# Evolution of Finger

## Refuse Connections

    from twisted.internet import reactor
    reactor.run()

Here, we just run the reactor. Nothing at all will happen, until we interrupt the program. It will not consume (almost) no CPU resources. Not very useful, perhaps -- but this is the skeleton inside which the Twisted program will grow.

## Do Nothing

    from twisted.internet import protocol, reactor
    class FingerProtocol(protocol.Protocol):
        pass
    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol
    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

Here, we start listening on port 1079 [which is supposed to be a reminder that eventually, we want to run on port 79, the port the finger server is supposed to run on. We define a protocol which does not respond to any events. Thus, connections to 1079 will be accepted, but the input ignored.

## Drop Connections

    from twisted.internet import protocol, reactor
    class FingerProtocol(protocol.Protocol):
        def connectionMade(self):
            self.transport.loseConnection()
    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol
    reactor.listenTCP(1079, 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 behaviour, but it is already not that far from behaving according to the letter of the protocol. After all, there is no requirement to send any data to the remote connection in the standard, is there. The only technical problem 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.

## Read Username, Drop Connections

    from twisted.internet import protocol, reactor
    from twisted.protocols import basic
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.transport.loseConnection()
    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol
    reactor.listenTCP(1079, 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. Congratulations, this is the first standard-compliant version of the code. However, usually people actually expect some data about users to be transmitted.

## Read Username, Output Error, Drop Connections

    from twisted.internet import protocol, reactor
    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
    reactor.listenTCP(1079, 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 devestating effect in honeypots, of course :)

## Output From Empty Factory

    # Read username, output from empty factory, drop connections
    from twisted.internet import protocol, reactor
    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"
    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

The same behaviour, but finally we see what usefuleness 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 backend changes.

## Output from Non-empty Factory

    # Read username, output from non-empty factory, drop connections
    from twisted.internet import protocol, reactor
    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")
    reactor.listenTCP(1079, 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.

## Use Deferreds

    # Read username, output from non-empty factory, drop connections
    # Use deferreds, to minimize synchronicity assumptions
    from twisted.internet import protocol, reactor, defer
    from twisted.protocols import basic
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    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"))
    reactor.listenTCP(1079, 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 result from a remote Oracle? Or from the web? Or, or...

## Run 'finger' Locally

    # Read username, output from factory interfacing to OS, drop connections
    from twisted.internet import protocol, reactor, defer, utils
    from twisted.protocols import basic
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol
        def getUser(self, user):
            return utils.getProcessOutput("finger", [user])
    reactor.listenTCP(1079, FingerFactory())
    reactor.run()

...from running a local command? Yes, this version (safely!) runs finger locally with whatever arguments it is given, and returns the standard output. This will do exactly what the standard version of the finger server does -- without the need for any remote buffer overflows, as the networking is done safely.

## Read Status from the Web

    # Read username, output from factory interfacing to web, drop connections
    from twisted.internet import protocol, reactor, defer, utils
    from twisted.protocols import basic
    from twisted.web import client
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    class FingerFactory(protocol.ServerFactory):
        protocol = FingerProtocol
        def __init__(self, prefix): self.prefix=prefix
        def getUser(self, user):
            return client.getPage(self.prefix+user)
    reactor.listenTCP(1079, FingerFactory(prefix='http://livejournal.com/~'))
    reactor.run()

The web. That invention which has infiltrated homes around the world finally gets through to our invention. Here we use the built-in Twisted web client, which also returns a deferred. Finally, we manage to have examples of three different database backends, which do not change the protocol class. In fact, we will not have to change the protocol again until the end of this talk: we have achieved, here, one truly usable class.

## Use Application

    # Read username, output from non-empty factory, drop connections
    # Use deferreds, to minimize synchronicity assumptions
    # Write application. Save in 'finger.tpy'
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    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 = app.Application('finger', uid=1, gid=1)
    application.listenTCP(79, FingerFactory(moshez='Happy and well'))

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 builtin way to do it. Create a snippet as above, defining 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.

## twistd

    root% twistd -ny finger.tpy # just like before
    root% twistd -y finger.tpy # daemonize, keep pid in twistd.pid
    root% twistd -y finger.tpy --pidfile=finger.pid
    root% twistd -y finger.tpy --rundir=/
    root% twistd -y finger.tpy --chroot=/var
    root% twistd -y finger.tpy -l /var/log/finger.log
    root% twistd -y finger.tpy --syslog # just log to syslog
    root% twistd -y finger.tpy --syslog --prefix=twistedfinger # use given prefix

This is how to run "Twisted Applications" -- files which define an 'application'. twistd (TWISTed Daemonizer) 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.

## Setting Message By Local Users

    # But let's try and fix setting away messages, shall we?
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    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"))
    class FingerSetterProtocol(basic.LineReceiver):
          def connectionMade(self): self.lines = []
          def lineReceived(self, line): self.lines.append(line)
          def connectionLost(self): self.factory.setUser(*self.lines)
    class FingerSetterFactory(protocol.ServerFactory):
          def __init__(self, ff): self.setUser = self.ff.users.__setitem__
    ff = FingerFactory(moshez='Happy and well')
    fsf = FingerSetterFactory(ff)
    application = app.Application('finger', uid=1, gid=1)
    application.listenTCP(79, ff)
    application.listenTCP(1079, fsf, interface='127.0.0.1')

Now that port 1079 is free, maybe we can run on it a different server, one which will let people set their messages. It does no access control, so anyone who can login to the machine can set any message. We assume this is the desired behaviour in our case. Testing it can be done by simply:

    % nc localhost 1079
    moshez
    Giving a talk now, sorry!
    ^D

## Use Services to Make Dependencies Sane

    # Fix asymmetry
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    class FingerSetterProtocol(basic.LineReceiver):
          def connectionMade(self): self.lines = []
          def lineReceived(self, line): self.lines.append(line)
          def connectionLost(self): self.factory.setUser(*self.lines)
    class FingerService(app.ApplicationService):
          def __init__(self, *args, **kwargs):
              app.ApplicationService.__init__(self, *args)
              self.users = kwargs
          def getUser(self, user):
              return defer.succeed(self.users.get(u, "No such user"))
          def getFingerFactory(self):
              f = protocol.ServerFactory()
              f.protocol, f.getUser = FingerProtocol, self.getUser
              return f
          def getFingerSetterFactory(self):
              f = protocol.ServerFactory()
              f.protocol, f.setUser = FingerSetterProtocol, self.users.__setitem__
              return f
    application = app.Application('finger', uid=1, gid=1)
    f = FingerService(application, 'finger', moshez='Happy and well')
    application.listenTCP(79, f.getFingerFactory())
    application.listenTCP(1079, f.getFingerSetterFactory(), interface='127.0.0.1')

The previous version had the setter poke at the innards of the finger factory. It's usually not a good idea: this version makes both factories symmetric by making them both look at a single object. Services are useful for when an object is needed which is not related to a specific network server. Here, we moved all responsibility for manufacturing factories into the service. Note that we stopped subclassing: the service simply puts useful methods and attributes inside the factories. We are getting better at protocol design: none of our protocol classes had to be changed, and neither will have to change until the end of the talk.

