# Authentication with Perspective Broker

## Motivation

In the examples shown in Using Perspective Broker there were some problems. You had to trust the user when they said their name was bob: no passwords or anything. If you wanted a direct-send one-to-one message feature, you might have implemented it by handing a User reference directly off to another User. (so they could invoke `.remote_sendMessage()` on the receiving User): but that lets them do anything else to that user too, things that should probably be restricted to the owner user, like `.remote_joinGroup()` or `.remote_quit()`.

And there were probably places where the easiest implementation was to have the client send a message that included their own name as an argument. Sending a message to the group could just be:

    class Group(pb.Referenceable):

    # ...

      def remote_sendMessage(self, from_user, message):
        for user in self.users:
          user.callRemote("sendMessage", "[%s]: %s" % (from_user, message))

But obviously this lets users spoof each other: there's no reason that Alice couldn't do:

    remotegroup.callRemote("sendMessage", "bob", "i like pork")

much to the horror of Bob's vegetarian friends.

(In general, learn to get suspicious if you see `groupName` or `userName` in the argument list of a remotely-invokable method).

You could fix this by adding more classes (with fewer remotely-invokable methods), and making sure that the reference you give to Alice won't let her pretend to be anybody else. You'd probably give Alice her own object, with her name buried inside:

    class User(pb.Referenceable):
      def __init__(self, name):
        self.name = name
      def remote_sendMessage(self, group, message):
        g = findgroup(group)
        for user in g.users:
          user.callRemote("sendMessage", "[%s]: %s" % (self.name, message))

This improves matters because, as long as Alice only has a reference to this object and nobody else's, she can't cause a different `self.name` to get used. Of course, you have to make sure that you don't give her a reference to the wrong object.

Note:

Third party references (there aren't any)

Note that the reference that the server gives to a client is only useable by that one client: if they try to hand it off to a third party, they'll get an exception (XXX: which? looks like an assert in pb.py:290 RemoteReference.jellyFor). This helps somewhat: only the client you gave the reference to can cause any damage with it. Of course, the client might be a brainless zombie, simply doing anything some third party wants. When it's not proxying `callRemote` invocations, it's probably terrorizing the living and searching out human brains for sustenance. In short, if you don't trust them, don't give them that reference.

Also note that the design of the serialization mechanism (implemented in `twisted.spread.jelly`: pb, jelly, spread.. get it? Also look for banana and marmalade. What other networking framework can claim API names based on sandwich ingredients?) makes it impossible for the client to obtain a reference that they weren't explicitly given. References passed over the wire are given id numbers and recorded in a per-connection dictionary. If you didn't give them the reference, the id number won't be in the dict, and no amount of id guessing by a malicious client will give them anything else. The dict goes away when the connection is dropped, limiting further the scope of those references.

Of course, everything you've ever given them over that connection can come back to you. If expect the client to invoke your method with some object A that you sent to them earlier, and instead they send you object B (that you also sent to them earlier), and you don't check it somehow, then you've just opened up a security hole. It may be better to keep such objects in a dictionary on the server side, and have the client send you an index string instead. Doing it that way makes it obvious that they can send you anything they want, and improves the chances that you'll remember to implement the right checks. (This is exactly what PB is doing underneath, with a per-connection dictionary of `Referenceable` objects, indexed by a number).

But now she could sneak into another group. So you might have to have an object per-group-per-user:

    class UserGroup(pb.Referenceable):
      def __init__(self, group, user):
        self.group = group
        self.user = user
      def remote_sendMessage(self, message):
        name = self.user.name
        for user in self.group.users:
          user.callRemote("sendMessage", "[%s]: %s" % (name, message))

But that means more code, and more code is bad, especially when it's a common problem (everybody designs with security in mind, right? Right??).

So we have a security problem. We need a way to ask for and verify a password, so we know that Bob is really Bob and not Alice wearing her Hi, my name is Bob t-shirt. And it would make the code cleaner (i.e.: fewer classes) if some methods could know reliably who is calling them.

## A sample application

As a framework for this chapter, we'll be referring to a hypothetical game implemented by several programs using the Twisted framework. This game will have multiple players, where users log in using their client programs, and there is a server, and users can do some things but not others1.

The players make moves in this game by invoking remote methods on objects that live in the server. The clients can't really be relied upon to tell the server who they are with each move they make: they might get it wrong, or (horrors!) lie to mess up the other player.

