# The Evolution of Finger: pluggable backends

## Introduction

This is the fifth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger .

In this part we will add new several new backends to our finger service using the component-based architecture developed in The Evolution of Finger: moving to a component based architecture . This will show just how convenient it is to implement new back-ends 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 Back-end

`finger19b_changes.py`

    import pwd

    from twisted.internet import defer, protocol, reactor, utils

    # Another back-end

    @implementer(IFingerService)
    class LocalFingerService(service.Service):
        def getUser(self, user):
            # need a local finger daemon running for this to work
            return utils.getProcessOutput("finger", [user])

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

    f = LocalFingerService()

Full source code here:

`finger19b.tac`

    # Do everything properly, and componentize
    import cgi
    import pwd

    from zope.interface import Interface, implementer

    from twisted.application import internet, service, strports
    from twisted.internet import defer, endpoints, protocol, reactor, utils
    from twisted.protocols import basic
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc
    from twisted.words.protocols import irc

    class IFingerService(Interface):
        def getUser(user):
            """
            Return a deferred returning L{bytes}.
            """

        def getUsers():
            """
            Return a deferred returning a L{list} of L{bytes}.
            """

    class IFingerSetterService(Interface):
        def setUser(user, status):
            """
            Set the user's status to something.
            """

    class IFingerSetterService(Interface):
        def setUser(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 + b"\r\n")
                self.transport.loseConnection()

            d.addCallback(writeValue)

    class IFingerFactory(Interface):
        def getUser(user):
            """
            Return a deferred returning a string.
            """

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

    @implementer(IFingerFactory)
    class FingerFactoryFromService(protocol.ServerFactory):
        protocol = FingerProtocol

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

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

    components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory)

    class FingerSetterProtocol(basic.LineReceiver):
        def connectionMade(self):
            self.lines = []

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

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

    class IFingerSetterFactory(Interface):
        def setUser(user, status):
            """
            Return a deferred returning L{bytes}.
            """

        def buildProtocol(addr):
            """
            Return a protocol returning L{bytes}.
            """

    @implementer(IFingerSetterFactory)
    class FingerSetterFactoryFromService(protocol.ServerFactory):
        protocol = FingerSetterProtocol

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

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

    components.registerAdapter(
        FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory
    )

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

        def privmsg(self, user, channel, msg):
            user = user.split("!")[0]
            if self.nickname.lower() == channel.lower():
                d = self.factory.getUser(msg)
                d.addErrback(catchError)
                d.addCallback(lambda m: f"Status of {msg}: {m}")
                d.addCallback(lambda m: self.msg(user, m))

    class IIRCClientFactory(Interface):

        """
        @ivar nickname
        """

        def getUser(user):
            """
            Return a deferred returning L{bytes}.
            """

        def buildProtocol(addr):
            """
            Return a protocol.
            """

    @implementer(IIRCClientFactory)
    class IRCClientFactoryFromService(protocol.ClientFactory):
        protocol = IRCReplyBot
        nickname = None

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

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

    components.registerAdapter(
        IRCClientFactoryFromService, IFingerService, IIRCClientFactory
    )

    @implementer(resource.IResource)
    class UserStatusTree(resource.Resource):
        def __init__(self, service):
            resource.Resource.__init__(self)
            self.service = service
            self.putChild("RPC2", UserStatusXR(self.service))

        def render_GET(self, request):
            d = self.service.getUsers()

            def formatUsers(users):
                l = [f'<li><a href="{user}">{user}</a></li>' 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):
            if path == "":
                return UserStatusTree(self.service)
            else:
                return UserStatus(path, self.service)

    components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)

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

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

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

    @implementer(IFingerService)
    class FingerService(service.Service):
        def __init__(self, filename):
            self.filename = filename
            self.users = {}

        def _read(self):
            self.users.clear()
            with open(self.filename, "rb") as f:
                for line in f:
                    user, status = line.split(b":", 1)
                    user = user.strip()
                    status = status.strip()
                    self.users[user] = status
            self.call = reactor.callLater(30, self._read)

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

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

        def startService(self):
            self._read()
            service.Service.startService(self)

        def stopService(self):
            service.Service.stopService(self)
            self.call.cancel()

    # Another back-end

    @implementer(IFingerService)
    class LocalFingerService(service.Service):
        def getUser(self, user):
            # need a local finger daemon running for this to work
            return utils.getProcessOutput(b"finger", [user])

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

    application = service.Application("finger", uid=1, gid=1)
    f = LocalFingerService()
    serviceCollection = service.IServiceCollection(application)
    strports.service("tcp:79", IFingerFactory(f)).setServiceParent(serviceCollection)
    strports.service("tcp:8000", server.Site(resource.IResource(f))).setServiceParent(
        serviceCollection
    )
    i = IIRCClientFactory(f)
    i.nickname = "fingerbot"
    internet.ClientService(
        endpoints.clientFromString(reactor, "tcp:irc.freenode.org:6667"), i
    ).setServiceParent(serviceCollection)

We’ve already written this, but now we get more for less work: the network code is completely separate from the back-end.

