This is the first part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .
If you’re not familiar with ‘finger’ it’s probably because it’s not used as
much nowadays as it used to be. Basically, if you run finger nail
or finger nail@example.com the target computer spits out some
information about the user named nail . For instance:
Login: nail Name: Nail Sharp
Directory: /home/nail Shell: /usr/bin/sh
Last login Wed Mar 31 18:32 2004 (PST)
New mail received Thu Apr 1 10:50 2004 (PST)
Unread since Thu Apr 1 10:50 2004 (PST)
No Plan.
If the target computer does not have
the fingerd daemon
running you’ll get a “Connection Refused” error. Paranoid sysadmins
keep fingerd off or limit the output to hinder crackers
and harassers. The above format is the standard fingerd
default, but an alternate implementation can output anything it wants,
such as automated responsibility status for everyone in an
organization. You can also define pseudo “users”, which are
essentially keywords.
This portion of the tutorial makes use of factories and protocols as introduced in the Writing a TCP Server howto and deferreds as introduced in Using Deferreds and Generating Deferreds . Services and applications are discussed in Using the Twisted Application Framework .
By the end of this section of the tutorial, our finger server will answer TCP finger requests on port 1079, and will read data from the web.
from twisted.internet import reactor
reactor.run()
This example only runs the reactor. It will consume almost no CPU
resources. As it is not listening on any port, it can’t respond to network
requests — nothing at all will happen until we interrupt the program. At
this point if you run finger nail or telnet localhost 1079 , you’ll get a “Connection refused” error since there’s no daemon
running to respond. Not very useful, perhaps — but this is the skeleton
inside which the Twisted program will grow.
As implied above, at various points in this tutorial you’ll want to
observe the behavior of the server being developed. Unless you have a
finger program which can use an alternate port, the easiest way to do this
is with a telnet client. telnet localhost 1079 will connect to
the local host on port 1079, where a finger server will eventually be
listening.
You don’t call Twisted, Twisted calls you. The reactor is Twisted’s main event loop, similar to the main loop in other toolkits available in Python (Qt, wx, and Gtk). There is exactly one reactor in any running Twisted application. Once started it loops over and over again, responding to network events and making scheduled calls to code.
Note that there are actually several different reactors to choose
from; from twisted.internet import reactor returns the
current reactor. If you haven’t chosen a reactor class yet, it
automatically chooses the default. See
the Reactor Basics HOWTO for
more information.
from twisted.internet import protocol, reactor, endpoints
class FingerProtocol(protocol.Protocol):
pass
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory())
reactor.run()
Here we use endpoints.serverFromString to create a Twisted endpoint. An
endpoint is a Twisted concept that encapsulates one end of a connection. There
are different endpoints for clients and servers. One of the great advantages of
endpoints is that they can be described textually using a kind of
domain-specific language. For example, here, we ask Twisted to create a TCP
endpoint for a server using the string "tcp:1079". That, along with the
call to serverFromString, tells Twisted to look for a TCP endpoint, and
pass it the port 1079. The endpoint returned from that function can then have
the listen() method invoked on it, which causes Twisted to start listening
on port 1079. (The number 1079 is a reminder that eventually we want to run on
port 79, the standard port for finger servers.) For more detail on endpoints,
check out the Getting Connected With Endpoints.
The factory given to the listen() method, FingerFactory , is used to
handle incoming requests on that port. Specifically, for each request, the
reactor calls the factory’s buildProtocol method, which in this
case causes FingerProtocol to be instantiated. Since the protocol
defined here does not actually respond to any events, connections to 1079 will
be accepted, but the input ignored.
A Factory is the proper place for data that you want to make available to the protocol instances, since the protocol instances are garbage collected when the connection is closed.
from twisted.internet import protocol, reactor, endpoints
class FingerProtocol(protocol.Protocol):
def connectionMade(self):
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory())
reactor.run()
Here we add to the protocol the ability to respond to the event of beginning
a connection — by terminating it. Perhaps not an interesting behavior,
but it is already close to behaving according to the letter of the standard
finger protocol. After all, there is no requirement to send any data to the
remote connection in the standard. The only problem, as far as the standard is
concerned, is that we terminate the connection too soon. A client which is slow
enough will see his send() of the username result in an error.
from twisted.internet import protocol, reactor, endpoints
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory())
reactor.run()
Here we make FingerProtocol inherit from LineReceiver , so that we get data-based
events on a line-by-line basis. We respond to the event of receiving the line
with shutting down the connection.