Let's simplify it to a server-based game of Go (if that can be considered simple). Go has two players, white and black, who take turns placing stones of their own color at the intersections of a 19x19 grid. If we represent the game and board as an object in the server called Game, then the players might interact with it using something like this:

    class Game(pb.Referenceable):
      def remote_getBoard(self):
        return self.board # a dict, with the state of the board
      def remote_move(self, playerName, x, y):
        self.board[x,y] = playerName

But Wait, you say, yes that method takes a playerName, which means they could cheat and move for the other player. So instead, do this:

    class Game(pb.Referenceable):
      def remote_getBoard(self):
        return self.board # a dict, with the state of the board
      def move(self, playerName, x, y):
        self.board[x,y] = playerName

and move the responsibility (and capability) for calling Game.move() out to a different class. That class is a `twisted.spread.pb.Perspective`.

## Perspectives

`pb.Perspective` (and some related classes: Identity, Authorizer, and Service) is a layer on top of the basic PB system that handles username/password checking. The basic idea is that there is a separate Perspective object (probably a subclass you've created) for each user2, and only the authorized user gets a remote reference to that Perspective object. You can store whatever permissions or capabilities the user possesses in that object, and then use them when the user invokes a remote method. You give the user access to the Perspective object instead of the objects that do the real work.

Your code can then look like this:

    class Game:
      def getBoard(self):
        return self.board # a dict, with the state of the board
      def move(self, playerName, x, y):
        self.board[x,y] = playerName

    class PlayerPerspective(pb.Perspective):
      def __init__(self, playerName, game):
        self.playerName = playerName
        self.game = game
      def perspective_move(self, x, y):
        self.game.move(self.playerName, x, y)
      def perspective_getBoard(self):
        return self.game.getBoard()

The code on the server side creates the PlayerPerspective object, giving it the right playerName and a reference to the Game object. The remote player doesn't get a reference to the Game object, only their own PlayerPerspective, so they don't have an opportunity to lie about their name: it comes from the `.playerName` attribute, not an argument of their remote method call.

Here is a brief example of using a Perspective. Most of the support code is magic for now: we'll explain it later.

Note:

This example has more support code than you'd actually need. If you only have one Service, then there's probably a one-to-one relationship between your Identities and your Perspectives. If that's the case, you can use a utility method called Perspective.makeIdentity() instead of creating the perspectives and identities in separate steps. This is shorter, but hides some of the details that are useful here to explain what's going on. Again, this will make more sense later.

    #! /usr/bin/python

    from twisted.spread import pb
    from twisted.cred.authorizer import DefaultAuthorizer
    import twisted.internet.app

    class MyPerspective(pb.Perspective):
        def perspective_foo(self, arg):
            print "I am", self.myname, "perspective_foo(",arg,") called on", self

    # much of the following is magic
    app = twisted.internet.app.Application("pb5server")
    auth = DefaultAuthorizer(app)
    # create the service, tell it to generate MyPerspective objects when asked
    s = pb.Service("myservice", app, auth)
    s.perspectiveClass = MyPerspective

    #  create a MyPerspective
    p1 = s.createPerspective("perspective1")
    p1.myname = "p1"
    # create an Identity, give it a name and password, and allow it access to
    # the MyPerspective we created before
    i1 = auth.createIdentity("user1")
    i1.setPassword("pass1")
    i1.addKeyByString("myservice", "perspective1")
    auth.addIdentity(i1)

    # start the application
    app.listenTCP(8800, pb.BrokerFactory(pb.AuthRoot(auth)))
    app.run(save=0)

Source listing - listings/pb/pb5server.py

    #! /usr/bin/python

    from twisted.spread import pb
    from twisted.internet import reactor

    def main():
        def1 = pb.connect("localhost", 8800,
                          "user1", "pass1",
                          "myservice", "perspective1",
                          timeout=30)
        def1.addCallbacks(connected)
        reactor.run()

    def connected(perspective):
        print "got perspective ref:", perspective
        print "asking it to foo(12)"
        perspective.callRemote("foo", 12)

    main()

Source listing - listings/pb/pb5client.py

Note that once this example has done the method call, you'll have to terminate both ends yourself. Also note that the Perspective's `.attached()` and `.detached()` methods are run when the client connects and disconnects. The base class implementations of these methods just prints a message.

Ok, so that wasn't really very exciting. It doesn't accomplish much more than the first PB example, and used a lot more code to do it. Let's try it again with two users this time, each with their own Perspective. We also override `.attached()` and `.detached()`, just to see how they are called.

