
visit http://localhost:8080 in your web browser, and the reverse
proxy will proxy your connection to www.yahoo.com.
In this example we use server.Site to serve
a ReverseProxyResource directly. There is
also a ReverseProxy family of classes
in twisted.web.proxy mirroring those of the Proxy
family:
Like Proxy , ReverseProxy inherits
from http.HTTPChannel . Each client request to the reverse proxy
generates a ReverseProxyRequest to the destination
server. Like ProxyRequest , ReverseProxyRequest uses a ProxyClientFactory to create an instance of
the ProxyClient protocol for
the connection.
Additional examples of proxies and reverse proxies can be found in the Twisted web examples
Non-trivial configurations of Twisted Web are achieved with Python
configuration files. This is a Python snippet which builds up a
variable called application. Usually,
the twisted.application.strports.service function will be used to build a
service instance that will be used to make the application listen on a TCP port
(80, in case direct web serving is desired), with the listener being
a twisted.web.server.Site . The resulting file
can then be run with twistd -y . Alternatively a reactor object can be used directly to make
a runnable script.
The Site will wrap a Resource object – the
root.
from twisted.application import internet, service, strports
from twisted.web import static, server
root = static.File("/var/www/htdocs")
application = service.Application('web')
site = server.Site(root)
sc = service.IServiceCollection(application)
i = strports.service("tcp:80", site)
i.setServiceParent(sc)
Most advanced configurations will be in the form of tweaking the root resource object.
Usually, the root’s children will be based on the filesystem’s contents.
It is possible to override the filesystem by explicit putChild
methods.
Here are two examples. The first one adds a /doc child
to serve the documentation of the installed packages, while the second
one adds a cgi-bin directory for CGI scripts.
from twisted.internet import reactor, endpoints
from twisted.web import static, server
root = static.File("/var/www/htdocs")
root.putChild("doc", static.File("/usr/share/doc"))
endpoint = endpoints.TCP4ServerEndpoint(reactor, 80)
endpoint.listen(server.Site(root))
reactor.run()
from twisted.internet import reactor, endpoints
from twisted.web import static, server, twcgi
root = static.File("/var/www/htdocs")
root.putChild("cgi-bin", twcgi.CGIDirectory("/var/www/cgi-bin"))
endpoint = endpoints.TCP4ServerEndpoint(reactor, 80)
endpoint.listen(server.Site(root))
reactor.run()
File resources, be they root object or children
thereof, have two important attributes that often need to be
modified: indexNames
and processors . indexNames determines which
files are treated as “index files” – served up when a directory
is rendered. processors determine how certain file
extensions are treated.
Here is an example for both, creating a site where all .rpy
extensions are Resource Scripts, and which renders directories by
searching for a index.rpy file.
from twisted.application import internet, service, strports
from twisted.web import static, server, script
root = static.File("/var/www/htdocs")
root.indexNames=['index.rpy']
root.processors = {'.rpy': script.ResourceScript}
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = strports.service("tcp:80", site)
i.setServiceParent(sc)
File objects also have a method called ignoreExt .
This method can be used to give extension-less URLs to users, so that
implementation is hidden. Here is an example:
from twisted.application import internet, service, strports
from twisted.web import static, server, script
root = static.File("/var/www/htdocs")
root.ignoreExt(".rpy")
root.processors = {'.rpy': script.ResourceScript}
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = strports.service("tcp:80", site)
i.setServiceParent(sc)
Now, a URL such as /foo might be served from a Resource
Script called foo.rpy , if no file by the name of foo
exists.
Virtual hosting is done via a special resource, that should be used
as the root resource
– NameVirtualHost . NameVirtualHost has an
attribute named default , which holds the default
website. If a different root for some other name is desired,
the addHost method should be called.
from twisted.application import internet, service, strports
from twisted.web import static, server, vhost, script
root = vhost.NameVirtualHost()
# Add a default -- htdocs
root.default=static.File("/var/www/htdocs")
# Add a simple virtual host -- foo.com
root.addHost("foo.com", static.File("/var/www/foo"))
# Add a simple virtual host -- bar.com
root.addHost("bar.com", static.File("/var/www/bar"))
# The "baz" people want to use Resource Scripts in their web site
baz = static.File("/var/www/baz")
baz.processors = {'.rpy': script.ResourceScript}
baz.ignoreExt('.rpy')
root.addHost('baz', baz)
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = strports.service("tcp:80", site)
i.setServiceParent(sc)
Since the configuration is a Python snippet, it is possible to use the full power of Python. Here are some simple examples:
# No need for configuration of virtual hosts -- just make sure
# a directory /var/vhosts/<vhost name> exists:
from twisted.web import vhost, static, server
from twisted.application import internet, service, strports
root = vhost.NameVirtualHost()
root.default = static.File("/var/www/htdocs")
for dir in os.listdir("/var/vhosts"):
root.addHost(dir, static.File(os.path.join("/var/vhosts", dir)))
application = service.Application('web')
sc = service.IServiceCollection(application)
site = server.Site(root)
i = strports.service("tcp:80", site)
i.setServiceParent(sc)
# Determine ports we listen on based on a file with numbers:
from twisted.web import vhost, static, server
from twisted.application import internet, service
root = static.File("/var/www/htdocs")
site = server.Site(root)
application = service.Application('web')
serviceCollection = service.IServiceCollection(application)
with open("/etc/web/ports") as f:
for num in map(int, f.read().split()):
serviceCollection.addCollection(
strports.service("tcp:%d" % num, site)
)
In many cases, you’ll end up repeating common usage patterns of twisted.web. In those cases you’ll probably want to use Twisted’s pre-configured web server setup.