## Read Status File

    # Read from file
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    class FingerSetterProtocol(basic.LineReceiver):
          def connectionMade(self): self.lines = []
          def lineReceived(self, line): self.lines.append(line)
          def connectionLost(self): self.factory.setUser(*self.lines)
    class FingerService(app.ApplicationService):
          def __init__(self, file, *args, **kwargs):
              app.ApplicationService.__init__(self, *args, **kwargs)
              self.file = file
          def startService(self):
              app.ApplicationService.startService(self)
              self._read()
          def _read(self):
              self.users = {}
              for line in file(self.file):
                  user, status = line.split(':', 1)
                  self.users[user] = status
              self.call = reactor.callLater(30, self._read)
          def stopService(self):
              app.ApplicationService.stopService(self)
              self.call.cancel()
          def getUser(self, user):
              return defer.succeed(self.users.get(u, "No such user"))
          def getFingerFactory(self):
              f = protocol.ServerFactory()
              f.protocol, f.getUser = FingerProtocol, self.getUser
              return f
    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, f.getFingerFactory())

This version shows how, instead of just letting users set their messages, we can read those from a centrally managed file. We cache results, and every 30 seconds we refresh it. Services are useful for such scheduled tasks.

## Announce on Web, Too

    # Read from file, announce on the web!
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic
    from twisted.web import resource, server, static
    import cgi
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    class FingerSetterProtocol(basic.LineReceiver):
          def connectionMade(self): self.lines = []
          def lineReceived(self, line): self.lines.append(line)
          def connectionLost(self): self.factory.setUser(*self.lines)
    class FingerService(app.ApplicationService):
          def __init__(self, file, *args, **kwargs):
              app.ApplicationService.__init__(self, *args, **kwargs)
              self.file = file
          def startService(self):
              app.ApplicationService.startService(self)
              self._read()
          def _read(self):
              self.users = {}
              for line in file(self.file):
                  user, status = line.split(':', 1)
                  self.users[user] = status
              self.call = reactor.callLater(30, self._read)
          def stopService(self):
              app.ApplicationService.stopService(self)
              self.call.cancel()
          def getUser(self, user):
              return defer.succeed(self.users.get(u, "No such user"))
          def getFingerFactory(self):
              f = protocol.ServerFactory()
              f.protocol, f.getUser = FingerProtocol, self.getUser
              return f
          def getResource(self):
              r = resource.Resource()
              r.getChild = (lambda path, request:
               static.Data('text/html',
                '<h1>%s</h1><p>%s</p>' %
                  tuple(map(cgi.escape,
                            [path,self.users.get(path, "No such user")]))))
    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, f.getFingerFactory())
    application.listenTCP(80, server.Site(f.getResource()))

The same kind of service can also produce things useful for other protocols. For example, in twisted.web, the factory itself (the site) is almost never subclassed -- instead, it is given a resource, which represents the tree of resources available via URLs. That hierarchy is navigated by site, and overriding it dynamically is possible with getChild.

## Announce on IRC, Too

    # Read from file, announce on the web, irc
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic, irc
    from twisted.web import resource, server, static
    import cgi
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    class FingerSetterProtocol(basic.LineReceiver):
          def connectionMade(self): self.lines = []
          def lineReceived(self, line): self.lines.append(line)
          def connectionLost(self): self.factory.setUser(*self.lines)
    class IRCReplyBot(irc.IRCClient):
        def connectionMade(self):
            self.nickname = self.factory.nickname
            irc.IRCClient.connectionMade(self)
        def privmsg(self, user, channel, msg):
            if user.lower() == channel.lower():
                self.factory.getUser(msg
                ).addErrback(lambda _: "Internal error in server"
                ).addCallback(lambda m: self.msg(user, m))
    class FingerService(app.ApplicationService):
          def __init__(self, file, *args, **kwargs):
              app.ApplicationService.__init__(self, *args, **kwargs)
              self.file = file
          def startService(self):
              app.ApplicationService.startService(self)
              self._read()
          def _read(self):
              self.users = {}
              for line in file(self.file):
                  user, status = line.split(':', 1)
                  self.users[user] = status
              self.call = reactor.callLater(30, self._read)
          def stopService(self):
              app.ApplicationService.stopService(self)
              self.call.cancel()
          def getUser(self, user):
              return defer.succeed(self.users.get(u, "No such user"))
          def getFingerFactory(self):
              f = protocol.ServerFactory()
              f.protocol, f.getUser = FingerProtocol, self.getUser
              return f
          def getResource(self):
              r = resource.Resource()
              r.getChild = (lambda path, request:
               static.Data('text/html',
                '<h1>%s</h1><p>%s</p>' %
                  tuple(map(cgi.escape,
                            [path,self.users.get(path, "No such user")]))))
          def getIRCBot(self, nickname):
              f = protocol.ReconnectingClientFactory()
              f.protocol,f.nickname,f.getUser = IRCReplyBot,nickname,self.getUser
              return f
    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, f.getFingerFactory())
    application.listenTCP(80, server.Site(f.getResource()))
    application.connectTCP('irc.freenode.org', 6667, f.getIRCBot('finger-bot'))

This is the first time there is client code. IRC clients often act a lot like servers: responding to events form the network. The reconnecting client factory will make sure that severed links will get re-established, with intelligent tweaked exponential backoff algorithms. The irc client itself is simple: the only real hack is getting the nickname from the factory in connectionMade.