If you use a telnet client to interact with this server, the result will look something like this:
$ telnet localhost 1079
Trying 127.0.0.1...
Connected to localhost.localdomain.
alice
Connection closed by foreign host.
Congratulations, this is the first standard-compliant version of the code. However, usually people actually expect some data about users to be transmitted.
from twisted.internet import protocol, reactor, endpoints
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.write("No such user\r\n")
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory())
reactor.run()
Finally, a useful version. Granted, the usefulness is somewhat limited by the fact that this version only prints out a “No such user” message. It could be used for devastating effect in honey-pots (decoy servers), of course.
# Read username, output from empty factory, drop connections
from twisted.internet import protocol, reactor, endpoints
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.write(self.factory.getUser(user)+"\r\n")
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def getUser(self, user):
return "No such user"
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory())
reactor.run()
The same behavior, but finally we see what usefulness the factory has: as something that does not get constructed for every connection, it can be in charge of the user database. In particular, we won’t have to change the protocol if the user database back-end changes.
# Read username, output from non-empty factory, drop connections
from twisted.internet import protocol, reactor, endpoints
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
self.transport.write(self.factory.getUser(user)+"\r\n")
self.transport.loseConnection()
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return self.users.get(user, "No such user")
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory(moshez='Happy and well'))
reactor.run()
Finally, a really useful finger database. While it does not supply information about logged in users, it could be used to distribute things like office locations and internal office numbers. As hinted above, the factory is in charge of keeping the user database: note that the protocol instance has not changed. This is starting to look good: we really won’t have to keep tweaking our protocol.
# Read username, output from non-empty factory, drop connections
# Use deferreds, to minimize synchronicity assumptions
from twisted.internet import protocol, reactor, defer, endpoints
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory(moshez='Happy and well'))
reactor.run()
But, here we tweak it just for the hell of it. Yes, while the previous version worked, it did assume the result of getUser is always immediately available. But what if instead of an in-memory database, we would have to fetch the result from a remote Oracle server? By allowing getUser to return a Deferred, we make it easier for the data to be retrieved asynchronously so that the CPU can be used for other tasks in the meanwhile.
As described in the Deferred HOWTO , Deferreds allow a program to be driven by events. For instance, if one task in a program is waiting on data, rather than have the CPU (and the program!) idly waiting for that data (a process normally called ‘blocking’), the program can perform other operations in the meantime, and waits for some signal that data is ready to be processed before returning to that process.
In brief, the code in FingerFactory above creates a
Deferred, to which we start to attach callbacks . The
deferred action in FingerFactory is actually a
fast-running expression consisting of one dictionary
method, get . Since this action can execute without
delay, FingerFactory.getUser
uses defer.succeed to create a Deferred which already has
a result, meaning its return value will be passed immediately to the
first callback function, which turns out to
be FingerProtocol.writeResponse . We’ve also defined
an errback (appropriately
named FingerProtocol.onError ) that will be called instead
of writeResponse if something goes wrong.
# Read username, output from factory interfacing to OS, drop connections
from twisted.internet import protocol, reactor, defer, utils, endpoints
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def getUser(self, user):
return utils.getProcessOutput("finger", [user])
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory())
reactor.run()
This example also makes use of a
Deferred. twisted.internet.utils.getProcessOutput is a
non-blocking version of Python’s commands.getoutput : it
runs a shell command (finger , in this case) and captures
its standard output. However, getProcessOutput returns a
Deferred instead of the output itself.
Since FingerProtocol.lineReceived is already expecting a
Deferred to be returned by getUser , it doesn’t need to be
changed, and it returns the standard output as the finger result.
Note that in this case the shell’s built-in finger command is
simply run with whatever arguments it is given. This is probably insecure, so
you probably don’t want a real server to do this without a lot more validation
of the user input. This will do exactly what the standard version of the finger
server does.
