# The Evolution of Finger: moving to a component based architecture

## Introduction

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

In this section of the tutorial, we'll move our code to a component architecture so that adding new features is trivial. See Interfaces and Adapters for a more complete discussion of components.

## Write Maintainable Code

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 rereading the file every 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 a call to `zope.interface.implements`. Then, adapters are used to make this service look like an appropriate class for various things: for supplying a finger factory to `TCPServer`, for supplying a resource to site's constructor, and to provide an IRC client factory for `TCPClient`. All the adapters use are the methods in `FingerService` they are declared to use: `getUser`/`getUsers`. We could, of course, skip 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.

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    # Do everything properly, and componentize
    from twisted.application import internet, service
    from twisted.internet import protocol, reactor, defer
    from twisted.words.protocols import irc
    from twisted.protocols import basic
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc
    from zope.interface import Interface, implements
    import cgi

    class IFingerService(Interface):

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

        def getUsers():
            """
            Return a deferred returning a list of strings.
            """

    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+'\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.
            """

    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,
                               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.
            """

    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,
                               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: "Status of %s: %s" % (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.
            """

    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(user)

    components.registerAdapter(IRCClientFactoryFromService,
                               IFingerService,
                               IIRCClientFactory)

    class UserStatusTree(resource.Resource):

        implements(resource.IResource)

        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 = ['<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):
            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)

    class FingerService(service.Service):

        implements(IFingerService)

        def __init__(self, filename):
            self.filename = filename
            self.users = {}

        def _read(self):
            self.users.clear()
            for line in file(self.filename):
                user, status = line.split(':', 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, "No such user"))

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

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

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

    application = service.Application('finger', uid=1, gid=1)
    f = FingerService('/etc/users')
    serviceCollection = service.IServiceCollection(application)
    internet.TCPServer(79, IFingerFactory(f)
                       ).setServiceParent(serviceCollection)
    internet.TCPServer(8000, server.Site(resource.IResource(f))
                       ).setServiceParent(serviceCollection)
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    internet.TCPClient('irc.freenode.org', 6667, i
                       ).setServiceParent(serviceCollection)

Source listing - listings/finger/finger19.tac

## 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

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    class IFingerSetterService(Interface):

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

    # Advantages of latest version

    class MemoryFingerService(service.Service):

        implements([IFingerService, IFingerSetterService])

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

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

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

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

    f = MemoryFingerService(moshez='Happy and well')
    serviceCollection = service.IServiceCollection(application)
    internet.TCPServer(1079, IFingerSetterFactory(f), interface='127.0.0.1'
                       ).setServiceParent(serviceCollection)

Source listing - listings/finger/finger19a_changes.py

Full source code here:

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    # Do everything properly, and componentize
    from twisted.application import internet, service
    from twisted.internet import protocol, reactor, defer
    from twisted.words.protocols import irc
    from twisted.protocols import basic
    from twisted.python import components
    from twisted.web import resource, server, static, xmlrpc
    from zope.interface import Interface, implements
    import cgi

    class IFingerService(Interface):

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

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

    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+'\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"""

    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,
                               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"""

    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,
                               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: "Status of %s: %s" % (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"""

    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(user)

    components.registerAdapter(IRCClientFactoryFromService,
                               IFingerService,
                               IIRCClientFactory)

    class UserStatusTree(resource.Resource):

        implements(resource.IResource)

        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 = ['<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):
            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)

    class MemoryFingerService(service.Service):

        implements([IFingerService, IFingerSetterService])

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

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

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

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

    application = service.Application('finger', uid=1, gid=1)
    f = MemoryFingerService(moshez='Happy and well')
    serviceCollection = service.IServiceCollection(application)
    internet.TCPServer(79, IFingerFactory(f)
                       ).setServiceParent(serviceCollection)
    internet.TCPServer(8000, server.Site(resource.IResource(f))
                       ).setServiceParent(serviceCollection)
    i = IIRCClientFactory(f)
    i.nickname = 'fingerbot'
    internet.TCPClient('irc.freenode.org', 6667, i
                       ).setServiceParent(serviceCollection)
    internet.TCPServer(1079, IFingerSetterFactory(f), interface='127.0.0.1'
                       ).setServiceParent(serviceCollection)

Source listing - listings/finger/finger19a.tac

## Aspect-Oriented Programming

At last, an example of aspect-oriented programming that isn't about logging or timing. This code is actually useful! Watch how aspect-oriented programming helps you write less code and have fewer dependencies!

Index

Version: 11.0.0