## Add XML-RPC Support

    # Read from file, announce on the web, irc, xml-rpc
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic, irc
    from twisted.web import resource, server, static, xmlrpc
    import cgi
    class FingerProtocol(basic.LineReceiver):
        def lineReceived(self, user):
            self.factory.getUser(user
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m:
                (self.transport.write(m+"\r\n"),self.transport.loseConnection()))
    class FingerSetterProtocol(basic.LineReceiver):
          def connectionMade(self): self.lines = []
          def lineReceived(self, line): self.lines.append(line)
          def connectionLost(self): self.factory.setUser(*self.lines)
    class IRCReplyBot(irc.IRCClient):
        def connectionMade(self):
            self.nickname = self.factory.nickname
            irc.IRCClient.connectionMade(self)
        def privmsg(self, user, channel, msg):
            if user.lower() == channel.lower():
                self.factory.getUser(msg
            ).addErrback(lambda _: "Internal error in server"
            ).addCallback(lambda m: self.msg(user, m))
    class FingerService(app.ApplicationService):
          def __init__(self, file, *args, **kwargs):
              app.ApplicationService.__init__(self, *args, **kwargs)
              self.file = file
          def startService(self):
              app.ApplicationService.startService(self)
              self._read()
          def _read(self):
              self.users = {}
              for line in file(self.file):
                  user, status = line.split(':', 1)
                  self.users[user] = status
              self.call = reactor.callLater(30, self._read)
          def stopService(self):
              app.ApplicationService.stopService(self)
              self.call.cancel()
          def getUser(self, user):
              return defer.succeed(self.users.get(u, "No such user"))
          def getFingerFactory(self):
              f = protocol.ServerFactory()
              f.protocol, f.getUser = FingerProtocol, self.getUser
              return f
          def getResource(self):
              r = resource.Resource()
              r.getChild = (lambda path, request:
               static.Data('text/html',
                '<h1>%s</h1><p>%s</p>' %
                  tuple(map(cgi.escape,
                            [path,self.users.get(path, "No such user")]))))
              x = xmlrpc.XMLRPRC()
              x.xmlrpc_getUser = self.getUser
              r.putChild('RPC2.0', x)
              return r
          def getIRCBot(self, nickname):
              f = protocol.ReconnectingClientFactory()
              f.protocol,f.nickname,f.getUser = IRCReplyBot,nickname,self.getUser
              return f
    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, f.getFingerFactory())
    application.listenTCP(80, server.Site(f.getResource()))
    application.connectTCP('irc.freenode.org', 6667, f.getIRCBot('finger-bot'))

In Twisted, XML-RPC support is handled just as though it was another resource. That resource will still support GET calls normally through render(), but that is usually left unimplemented. Note that it is possible to return deferreds from XML-RPC methods. The client, of course, will not get the answer until the deferred is triggered.

## Write Readable Code

    # Do everything properly
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic, irc
    from twisted.web import resource, server, static, xmlrpc
    import cgi

    def catchError(err):
        return "Internal error in server"

    class FingerProtocol(basic.LineReceiver):

        def lineReceived(self, user):
            d = self.factory.getUser(user)
            d.addErrback(catchError)
            def writeValue(value):
                self.transport.write(value)
                self.transport.loseConnection()
            d.addCallback(writeValue)

    class FingerSetterProtocol(basic.LineReceiver):

          def connectionMade(self):
              self.lines = []

          def lineReceived(self, line):
              self.lines.append(line)

          def connectionLost(self):
              if len(self.lines) == 2:
                  self.factory.setUser(*self.lines)

    class IRCReplyBot(irc.IRCClient):

        def connectionMade(self):
            self.nickname = self.factory.nickname
            irc.IRCClient.connectionMade(self)

        def privmsg(self, user, channel, msg):
            if user.lower() == channel.lower():
                d = self.factory.getUser(msg)
                d.addErrback(catchError)
                d.addCallback(lambda m: "Status of %s: %s" % (user, m))
                d.addCallback(lambda m: self.msg(user, m))

    class UserStatusTree(resource.Resource):

        def __init__(self, service):
            resource.Resource.__init__(self):
            self.service = service

        def render(self, request):
            d = self.service.getUsers()
            def formatUsers(users):
                l = ["<li><a href="%s">%s</a></li> % (user, user)
                    for user in users]
                return '<ul>'+''.join(l)+'</ul>'
            d.addCallback(formatUsers)
            d.addCallback(request.write)
            d.addCallback(lambda _: request.finish())
            return server.NOT_DONE_YET

        def getChild(self, path, request):
            return UserStatus(path, self.service)

    class UserStatus(resource.Resource):

        def __init__(self, user, service):
            resource.Resource.__init__(self):
            self.user = user
            self.service = service

        def render(self, request):
            d = self.service.getUser(self.user)
            d.addCallback(cgi.escape)
            d.addCallback(lambda m:
                         '<h1>%s</h1>'%self.user+'<p>%s</p>'%m)
            d.addCallback(request.write)
            d.addCallback(lambda _: request.finish())
            return server.NOT_DONE_YET

    class UserStatusXR(xmlrpc.XMLPRC):

        def __init__(self, service):
            xmlrpc.XMLRPC.__init__(self)
            self.service = service

        def xmlrpc_getUser(self, user):
            return self.service.getUser(user)

    class FingerService(app.ApplicationService):

          def __init__(self, file, *args, **kwargs):
              app.ApplicationService.__init__(self, *args, **kwargs)
              self.file = file

          def startService(self):
              app.ApplicationService.startService(self)
              self._read()

          def _read(self):
              self.users = {}
              for line in file(self.file):
                  user, status = line.split(':', 1)
                  self.users[user] = status
              self.call = reactor.callLater(30, self._read)

          def stopService(self):
              app.ApplicationService.stopService(self)
              self.call.cancel()

          def getUser(self, user):
              return defer.succeed(self.users.get(u, "No such user"))

          def getUsers(self):
              return defer.succeed(self.users.keys())

          def getFingerFactory(self):
              f = protocol.ServerFactory()
              f.protocol = FingerProtocol
              f.getUser = self.getUser
              return f

          def getResource(self):
              r = UserStatusTree(self)
              x = UserStatusXR(self)
              r.putChild('RPC2.0', x)
              return r

          def getIRCBot(self, nickname):
              f = protocol.ReconnectingClientFactory()
              f.protocol = IRCReplyBot
              f.nickname = nickname
              f.getUser = self.getUser
              return f

    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, f.getFingerFactory())
    application.listenTCP(80, server.Site(f.getResource()))
    application.connectTCP('irc.freenode.org', 6667, f.getIRCBot('finger-bot'))

The last version of the application had a lot of hacks. We avoided subclassing, did not support things like user listings in the web support, and removed all blank lines -- all in the interest of code which is shorter. Here we take a step back, subclass what is more naturally a subclass, make things which should take multiple lines take them, etc. This shows a much better style of developing Twisted applications, though the hacks in the previous stages are sometimes used in throw-away prototypes.