Note:

The Perspective object is usually expected to outlast the user's connection to it: it is nominally created some time before the user connects, and survives after they disconnect. `.attached()` and `.detached()` are invoked to let the Perspective know when the user has connected and disconnected.

When the client runs `pb.connect` to establish the connection, they can provide it with an optional `client` argument (which must be a `pb.Referenceable` object). If they do, then a reference to that object will be handed to the server-side Perspective's `.attached` method, in the `clientref` argument.

The server-side Perspective can use it to invoke remote methods on something in the client, so that the client doesn't always have to drive the interaction. In a chat server, the client object would be the one to which display text messages were sent. In a game, this would provide a way to tell the clients that someone has made a move, so they can update their game boards. To actually use it, you'd probably want to subclass Perspective and change the .attached method to stash the clientref somewhere, because the default implementation just drops it.

`.attached()` also receives a reference to the `Identity` object that represents the user. (The user has proved, by using a password of some sort, that they are that `Identity`, and then they can access any service/perspective on the Identity's keyring). The method can use that reference to extract more information about the user.

In addition, `.attached()` has the opportunity to return a different Perspective, if it so chooses. You could have all users initially access the same Perspective, but then as they connect (and `.attached()` gets called), give them unique Perspectives based upon their individual Identities. The client will get a reference to whatever `.attached()` returns, so the default case is to 'return self'.

Finally, when the client goes away (i.e., the network connection has been closed), `.detached()` will be called. The Perspective can use this to mark the user as having gone away: this may mean that outgoing messages should be queued in the Perspective until they reconnect, or callers should be given an error message because they messages cannot be delivered, etc. It can also be used to terminate or suspend any sessions the user was participating in. `detached` is called with the same 'clientref' and Identity objects that were given to the original 'attached' call. It will be invoked on the Perspective object that was returned by `.attached()`.

    #! /usr/bin/python

    from twisted.spread import pb
    from twisted.cred.authorizer import DefaultAuthorizer
    import twisted.internet.app

    class MyPerspective(pb.Perspective):
        def attached(self, clientref, identity):
            print "client attached! they are:", identity
            return self
        def detached(self, ref, identity):
            print "client detached! they were:", identity
        def perspective_foo(self, arg):
            print "I am", self.myname, "perspective_foo(",arg,") called on", self

    # much of the following is magic
    app = twisted.internet.app.Application("pb6server")
    auth = DefaultAuthorizer(app)
    # create the service, tell it to generate MyPerspective objects when asked
    s = pb.Service("myservice", app, auth)
    s.perspectiveClass = MyPerspective

    #  create one MyPerspective
    p1 = s.createPerspective("perspective1")
    p1.myname = "p1"
    # create an Identity, give it a name and password, and allow it access to
    # the MyPerspective we created before
    i1 = auth.createIdentity("user1")
    i1.setPassword("pass1")
    i1.addKeyByString("myservice", "perspective1")
    auth.addIdentity(i1)

    #  create another MyPerspective
    p2 = s.createPerspective("perspective2")
    p2.myname = "p2"
    i2 = auth.createIdentity("user2")
    i2.setPassword("pass2")
    i2.addKeyByString("myservice", "perspective2")
    auth.addIdentity(i2)

    # start the application
    app.listenTCP(8800, pb.BrokerFactory(pb.AuthRoot(auth)))
    app.run(save=0)

Source listing - listings/pb/pb6server.py

    #! /usr/bin/python

    from twisted.spread import pb
    from twisted.internet import reactor

    def main():
        def1 = pb.connect("localhost", 8800,
                          "user1", "pass1",
                          "myservice", "perspective1",
                          timeout=30)
        def1.addCallbacks(connected)
        reactor.run()

    def connected(perspective):
        print "got perspective1 ref:", perspective
        print "asking it to foo(13)"
        perspective.callRemote("foo", 13)

    main()

Source listing - listings/pb/pb6client1.py

    #! /usr/bin/python

    from twisted.spread import pb
    from twisted.internet import reactor

    def main():
        def1 = pb.connect("localhost", 8800,
                          "user2", "pass2",
                          "myservice", "perspective2",
                          timeout=30)
        def1.addCallbacks(connected)
        reactor.run()

    def connected(perspective):
        print "got perspective2 ref:", perspective
        print "asking it to foo(14)"
        perspective.callRemote("foo", 14)

    main()

Source listing - listings/pb/pb6client2.py

While pb6server.py is running, try starting pb6client1, then pb6client2. Compare the argument passed by the `.callRemote()` in each client. You can see how each client logs into a different Perspective.