The easiest way to run a Twisted Web server is with the Twisted Daemonizer. For example, this command will run a web server which serves static files from a particular directory:
% twistd web --path /path/to/web/content
If you just want to serve content from your own home directory, the following will do:
% twistd web --path ~/public_html/
You can stop the server at any time by going back to the directory you started it in and running the command:
% kill `cat twistd.pid`
Some other configuration options are available as well:
--port : Specify the port for the web
server to listen on. This defaults to 8080.
--logfile : Specify the path to the
log file.
The full set of options that are available can be seen with:
% twistd web --help
Twisted Web serves flat HTML files just as it does any other flat file.
A Resource script is a Python file ending with the extension .rpy , which is required to create an instance of a (subclass of a) twisted.web.resource.Resource .
Resource scripts have 3 special variables:
__file__ : The name of the .rpy file, including the full path. This variable is automatically defined and present within the namespace.
registry : An object of class static.Registry . It can be used to access and set persistent data keyed by a class.
resource : The variable which must be defined by the script and set to the resource instance that will be used to render the page.
A very simple Resource Script might look like:
from twisted.web import resource
class MyGreatResource(resource.Resource):
def render_GET(self, request):
return "<html>foo</html>"
resource = MyGreatResource()
A slightly more complicated resource script, which accesses some persistent data, might look like:
from twisted.web import resource
from SillyWeb import Counter
counter = registry.getComponent(Counter)
if not counter:
registry.setComponent(Counter, Counter())
counter = registry.getComponent(Counter)
class MyResource(resource.Resource):
def render_GET(self, request):
counter.increment()
return "you are visitor %d" % counter.getValue()
resource = MyResource()
This is assuming you have the SillyWeb.Counter module,
implemented something like the following:
class Counter:
def __init__(self):
self.value = 0
def increment(self):
self.value += 1
def getValue(self):
return self.value
The Nevow framework, available as part of the Quotient project, is an advanced system for giving Web UIs to your application. Nevow uses Twisted Web but is not itself part of Twisted.
One of the most interesting applications of Twisted Web is the distributed webserver; multiple servers can all answer requests on the same port, using the twisted.spread package for “spreadable” computing. In two different directories, run the commands:
% twistd web --user
% twistd web --personal [other options, if you desire]
Once you’re running both of these instances, go to http://localhost:8080/your_username.twistd/ – you will see the front page from the server you created with the --personal option. What’s happening here is that the request you’ve sent is being relayed from the central (User) server to your own (Personal) server, over a PB connection. This technique can be highly useful for small “community” sites; using the code that makes this demo work, you can connect one HTTP port to multiple resources running with different permissions on the same machine, on different local machines, or even over the internet to a remote site.
By default, a personal server listens on a UNIX socket in the owner’s home
directory. The --port option can be used to make
it listen on a different address, such as a TCP or SSL server or on a UNIX
server in a different location. If you use this option to make a personal
server listen on a different address, the central (User) server won’t be
able to find it, but a custom server which uses the same APIs as the central
server might. Another use of the --port option
is to make the UNIX server robust against system crashes. If the server
crashes and the UNIX socket is left on the filesystem, the personal server
will not be able to restart until it is removed. However, if --port unix:/home/username/.twistd-web-pb:wantPID=1 is
supplied when creating the personal server, then a lockfile will be used to
keep track of whether the server socket is in use and automatically delete
it when it is not.
Everything related to CGI is located in
the twisted.web.twcgi , and it’s here you’ll find the
classes that you need to subclass in order to support the language of
your (or somebody elses) taste. You’ll also need to create your own
kind of resource if you are using a non-unix operating system (such as
Windows), or if the default resources has wrong pathnames to the
parsers.
The following snippet is a .rpy that serves perl-files. Look at twisted.web.twcgi
for more examples regarding twisted.web and CGI.
from twisted.web import static, twcgi
class PerlScript(twcgi.FilteredScript):
filter = '/usr/bin/perl' # Points to the perl parser
resource = static.File("/perlsite") # Points to the perl website
resource.processors = {".pl": PerlScript} # Files that end with .pl will be
# processed by PerlScript
resource.indexNames = ['index.pl']
WSGI is the Web Server Gateway Interface. It is a specification for web servers and application servers to communicate with Python web applications. All modern Python web frameworks support the WSGI interface.