## Write Maintainable Code

    # Do everything properly, and componentize
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic, irc
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc
    import cgi

    class IFingerService(components.Interface):

        def getUser(self, user):
            '''Return a deferred returning a string'''

        def getUsers(self):
            '''Return a deferred returning a list of strings'''

    class IFingerSettingService(components.Interface):

        def setUser(self, user, status):
            '''Set the user's status to something'''

    def catchError(err):
        return "Internal error in server"

    class FingerProtocol(basic.LineReceiver):

        def lineReceived(self, user):
            d = self.factory.getUser(user)
            d.addErrback(catchError)
            def writeValue(value):
                self.transport.write(value)
                self.transport.loseConnection()
            d.addCallback(writeValue)

    class IFingerFactory(components.Interface):

        def getUser(self, user):
            """Return a deferred returning a string""""

        def buildProtocol(self, addr):
            """Return a protocol returning a string""""

    class FingerFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerFactory,

        protocol = FingerProtocol

        def __init__(self, service):
            self.service = service

        def getUser(self, user):
            return self.service.getUser(user)

    components.registerAdapter(FingerFactoryFromService, IFingerService)

    class FingerSetterProtocol(basic.LineReceiver):

          def connectionMade(self):
              self.lines = []

          def lineReceived(self, line):
              self.lines.append(line)

          def connectionLost(self):
              if len(self.lines) == 2:
                  self.factory.setUser(*self.lines)

    class IFingerSetterFactory(components.Interface):

        def setUser(self, user, status):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol returning a string"""

    class FingerSetterFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerSetterFactory,

        protocol = FingerSetterProtocol

        def __init__(self, service):
            self.service = service

        def setUser(self, user, status):
            self.service.setUser(user, status)

    components.registerAdapter(FingerSetterFactoryFromService,
                               IFingerSettingService)

    class IRCReplyBot(irc.IRCClient):

        def connectionMade(self):
            self.nickname = self.factory.nickname
            irc.IRCClient.connectionMade(self)

        def privmsg(self, user, channel, msg):
            if user.lower() == channel.lower():
                d = self.factory.getUser(msg)
                d.addErrback(catchError)
                d.addCallback(lambda m: "Status of %s: %s" % (user, m))
                d.addCallback(lambda m: self.msg(user, m))

    class IIRCClientFactory(components.Interface):

        '''
        @ivar nickname
        '''

        def getUser(self, user):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol"""

    class IRCClientFactoryFromService(protocol.ClientFactory):

       __implements__ = IIRCClientFactory,

       protocol = IRCReplyBot
       nickname = None

       def __init__(self, service):
           self.service = service

        def getUser(self, user):
            return self.service.getUser()

    components.registerAdapter(IRCClientFactoryFromService, IFingerService)

    class UserStatusTree(resource.Resource):

        def __init__(self, service):
            resource.Resource.__init__(self):
            self.putChild('RPC2.0', UserStatusXR(self.service))
            self.service = service

        def render(self, request):
            d = self.service.getUsers()
            def formatUsers(users):
                l = ["<li><a href="%s">%s</a></li> % (user, user)
                    for user in users]
                return '<ul>'+''.join(l)+'</ul>'
            d.addCallback(formatUsers)
            d.addCallback(request.write)
            d.addCallback(lambda _: request.finish())
            return server.NOT_DONE_YET

        def getChild(self, path, request):
            return UserStatus(path, self.service)

    components.registerAdapter(UserStatusTree, IFingerService)

    class UserStatus(resource.Resource):

        def __init__(self, user, service):
            resource.Resource.__init__(self):
            self.user = user
            self.service = service

        def render(self, request):
            d = self.service.getUser(self.user)
            d.addCallback(cgi.escape)
            d.addCallback(lambda m:
                          '<h1>%s</h1>'%self.user+'<p>%s</p>'%m)
            d.addCallback(request.write)
            d.addCallback(lambda _: request.finish())
            return server.NOT_DONE_YET

    class UserStatusXR(xmlrpc.XMLPRC):

        def __init__(self, service):
            xmlrpc.XMLRPC.__init__(self)
            self.service = service

        def xmlrpc_getUser(self, user):
            return self.service.getUser(user)

    class FingerService(app.ApplicationService):

        __implements__ = IFingerService,

        def __init__(self, file, *args, **kwargs):
            app.ApplicationService.__init__(self, *args, **kwargs)
            self.file = file

        def startService(self):
            app.ApplicationService.startService(self)
            self._read()

        def _read(self):
            self.users = {}
            for line in file(self.file):
                user, status = line.split(':', 1)
                self.users[user] = status
            self.call = reactor.callLater(30, self._read)

        def stopService(self):
            app.ApplicationService.stopService(self)
            self.call.cancel()

        def getUser(self, user):
            return defer.succeed(self.users.get(u, "No such user"))

        def getUsers(self):
            return defer.succeed(self.users.keys())

    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, IFingerFactory(f))
    application.listenTCP(80, server.Site(resource.IResource(f)))
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)

In the last version, the service class was three times longer than any other class, and was hard to understand. This was because it turned out to have multiple responsibilities. It had to know how to access user information, by scheduling a reread of the file ever half minute, but also how to display itself in a myriad of protocols. Here, we used the component-based architecture that Twisted provides to achieve a separation of concerns. All the service is responsible for, now, is supporting getUser/getUsers. It declares its support via the __implements__ keyword. Then, adapters are used to make this service look like an appropriate class for various things: for supplying a finger factory to listenTCP, for supplying a resource to site's constructor, and to provide an IRC client factory for connectTCP. All the adapters use are the methods in FingerService they are declared to use: getUser/getUsers. We could, of course, skipped the interfaces and let the configuration code use things like FingerFactoryFromService(f) directly. However, using interfaces provides the same flexibility inheritance gives: future subclasses can override the adapters.

## Advantages of Latest Version

- Readable -- each class is short
- Maintainable -- each class knows only about interfaces
- Dependencies between code parts are minimized
- Example: writing a new IFingerService is easy

    class MemoryFingerService(app.ApplicationService):
          __implements__ = IFingerService, IFingerSetterService

          def __init__(self, *args, **kwargs):
              app.ApplicationService.__init__(self, *args)
              self.users = kwargs

          def getUser(self, user):
              return defer.succeed(self.users.get(u, "No such user"))

          def getUsers(self):
              return defer.succeed(self.users.keys())

          def setUser(self, user, status):
              self.users[user] = status

    application = app.Application('finger', uid=1, gid=1)
    # New constructor call
    f = MemoryFingerService(application, 'finger', moshez='Happy and well')
    application.listenTCP(79, IFingerFactory(f))
    application.listenTCP(80, server.Site(resource.IResource(f)))
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)
    # New: run setter too
    application.listenTCP(1079, IFingerSetterFactory(f), interface='127.0.0.1')

Here we show just how convenient it is to implement new backends when we move to a component based architecture. Note that here we also use an interface we previously wrote, FingerSetterFactory, by supporting one single method. We manage to preserve the service's ignorance of the network.