## Class Overview

Now that we've seen some of the motivation behind the Perspective class, let's start to de-mystify some of the parts labeled magic in `pb6server.py`. Here are the major classes involved:

- `twisted.internet.app.Application`: `twisted/internet/app.py`
- `twisted.cred.service.Service`: `twisted/cred/service.py`
- `twisted.cred.authorizer.Authorizer`: `twisted/cred/authorizer.py`
- `twisted.cred.identity.Identity`: `twisted/cred/identity.py`
- `twisted.cred.perspective.Perspective`: `twisted/cred/pb.py`

You've already seen `Application`. It holds the program-wide settings, like which uid/gid it should run under, and contains a list of ports that it should listen on (with a Factory for each one to create Protocol objects). When used for PB, we put a pb.BrokerFactory on the port. The `Application` also holds a list of Services.

A `Service` is, well, a service. A web server would be a `Service`, as would a chat server, or any other kind of server you might choose to run. What's the difference between a `Service` and an `Application`? You can have multiple `Service`s in a single `Application`: perhaps both a web-based chat service and an IM server in the same program, that let you exchange messages between the two. Or your program might provide different kinds of interfaces to different classes of users: administrators could get one `Service`, while mere end-users get a less-powerful `Service`.

Note:

Note that the `Service` is a server of some sort, but that doesn't mean there's a one-to-one relationship between the `Service` and the TCP port that's being listened to. In theory, several different `Service`s can hang off the same TCP port. Look at the MultiService class for details.

The `Service` is reponsible for providing `Perspective` objects. More on that later.

The `Authorizer` is a class that provides `Identity` objects. The abstract base class is `twisted.cred.authorizer.Authorizer`, and for simple purposes you can just use `twisted.cred.authorizer.DefaultAuthorizer`, which is a subclass that stores pre-generated Identities in a simple dict (indexed by username). The `Authorizer`'s purpose in life is to implement the `.getIdentityRequest()` method, which takes a user name and (eventually) returns the corresponding `Identity` object.

Each `Identity` object represents a single user, with a username and a password of some sort. Its job is to talk to the as-yet-anonymous remote user and verify that they really are who they claim to be. The default `twisted.cred.authorizer.Identity` class implements MD5-hashed challenge-response password authorization, much like the HTTP MD5-Authentication method: the server sends a random challenge string, the client concatenates a hash of their password with the challenge string, and sends back a hash of the result. At this point the client is said to be authorized for access to that `Identity`, and they are given a remote reference to the `Identity` (actually a wrapper around it), giving them all the privileges of that `Identity`.

Those privileges are limited to requesting `Perspective`s. The `Identity` object also has a keyring, which is a list of (serviceName, perspectiveName) pairs that the corresponding authorized user is allowed to access. Once the user has been authenticated, the `Identity`'s job is to implement `.requestPerspectiveForKey()`, which it does by verifying the key exists on the keyring, then asking the matching `Service` to do `.getPerspectiveForIdentity()`.

Finally, the `Perspective` is the subclass of pb.Perspective that implements whatever `perspective_*` methods you wish to expose to an authenticated remote user. It also implements `.attached()` and `.detached()`, which are run when the user connects (actually when they finish the authentication sequence) or disconnects. Each `Perspective` has a name, which is scoped to the `Service` which owns the `Perspective`.

## Class Responsibilities

Now that we've gone over the classes and objects involved, let's look at the specific responsibilities of each. Most of these classes are on the hook to implement just one or two particular methods, and the rest of the class is just support code (or the main method has been broken up for ease of subclassing). This section indicates what those main methods are and when they get called.

### Authorizer

The `Authorizer` has to provide `Identity` objects (requested by name) by implementing `.getIdentityRequest()`. The `DefaultAuthorizer` class just looks up the name in a dict called `self.identities`, so when you use it, you have to make the Identities ahead of time (using `i = auth.createIdentity()`) and store them in that dict (by handing them to `auth.addIdentity(i)`).

However, you can make a subclass of `Authorizer` with a `.getIdentityRequest` method that behaves differently: your version could look in `/etc/passwd`, or do an SQL database lookup3, or create new Identities for anyone that asks (with a really secret password like '1234' that the user will probably never change, even if you ask them to). The Identities could be created by your server at startup time and stored in a dict, or they could be pickled and stored in a file until needed (in which case `.getIdentityRequest()` would use the username to find a file, unpickle the contents, and return the resulting `Identity` object), or created brand-new based upon whatever data you want. Any function that returns a Deferred (that will eventually get called back with the `Identity` object) can be used here.