The easiest way to get started with WSGI application is to use the twistd command:
% twistd -n web --wsgi=helloworld.application
This assumes that you have a WSGI application called application in your helloworld module/package, which might look like this:
def application(environ, start_response):
"""Basic WSGI Application"""
start_response('200 OK', [('Content-type','text/plain')])
return ['Hello World!']
The above setup will be suitable for many applications where all that is
needed is to server the WSGI application at the site’s root. However, for
greater control, Twisted provides support for using WSGI applications as
resources twisted.web.wsgi.WSGIResource .
Here is an example of a WSGI application being served as the root resource for a site, in the following tac file:
from twisted.web import server
from twisted.web.wsgi import WSGIResource
from twisted.python.threadpool import ThreadPool
from twisted.internet import reactor
from twisted.application import service, strports
# Create and start a thread pool,
wsgiThreadPool = ThreadPool()
wsgiThreadPool.start()
# ensuring that it will be stopped when the reactor shuts down
reactor.addSystemEventTrigger('after', 'shutdown', wsgiThreadPool.stop)
def application(environ, start_response):
"""A basic WSGI application"""
start_response('200 OK', [('Content-type','text/plain')])
return ['Hello World!']
# Create the WSGI resource
wsgiAppAsResource = WSGIResource(reactor, wsgiThreadPool, application)
# Hooks for twistd
application = service.Application('Twisted.web.wsgi Hello World Example')
server = strports.service('tcp:8080', server.Site(wsgiAppAsResource))
server.setServiceParent(application)
This can then be run like any other .tac file:
% twistd -ny myapp.tac
Because of the synchronous nature of WSGI, each application call (for
each request) is called within a thread, and the result is written back to the
web server. For this, a twisted.python.threadpool.ThreadPool
instance is used.
It is common to use one server (for example, Apache) on a site with multiple
names which then uses reverse proxy (in Apache, via mod_proxy ) to different
internal web servers, possibly on different machines. However, naive
configuration causes miscommunication: the internal server firmly believes it
is running on “internal-name:port” , and will generate URLs to that effect,
which will be completely wrong when received by the client.
While Apache has the ProxyPassReverse directive, it is really a hack and is nowhere near comprehensive enough. Instead, the recommended practice in case the internal web server is Twisted Web is to use VHostMonster.
From the Twisted side, using VHostMonster is easy: just drop a file named
(for example) vhost.rpy containing the following:
from twisted.web import vhost
resource = vhost.VHostMonsterResource()
Make sure the web server is configured with the correct processors
for the rpy extensions (the web server twistd web --path generates by default is so configured).
From the Apache side, instead of using the following ProxyPass directive:
<VirtualHost ip-addr>
ProxyPass / http://localhost:8538/
ServerName example.com
</VirtualHost>
Use the following directive:
<VirtualHost ip-addr>
ProxyPass / http://localhost:8538/vhost.rpy/http/example.com:80/
ServerName example.com
</VirtualHost>
Here is an example for Twisted Web’s reverse proxy:
from twisted.application import internet, service, strports
from twisted.web import proxy, server, vhost
vhostName = 'example.com'
reverseProxy = proxy.ReverseProxyResource('internal', 8538,
'/vhost.rpy/http/'+vhostName+'/')
root = vhost.NameVirtualHost()
root.addHost(vhostName, reverseProxy)
site = server.Site(root)
application = service.Application('web-proxy')
sc = service.IServiceCollection(application)
i = strports.service("tcp:80", site)
i.setServiceParent(sc)
Sometimes it is convenient to modify the content of
the Request object
before passing it on. Because this is most often used to rewrite
either the URL, the similarity to Apache’s mod_rewrite
has inspired the twisted.web.rewrite
module. Using this module is done via wrapping a resource with
a twisted.web.rewrite.RewriterResource which
then has rewrite rules. Rewrite rules are functions which accept a
request object, and possible modify it. After all rewrite rules run,
the child resolution chain continues as if the wrapped resource,
rather than the RewriterResource , was the child.
Here is an example, using the only rule currently supplied by Twisted itself:
default_root = rewrite.RewriterResource(default, rewrite.tildeToUsers)
This causes the URL /~foo/bar.html to be treated
like /users/foo/bar.html . If done after setting
default’s users child to a distrib.UserDirectory , it gives a
configuration similar to the classical configuration of web server,
common since the first NCSA servers.
Sometimes it is useful to know when the other side has broken the connection. Here is an example which does that:
from twisted.web.resource import Resource
from twisted.web import server
from twisted.internet import reactor
from twisted.python.util import println
class ExampleResource(Resource):
def render_GET(self, request):
request.write("hello world")
d = request.notifyFinish()
d.addCallback(lambda _: println("finished normally"))
d.addErrback(println, "error")
reactor.callLater(10, request.finish)
return server.NOT_DONE_YET
resource = ExampleResource()
This will allow us to run statistics on the log-file to see how many users are frustrated after merely 10 seconds.
Sometimes, you want to be able to send headers and status
directly. While you can do this with a ResourceScript , an easier way is to
use ASISProcessor .
Use it by, for example, adding it as a processor for
the .asis extension. Here is a sample file:
HTTP/1.0 200 OK
Content-Type: text/html
Hello world