## Another Backend

    class LocalFingerService(app.ApplicationService):
          __implements__ = IFingerService

          def getUser(self, user):
              return utils.getProcessOutput("finger", [user])

          def getUsers(self):
              return defer.succeed([])

    application = app.Application('finger', uid=1, gid=1)
    f = LocalFingerService(application, 'finger')
    application.listenTCP(79, IFingerFactory(f))
    application.listenTCP(80, server.Site(resource.IResource(f)))
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)

We have already wrote this, but now we get more for less work: the network code is completely separate from the backend.

## Yet Another Backend: Doing the Standard Thing

    import pwd

    class LocalFingerService(app.ApplicationService):
          __implements__ = IFingerService

          def getUser(self, user):
              try:
                  entry = pwd.getpwnam(user)
              except KeyError:
                  return "No such user"
              try:
                  f=file(os.path.join(entry[5],'.plan'))
              except (IOError, OSError):
                  return "No such user"
              data = f.read()
              f.close()
              return data

          def getUsers(self):
              return defer.succeed([])

    application = app.Application('finger', uid=1, gid=1)
    f = LocalFingerService(application, 'finger')
    application.listenTCP(79, IFingerFactory(f))
    application.listenTCP(80, server.Site(resource.IResource(f)))
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)

Not much to say about that, except to indicate that by now we can be churning out backends like crazy. Feel like doing a backend for advogato, for example? Dig out the XML-RPC client support Twisted has, and get to work!

## Aspect Oriented Programming

- This is an example...
- ...with something actually useful...
- ...not logging and timing.
- Write less code, have less dependencies!

## Use Woven

    # Do everything properly, and componentize
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic, irc
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc, microdom
    from twisted.web.woven import page, widget
    import cgi

    class IFingerService(components.Interface):

        def getUser(self, user):
            '''Return a deferred returning a string'''

        def getUsers(self):
            '''Return a deferred returning a list of strings'''

    class IFingerSettingService(components.Interface):

        def setUser(self, user, status):
            '''Set the user's status to something'''

    def catchError(err):
        return "Internal error in server"

    class FingerProtocol(basic.LineReceiver):

        def lineReceived(self, user):
            d = self.factory.getUser(user)
            d.addErrback(catchError)
            def writeValue(value):
                self.transport.write(value)
                self.transport.loseConnection()
            d.addCallback(writeValue)

    class IFingerFactory(components.Interface):

        def getUser(self, user):
            """Return a deferred returning a string""""

        def buildProtocol(self, addr):
            """Return a protocol returning a string""""

    class FingerFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerFactory,

        protocol = FingerProtocol

        def __init__(self, service):
            self.service = service

        def getUser(self, user):
            return self.service.getUser(user)

    components.registerAdapter(FingerFactoryFromService, IFingerService)

    class FingerSetterProtocol(basic.LineReceiver):

          def connectionMade(self):
              self.lines = []

          def lineReceived(self, line):
              self.lines.append(line)

          def connectionLost(self):
              if len(self.lines) == 2:
                  self.factory.setUser(*self.lines)

    class IFingerSetterFactory(components.Interface):

        def setUser(self, user, status):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol returning a string"""

    class FingerSetterFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerSetterFactory,

        protocol = FingerSetterProtocol

        def __init__(self, service):
            self.service = service

        def setUser(self, user, status):
            self.service.setUser(user, status)

    components.registerAdapter(FingerSetterFactoryFromService,
                               IFingerSettingService)

    class IRCReplyBot(irc.IRCClient):

        def connectionMade(self):
            self.nickname = self.factory.nickname
            irc.IRCClient.connectionMade(self)

        def privmsg(self, user, channel, msg):
            if user.lower() == channel.lower():
                d = self.factory.getUser(msg)
                d.addErrback(catchError)
                d.addCallback(lambda m: "Status of %s: %s" % (user, m))
                d.addCallback(lambda m: self.msg(user, m))

    class IIRCClientFactory(components.Interface):

        '''
        @ivar nickname
        '''

        def getUser(self, user):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol"""

    class IRCClientFactoryFromService(protocol.ClientFactory):

       __implements__ = IIRCClientFactory,

       protocol = IRCReplyBot
       nickname = None

       def __init__(self, service):
           self.service = service

        def getUser(self, user):
            return self.service.getUser()

    components.registerAdapter(IRCClientFactoryFromService, IFingerService)

    class UsersModel(model.MethodModel):

        def __init__(self, service):
            self.service = service

        def wmfactory_users(self):
            return self.service.getUsers()

    components.registerAdapter(UsersModel, IFingerService)

    class UserStatusTree(page.Page):

        template = """<html><head><title>Users</title><head><body>
                      <h1>Users</h1>
                      <ul model="users" view="List">
                      <li pattern="listItem" /><a view="Link" model="."
                      href="dummy"><span model="." view="Text" /></a>
                      </ul></body></html>"""

        def initialize(self, **kwargs):
            self.putChild('RPC2.0', UserStatusXR(self.model.service))

        def getDynamicChild(self, path, request):
            return UserStatus(user=path, service=self.model.service)

    components.registerAdapter(UserStatusTree, IFingerService)

    class UserStatus(page.Page):

        template='''<html><head><title view="Text" model="user"/></heaD>
        <body><h1 view="Text" model="user"/>
        <p mode="status" view="Text" />
        </body></html>'''

        def initialize(self, **kwargs):
            self.user = kwargs['user']
            self.service = kwargs['service']

        def wmfactory_user(self):
            return self.user

        def wmfactory_status(self):
            return self.service.getUser(self.user)

    class UserStatusXR(xmlrpc.XMLPRC):

        def __init__(self, service):
            xmlrpc.XMLRPC.__init__(self)
            self.service = service

        def xmlrpc_getUser(self, user):
            return self.service.getUser(user)

    class FingerService(app.ApplicationService):

        __implements__ = IFingerService,

        def __init__(self, file, *args, **kwargs):
            app.ApplicationService.__init__(self, *args, **kwargs)
            self.file = file

        def startService(self):
            app.ApplicationService.startService(self)
            self._read()

        def _read(self):
            self.users = {}
            for line in file(self.file):
                user, status = line.split(':', 1)
                self.users[user] = status
            self.call = reactor.callLater(30, self._read)

        def stopService(self):
            app.ApplicationService.stopService(self)
            self.call.cancel()

        def getUser(self, user):
            return defer.succeed(self.users.get(u, "No such user"))

        def getUsers(self):
            return defer.succeed(self.users.keys())

    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, IFingerFactory(f))
    application.listenTCP(80, server.Site(resource.IResource(f)))
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)

Here we convert to using Woven, instead of manually constructing HTML snippets. Woven is a sophisticated web templating system. Its main features are to disallow any code inside the HTML, and transparent integration with deferred results.