## Yet Another Back-end: Doing the Standard Thing

`finger19c_changes.py`

    import os
    import pwd

    from twisted.internet import defer, protocol, reactor, utils

    # Yet another back-end

    @implementer(IFingerService)
    class LocalFingerService(service.Service):
        def getUser(self, user):
            user = user.strip()
            try:
                entry = pwd.getpwnam(user)
            except KeyError:
                return defer.succeed("No such user")
            try:
                f = open(os.path.join(entry[5], ".plan"))
            except OSError:
                return defer.succeed("No such user")
            with f:
                data = f.read()
            data = data.strip()
            return defer.succeed(data)

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

    f = LocalFingerService()

Full source code here:

`finger19c.tac`

    # Do everything properly, and componentize
    import cgi
    import os
    import pwd

    from zope.interface import Interface, implementer

    from twisted.application import internet, service, strports
    from twisted.internet import defer, endpoints, protocol, reactor, utils
    from twisted.protocols import basic
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc
    from twisted.words.protocols import irc

    class IFingerService(Interface):
        def getUser(user):
            """
            Return a deferred returning L{bytes}.
            """

        def getUsers():
            """
            Return a deferred returning a L{list} of L{bytes}.
            """

    class IFingerSetterService(Interface):
        def setUser(user, status):
            """
            Set the user's status to something.
            """

    class IFingerSetterService(Interface):
        def setUser(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 + b"\r\n")
                self.transport.loseConnection()

            d.addCallback(writeValue)

    class IFingerFactory(Interface):
        def getUser(user):
            """
            Return a deferred returning a string.
            """

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

    @implementer(IFingerFactory)
    class FingerFactoryFromService(protocol.ServerFactory):

        protocol = FingerProtocol

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

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

    components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory)

    class FingerSetterProtocol(basic.LineReceiver):
        def connectionMade(self):
            self.lines = []

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

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

    class IFingerSetterFactory(Interface):
        def setUser(user, status):
            """
            Return a deferred returning a string.
            """

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

    @implementer(IFingerSetterFactory)
    class FingerSetterFactoryFromService(protocol.ServerFactory):

        protocol = FingerSetterProtocol

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

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

    components.registerAdapter(
        FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory
    )

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

        def privmsg(self, user, channel, msg):
            user = user.split("!")[0]
            if self.nickname.lower() == channel.lower():
                d = self.factory.getUser(msg)
                d.addErrback(catchError)
                d.addCallback(lambda m: f"Status of {msg}: {m}")
                d.addCallback(lambda m: self.msg(user, m))

    class IIRCClientFactory(Interface):

        """
        @ivar nickname
        """

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

        def buildProtocol(addr):
            """
            Return a protocol.
            """

    @implementer(IIRCClientFactory)
    class IRCClientFactoryFromService(protocol.ClientFactory):

        protocol = IRCReplyBot
        nickname = None

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

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

    components.registerAdapter(
        IRCClientFactoryFromService, IFingerService, IIRCClientFactory
    )

    @implementer(resource.IResource)
    class UserStatusTree(resource.Resource):
        def __init__(self, service):
            resource.Resource.__init__(self)
            self.service = service
            self.putChild("RPC2", UserStatusXR(self.service))

        def render_GET(self, request):
            d = self.service.getUsers()

            def formatUsers(users):
                l = [f'<li><a href="{user}">{user}</a></li>' 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):
            if path == "":
                return UserStatusTree(self.service)
            else:
                return UserStatus(path, self.service)

    components.registerAdapter(UserStatusTree, IFingerService, resource.IResource)

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

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

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

    @implementer(IFingerService)
    class FingerService(service.Service):
        def __init__(self, filename):
            self.filename = filename
            self.users = {}

        def _read(self):
            self.users.clear()
            with open(self.filename, "rb") as f:
                for line in f:
                    user, status = line.split(b":", 1)
                    user = user.strip()
                    status = status.strip()
                    self.users[user] = status
            self.call = reactor.callLater(30, self._read)

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

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

        def startService(self):
            self._read()
            service.Service.startService(self)

        def stopService(self):
            service.Service.stopService(self)
            self.call.cancel()

    # Yet another back-end

    @implementer(IFingerService)
    class LocalFingerService(service.Service):
        def getUser(self, user):
            user = user.strip()
            try:
                entry = pwd.getpwnam(user)
            except KeyError:
                return defer.succeed(b"No such user")
            try:
                f = open(os.path.join(entry[5], ".plan"))
            except OSError:
                return defer.succeed(b"No such user")
            with f:
                data = f.read()
            data = data.strip()
            return defer.succeed(data)

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

    application = service.Application("finger", uid=1, gid=1)
    f = LocalFingerService()
    serviceCollection = service.IServiceCollection(application)
    strports.service("tcp:79", IFingerFactory(f)).setServiceParent(serviceCollection)
    strports.service("tcp:8000", server.Site(resource.IResource(f))).setServiceParent(
        serviceCollection
    )
    i = IIRCClientFactory(f)
    i.nickname = "fingerbot"
    internet.ClientService(
        endpoints.clientFromString(reactor, "tcp:irc.freenode.org:6667"), i
    ).setServiceParent(serviceCollection)

Not much to say except that now we can be churn out backends like crazy. Feel like doing a back-end for Advogato , for example? Dig out the XML-RPC client support Twisted has, and get to work!