The web. That invention which has infiltrated homes around the
world finally gets through to our invention. In this case we use the
built-in Twisted web client
via twisted.web.client.getPage , a non-blocking version of
Python’s urllib2.urlopen(URL).read .
Like getProcessOutput it returns a Deferred which will be
called back with a string, and can thus be used as a drop-in
replacement.
Thus, we have examples of three different database back-ends, none of which change the protocol class. In fact, we will not have to change the protocol again until the end of this tutorial: we have achieved, here, one truly usable class.
# Read username, output from factory interfacing to web, drop connections
from twisted.internet import protocol, reactor, defer, utils, endpoints
from twisted.protocols import basic
from twisted.web import client
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, prefix):
self.prefix=prefix
def getUser(self, user):
return client.getPage(self.prefix+user)
fingerEndpoint = endpoints.serverFromString(reactor, "tcp:1079")
fingerEndpoint.listen(FingerFactory(prefix='http://livejournal.com/~'))
reactor.run()
Up until now, we faked. We kept using port 1079, because really, who wants to
run a finger server with root privileges? Well, the common solution
is “privilege shedding” : after binding to the network, become a different,
less privileged user. We could have done it ourselves, but Twisted has a
built-in way to do it. We will create a snippet as above, but now we will define
an application object. That object will have uid
and gid attributes. When running it (later we will see how) it will
bind to ports, shed privileges and then run.
Read on to find out how to run this code using the twistd utility.
This is how to run “Twisted Applications” — files which define an ‘application’. A daemon is expected to adhere to certain behavioral standards so that standard tools can stop/start/query them. If a Twisted application is run via twistd, the TWISTed Daemonizer, all this behavioral stuff will be handled for you. twistd does everything a daemon can be expected to — shuts down stdin/stdout/stderr, disconnects from the terminal and can even change runtime directory, or even the root filesystems. In short, it does everything so the Twisted application developer can concentrate on writing his networking code.
root% twistd -ny finger11.tac # just like before
root% twistd -y finger11.tac # daemonize, keep pid in twistd.pid
root% twistd -y finger11.tac --pidfile=finger.pid
root% twistd -y finger11.tac --rundir=/
root% twistd -y finger11.tac --chroot=/var
root% twistd -y finger11.tac -l /var/log/finger.log
root% twistd -y finger11.tac --syslog # just log to syslog
root% twistd -y finger11.tac --syslog --prefix=twistedfinger # use given prefix
There are several ways to tell twistd where your application is; here we
show how it is done using the application global variable in a
Python source file (a Twisted Application
Configuration file).
# Read username, output from non-empty factory, drop connections
# Use deferreds, to minimize synchronicity assumptions
# Write application. Save in 'finger.tpy'
from twisted.application import service, strports
from twisted.internet import protocol, reactor, defer
from twisted.protocols import basic
class FingerProtocol(basic.LineReceiver):
def lineReceived(self, user):
d = self.factory.getUser(user)
def onError(err):
return 'Internal error in server'
d.addErrback(onError)
def writeResponse(message):
self.transport.write(message + '\r\n')
self.transport.loseConnection()
d.addCallback(writeResponse)
class FingerFactory(protocol.ServerFactory):
protocol = FingerProtocol
def __init__(self, **kwargs):
self.users = kwargs
def getUser(self, user):
return defer.succeed(self.users.get(user, "No such user"))
application = service.Application('finger', uid=1, gid=1)
factory = FingerFactory(moshez='Happy and well')
strports.service("tcp:79", factory, reactor=reactor).setServiceParent(
service.IServiceCollection(application))
Instead of using endpoints.serverFromString as in the above
examples, here we are using its application-aware
counterpart, strports.service . Notice that when it is
instantiated, the application object itself does not reference either
the protocol or the factory. Any services (such as the one we created with
strports.service) which have the application as their parent will be
started when the application is started by twistd. The application object is
more useful for returning an object that supports the
IService , IServiceCollection , IProcess ,
and sob.IPersistable
interfaces with the given parameters; we’ll be seeing these in the
next part of the tutorial. As the parent of the endpoint we opened, the
application lets us manage the endpoint.
With the daemon running on the standard finger port, you can test it with
the standard finger command: finger moshez .