## Use Perspective Broker

    # Do everything properly, and componentize
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic, irc
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc, microdom
    from twisted.web.woven import page, widget
    from twisted.spread import pb
    import cgi

    class IFingerService(components.Interface):

        def getUser(self, user):
            '''Return a deferred returning a string'''

        def getUsers(self):
            '''Return a deferred returning a list of strings'''

    class IFingerSettingService(components.Interface):

        def setUser(self, user, status):
            '''Set the user's status to something'''

    def catchError(err):
        return "Internal error in server"

    class FingerProtocol(basic.LineReceiver):

        def lineReceived(self, user):
            d = self.factory.getUser(user)
            d.addErrback(catchError)
            def writeValue(value):
                self.transport.write(value)
                self.transport.loseConnection()
            d.addCallback(writeValue)

    class IFingerFactory(components.Interface):

        def getUser(self, user):
            """Return a deferred returning a string""""

        def buildProtocol(self, addr):
            """Return a protocol returning a string""""

    class FingerFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerFactory,

        protocol = FingerProtocol

        def __init__(self, service):
            self.service = service

        def getUser(self, user):
            return self.service.getUser(user)

    components.registerAdapter(FingerFactoryFromService, IFingerService)

    class FingerSetterProtocol(basic.LineReceiver):

          def connectionMade(self):
              self.lines = []

          def lineReceived(self, line):
              self.lines.append(line)

          def connectionLost(self):
              if len(self.lines) == 2:
                  self.factory.setUser(*self.lines)

    class IFingerSetterFactory(components.Interface):

        def setUser(self, user, status):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol returning a string"""

    class FingerSetterFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerSetterFactory,

        protocol = FingerSetterProtocol

        def __init__(self, service):
            self.service = service

        def setUser(self, user, status):
            self.service.setUser(user, status)

    components.registerAdapter(FingerSetterFactoryFromService,
                               IFingerSettingService)

    class IRCReplyBot(irc.IRCClient):

        def connectionMade(self):
            self.nickname = self.factory.nickname
            irc.IRCClient.connectionMade(self)

        def privmsg(self, user, channel, msg):
            if user.lower() == channel.lower():
                d = self.factory.getUser(msg)
                d.addErrback(catchError)
                d.addCallback(lambda m: "Status of %s: %s" % (user, m))
                d.addCallback(lambda m: self.msg(user, m))

    class IIRCClientFactory(components.Interface):

        '''
        @ivar nickname
        '''

        def getUser(self, user):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol"""

    class IRCClientFactoryFromService(protocol.ClientFactory):

       __implements__ = IIRCClientFactory,

       protocol = IRCReplyBot
       nickname = None

       def __init__(self, service):
           self.service = service

        def getUser(self, user):
            return self.service.getUser()

    components.registerAdapter(IRCClientFactoryFromService, IFingerService)

    class UsersModel(model.MethodModel):

        def __init__(self, service):
            self.service = service

        def wmfactory_users(self):
            return self.service.getUsers()

    components.registerAdapter(UsersModel, IFingerService)

    class UserStatusTree(page.Page):

        template = """<html><head><title>Users</title><head><body>
                      <h1>Users</h1>
                      <ul model="users" view="List">
                      <li pattern="listItem" /><a view="Link" model="."
                      href="dummy"><span model="." view="Text" /></a>
                      </ul></body></html>"""

        def initialize(self, **kwargs):
            self.putChild('RPC2.0', UserStatusXR(self.model.service))

        def getDynamicChild(self, path, request):
            return UserStatus(user=path, service=self.model.service)

    components.registerAdapter(UserStatusTree, IFingerService)

    class UserStatus(page.Page):

        template='''<html><head><<title view="Text" model="user"/></heaD>
        <body><h1 view="Text" model="user"/>
        <p mode="status" view="Text" />
        </body></html>'''

        def initialize(self, **kwargs):
            self.user = kwargs['user']
            self.service = kwargs['service']

        def wmfactory_user(self):
            return self.user

        def wmfactory_status(self):
            return self.service.getUser(self.user)

    class UserStatusXR(xmlrpc.XMLPRC):

        def __init__(self, service):
            xmlrpc.XMLRPC.__init__(self)
            self.service = service

        def xmlrpc_getUser(self, user):
            return self.service.getUser(user)

    class IPerspectiveFinger(components.Interface):

        def remote_getUser(self, username):
            """return a user's status"""

        def remote_getUsers(self):
            """return a user's status"""

    class PerspectiveFingerFromService(pb.Root):

        __implements__ = IPerspectiveFinger,

        def __init__(self, service):
            self.service = service

        def remote_getUser(self, username):
            return self.service.getUser(username)

        def remote_getUsers(self):
            return self.service.getUsers()

    components.registerAdapter(PerspectiveFingerFromService, IFingerService)

    class FingerService(app.ApplicationService):

        __implements__ = IFingerService,

        def __init__(self, file, *args, **kwargs):
            app.ApplicationService.__init__(self, *args, **kwargs)
            self.file = file

        def startService(self):
            app.ApplicationService.startService(self)
            self._read()

        def _read(self):
            self.users = {}
            for line in file(self.file):
                user, status = line.split(':', 1)
                self.users[user] = status
            self.call = reactor.callLater(30, self._read)

        def stopService(self):
            app.ApplicationService.stopService(self)
            self.call.cancel()

        def getUser(self, user):
            return defer.succeed(self.users.get(u, "No such user"))

        def getUsers(self):
            return defer.succeed(self.users.keys())

    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, IFingerFactory(f))
    application.listenTCP(80, server.Site(resource.IResource(f)))
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)
    application.listenTCP(8889, pb.BrokerFactory(IPerspectiveFinger(f))

We add support for perspective broker, Twisted's native remote object protocol. Now, Twisted clients will not have to go through XML-RPCish contortions to get information about users.

## Support HTTPS

    # Do everything properly, and componentize
    from twisted.internet import protocol, reactor, defer, app
    from twisted.protocols import basic, irc
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc, microdom
    from twisted.web.woven import page, widget
    from twisted.spread import pb
    from OpenSSL import SSL
    import cgi

    class IFingerService(components.Interface):

        def getUser(self, user):
            '''Return a deferred returning a string'''

        def getUsers(self):
            '''Return a deferred returning a list of strings'''

    class IFingerSettingService(components.Interface):

        def setUser(self, user, status):
            '''Set the user's status to something'''

    def catchError(err):
        return "Internal error in server"

    class FingerProtocol(basic.LineReceiver):

        def lineReceived(self, user):
            d = self.factory.getUser(user)
            d.addErrback(catchError)
            def writeValue(value):
                self.transport.write(value)
                self.transport.loseConnection()
            d.addCallback(writeValue)

    class IFingerFactory(components.Interface):

        def getUser(self, user):
            """Return a deferred returning a string""""

        def buildProtocol(self, addr):
            """Return a protocol returning a string""""

    class FingerFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerFactory,

        protocol = FingerProtocol

        def __init__(self, service):
            self.service = service

        def getUser(self, user):
            return self.service.getUser(user)

    components.registerAdapter(FingerFactoryFromService, IFingerService)

    class FingerSetterProtocol(basic.LineReceiver):

          def connectionMade(self):
              self.lines = []

          def lineReceived(self, line):
              self.lines.append(line)

          def connectionLost(self):
              if len(self.lines) == 2:
                  self.factory.setUser(*self.lines)

    class IFingerSetterFactory(components.Interface):

        def setUser(self, user, status):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol returning a string"""

    class FingerSetterFactoryFromService(protocol.ServerFactory):

        __implements__ = IFingerSetterFactory,

        protocol = FingerSetterProtocol

        def __init__(self, service):
            self.service = service

        def setUser(self, user, status):
            self.service.setUser(user, status)

    components.registerAdapter(FingerSetterFactoryFromService,
                               IFingerSettingService)

    class IRCReplyBot(irc.IRCClient):

        def connectionMade(self):
            self.nickname = self.factory.nickname
            irc.IRCClient.connectionMade(self)

        def privmsg(self, user, channel, msg):
            if user.lower() == channel.lower():
                d = self.factory.getUser(msg)
                d.addErrback(catchError)
                d.addCallback(lambda m: "Status of %s: %s" % (user, m))
                d.addCallback(lambda m: self.msg(user, m))

    class IIRCClientFactory(components.Interface):

        '''
        @ivar nickname
        '''

        def getUser(self, user):
            """Return a deferred returning a string"""

        def buildProtocol(self, addr):
            """Return a protocol"""

    class IRCClientFactoryFromService(protocol.ClientFactory):

       __implements__ = IIRCClientFactory,

       protocol = IRCReplyBot
       nickname = None

       def __init__(self, service):
           self.service = service

        def getUser(self, user):
            return self.service.getUser()

    components.registerAdapter(IRCClientFactoryFromService, IFingerService)

    class UsersModel(model.MethodModel):

        def __init__(self, service):
            self.service = service

        def wmfactory_users(self):
            return self.service.getUsers()

    components.registerAdapter(UsersModel, IFingerService)

    class UserStatusTree(page.Page):

        template = """<html><head><title>Users</title><head><body>
                      <h1>Users</h1>
                      <ul model="users" view="List">
                      <li pattern="listItem" /><a view="Link" model="."
                      href="dummy"><span model="." view="Text" /></a>
                      </ul></body></html>"""

        def initialize(self, **kwargs):
            self.putChild('RPC2.0', UserStatusXR(self.model.service))

        def getDynamicChild(self, path, request):
            return UserStatus(user=path, service=self.model.service)

    components.registerAdapter(UserStatusTree, IFingerService)

    class UserStatus(page.Page):

        template='''<html><head><title view="Text" model="user"/></heaD>
        <body><h1 view="Text" model="user"/>
        <p mode="status" view="Text" />
        </body></html>'''

        def initialize(self, **kwargs):
            self.user = kwargs['user']
            self.service = kwargs['service']

        def wmfactory_user(self):
            return self.user

        def wmfactory_status(self):
            return self.service.getUser(self.user)

    class UserStatusXR(xmlrpc.XMLPRC):

        def __init__(self, service):
            xmlrpc.XMLRPC.__init__(self)
            self.service = service

        def xmlrpc_getUser(self, user):
            return self.service.getUser(user)

    class IPerspectiveFinger(components.Interface):

        def remote_getUser(self, username):
            """return a user's status"""

        def remote_getUsers(self):
            """return a user's status"""

    class PerspectiveFingerFromService(pb.Root):

        __implements__ = IPerspectiveFinger,

        def __init__(self, service):
            self.service = service

        def remote_getUser(self, username):
            return self.service.getUser(username)

        def remote_getUsers(self):
            return self.service.getUsers()

    components.registerAdapter(PerspectiveFingerFromService, IFingerService)

    class FingerService(app.ApplicationService):

        __implements__ = IFingerService,

        def __init__(self, file, *args, **kwargs):
            app.ApplicationService.__init__(self, *args, **kwargs)
            self.file = file

        def startService(self):
            app.ApplicationService.startService(self)
            self._read()

        def _read(self):
            self.users = {}
            for line in file(self.file):
                user, status = line.split(':', 1)
                self.users[user] = status
            self.call = reactor.callLater(30, self._read)

        def stopService(self):
            app.ApplicationService.stopService(self)
            self.call.cancel()

        def getUser(self, user):
            return defer.succeed(self.users.get(u, "No such user"))

        def getUsers(self):
            return defer.succeed(self.users.keys())

    class ServerContextFactory:

        def getContext(self):
            """Create an SSL context.

            This is a sample implementation that loads a certificate from a file
            called 'server.pem'."""
            ctx = SSL.Context(SSL.SSLv23_METHOD)
            ctx.use_certificate_file('server.pem')
            ctx.use_privatekey_file('server.pem')
            return ctx

    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, IFingerFactory(f))
    site = server.Site(resource.IResource(f))
    application.listenTCP(80, site)
    application.listenSSL(443, site, ServerContextFactory())
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)
    application.listenTCP(8889, pb.BrokerFactory(IPerspectiveFinger(f))

All we need to do to code an HTTPS site is just write a context factory (in this case, which loads the certificate from a certain file) and then use the listenSSL method. Note that one factory (in this case, a site) can listen on multiple ports with multiple protocols.

## Finger Proxy

    class FingerClient(protocol.Protocol):

        def connectionMade(self):
            self.transport.write(self.factory.user+"\r\n")
            self.buf = []

        def dataReceived(self, data):
            self.buf.append(data)

        def connectionLost(self):
            self.factory.gotData(''.join(self.buf))

    class FingerClientFactory(protocol.ClientFactory):

        protocol = FingerClient

        def __init__(self, user):
            self.user = user
            self.d = defer.Deferred()

        def clientConnectionFailed(self, _, reason):
            self.d.errback(reason)

        def gotData(self, data):
            self.d.callback(data)

    def finger(user, host, port=79):
        f = FingerClientFactory(user)
        reactor.connectTCP(host, port, f)
        return f.d

    class ProxyFingerService(app.ApplicationService):
        __implements__ = IFingerService

        def getUser(self, user):
            user, host = user.split('@', 1)
            ret = finger(user, host)
            ret.addErrback(lambda _: "Could not connect to remote host")
            return ret

        def getUsers(self):
            return defer.succeed([])

    application = app.Application('finger', uid=1, gid=1)
    f = ProxyFingerService(application, 'finger')
    application.listenTCP(79, IFingerFactory(f))

Writing new clients with Twisted is much like writing new servers. We implement the protocol, which just gathers up all the data, and give it to the factory. The factory keeps a deferred which is triggered if the connection either fails or succeeds. When we use the client, we first make sure the deferred will never fail, by producing a message in that case. Implementing a wrapper around client which just returns the deferred is a common pattern. While being less flexible than using the factory directly, it is also more convenient.

## Organization

- Code belongs in modules: everything above the `application=` line.
- Templates belong in separate files. The templateFile attribute can be used to indicate the file.
- The templateDirectory attribute will be used to indicate where to look for the files.

    from twisted.internet import app
    from finger import FingerService, IIRCclient, ServerContextFactory, \
                       IFingerFactory, IPerspectiveFinger
    from twisted.web import resource, server
    from twisted.spread import pb

    application = app.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users', application, 'finger')
    application.listenTCP(79, IFingerFactory(f))
    r = resource.IResource(f)
    r.templateDirectory = '/usr/share/finger/templates/'
    site = server.Site(r)
    application.listenTCP(80, site)
    application.listenSSL(443, site, ServerContextFactory())
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    application.connectTCP('irc.freenode.org', 6667, i)
    application.listenTCP(8889, pb.BrokerFactory(IPerspectiveFinger(f))

- Seperaration between: code (module), configuration (file above), presentation (templates), contents (/etc/users), deployment (twistd)
- Examples, early prototypes don't need that.
- But when writing correctly, easy to do!

## Easy Configuration

We can also supply easy configuration for common cases

    # in finger.py moudle
    def updateApplication(app, **kwargs):
         f = FingerService(kwargs['users'], application, 'finger')
         application.listenTCP(79, IFingerFactory(f))
         r = resource.IResource(f)
         r.templateDirectory = kwargs['templates']
         site = server.Site(r)
         app.listenTCP(80, site)
         if kwargs.get('ssl'):
             app.listenSSL(443, site, ServerContextFactory())
         if kwargs.has_key('ircnick'):
             i = IIRCClientFactory(f)
             i.nickname = kwargs['ircnick']
             ircServer = kwargs['ircserver']
             application.connectTCP(ircserver, 6667, i)
         if kwargs.has_key('pbport'):
             application.listenTCP(int(kwargs['pbport']),
                                   pb.BrokerFactory(IPerspectiveFinger(f))

And we can write simpler files now:

    # simple-finger.tpy
    from twisted.internet import app
    import finger

    application = app.Application('finger', uid=1, gid=1)
    finger.updateApplication(application,
       users='/etc/users',
       templatesDirectory='/usr/share/finger/templates',
       ssl=1,
       ircnick='fingerbot',
       ircserver='irc.freenode.net',
       pbport=8889
    )

Note: the finger user still has ultimate power: he can use updateApplication, or he can use the lower-level interface if he has specific needs (maybe an ircserver on some other port? maybe we want the non-ssl webserver to listen only locally? etc. etc.) This is an important design principle: never force a layer of abstraction: allow usage of layers of abstractions.

The pasta theory of design:

- Spaghetti: each piece of code interacts with every other piece of code [can be implemented with GOTO, functions, objects]
- Lasagna: code has carefully designed layers. Each layer is, in theory independent. However low-level layers usually cannot be used easily, and high-level layers depend on low-level layers.
- Raviolli: each part of the code is useful by itself. There is a thin layer of interfaces between various parts [the sauce]. Each part can be usefully be used elsewhere.
- ...but sometimes, the user just wants to order "Raviolli", so one coarse-grain easily definable layer of abstraction on top of it all can be useful.

## Plugins

So far, the user had to be somewhat of a programmer to use this. Maybe we can eliminate even that? Move old code to "finger/service.py", put empty "__init__.py" and...

    # finger/tap.py
    from twisted.python import usage
    from finger import service

    class Options(usage.Options):

        optParams = [
          ['users', 'u', '/etc/users'],
          ['templatesDirectory', 't', '/usr/share/finger/templates'],
          ['ircnick', 'n', 'fingerbot'],
          ['ircserver', None, 'irc.freenode.net'],
          ['pbport', 'p', 8889],
        ]

        optFlags = [['ssl', 's']]

    def updateApplication(app, config):
        service.updateApplication(app, **config)

And register it all:

    #finger/plugins.tml
    register('Finger', 'finger.tap', type='tap', tapname='finger')

And now, the following works

    % mktap finger --users=/usr/local/etc/users --ircnick=moshez-finger
    % sudo twistd -f finger.tap

## OS Integration

If we already have the "finger" package installed, we can achieve easy integration:

on Debian--

    % tap2deb --unsigned -m "Foo <foo@example.com>" --type=python finger.tpy
    % sudo dpkg -i .build/*.deb

On Red Hat [or Mandrake]

    % tap2rpm --type=python finger.tpy #[maybe other options needed]
    % sudo rpm -i .build/*.rpm

Will properly register configuration files, init.d sripts, etc. etc.

If it doesn't work on your favourite OS: patches accepted!

## Summary

- Twisted is asynchronous
- Twisted has implementations of every useful protocol
- In Twisted, implementing new protocols is easy [we just did three]
- In Twisted, achieving tight integration of servers and clients is easy.
- In Twisted, achieving high code usability is easy.
- Ease of use of Twisted follows, in a big part, from that of Python.
- Bonus: No buffer overflows. Ever. No matter what.

## Motto

- "Twisted is not about forcing. It's about mocking you when you use the technology in suboptimal ways."
- You're not forced to use anything except the reactor...
- ...not the protocol implementations...
- ...not application...
- ...not services...
- ...not components...
- ...not woven...
- ...not perspective broker...
- ...etc.
- But you should!
- Reinventing the wheel is not a good idea, especially if you form some vaguely squarish lump of glue and poison and try and attach it to your car.
- The Twisted team solved many of the problems you are likely to come across...
- ...several times...
- ...getting it right the nth time.