Note:

For static Identities that are available right away, the Deferred's callback() method is called right away. This is why the interface of `.getIdentityRequest()` specifies that its Deferred is returned unarmed, so that the caller has a chance to actually add a callback to it before the callback gets run. (XXX: check, I think armed/unarmed is an outdated concept)

### Identity

The `Identity` object thus returned has two responsibilities. The first is to authenticate the user, because so far they are unverified: they have claimed to be somebody (by giving a username to the Authorizer), but have not yet proved that claim. It does this by implementing `.verifyPassword`, which is called by IdentityWrapper (described later) as part of the challenge-response sequence. If the password is valid, `.verifyPassword` should return a Deferred and run its callback. If the password is wrong, the Deferred should have the error-back run instead.

The second responsibility is to provide `Perspective` objects to users who are allowed to access them. The authenticated user gives a service name and a perspective name, and `.requestPerspectiveForKey()` is invoked to retrieve the given `Perspective`. The `Identity` is the one who decides which services/perspectives the user is allowed to access. Unless you override it in a subclass, the default implementation uses a simple dict called `.keyring`, which has keys that are (servicename, perspectivename) pairs. If the requested name pair is in the keyring, access is allowed, and the `Identity` will proceed to ask the `Service` to give back the specified `Perspective` to the user. `.requestPerspectiveForKey()` is required to return a Deferred, which will eventually be called back with a `Perspective` object, or error-backed with a `Failure` object if they were not allowed access.

XXX: explain perspective names being scoped to services better

You could subclass `Identity` to change the behavior of either of these, but chances are you won't bother. The only reason to change `.verifyPassword()` would be to replace it with some kind of public-key verification scheme, but that would require changes to `twisted.spread.pb.IdentityWrapper` too, as well as significant changes on the client side. Any changes you might want to make to `.requestPerspectiveForKey()` are probably more appropriate to put in the Service's `.getPerspectiveForIdentity` method instead. The Identity simply passes all requests for Perspectives off to the Service.

The default `Identity` objects are created with a username and password, and a keyring of valid service/perspective name pairs. They are children of an `Authorizer` object. The best way to create them is to have the `Authorizer` do it for you, then fill in the details, by doing the following:

    i = auth.createIdentity("username")
    i.setPassword("password")
    i.addKeyByString("service", "perspective")
    auth.addIdentity(i)

### Service

The `Service` object's job is to provide `Perspective` instances, by implementing `.getPerspectiveForIdentity()`. This function takes a Perspective name, and is expected to return a Deferred which will (eventually) be called back with an instance of `Perspective` (or a subclass).

The default implementation (in `twisted.spread.pb.Service`) retrieves static pre-generated `Perspective`s from a dict (indexed by perspective name), much like DefaultAuthorizer does with Identities. And like `Authorizer`, it is very useful to subclass `pb.Service` to change the way `.getPerspectiveForIdentity()` works: to create `Perspective`s out of persistent data or database lookups, to set extra attributes in the `Perspective`, etc.

When using the default implementation, you have to create the `Perspective`s at startup time. Each `Service` object has an attribute named `.perspectiveClass`, which helps it to create the `Perspective` objects for you. You do this by running `p = svc.createPerspective("perspective_name")`.

You should use `.createPerspective()` rather than running the constructor of your Perspective-subclass by hand, because the Perspective object needs a pointer to its parent `Service` object, and the `Service` needs to have a list of all the `Perspective`s that it contains.

## How that example worked

Ok, so that's what everything is supposed to do. Now you can walk through the previous example and see what was going on: we created a subclass called `MyPerspective`, made a `DefaultAuthorizer` and added it to the `Application`, created a `Service` and told it to make `MyPerspective`s, used `.createPerspective()` to build a few, for each one we made an `Identity` (with a username and password), and allowed that `Identity` to access a single `MyPerspective` by adding it to the keyring. We added the `Identity` objects to the `Authorizer`, and then glued the authorizer to the `pb.BrokerFactory`.

How did that last bit of magic glue work? I won't tell you here, because it isn't very useful to override it, but you effectively hang an `Authorizer` off of a TCP port. The combination of the object and methods exported by the `twisted.spread.pb.AuthRoot` object works together with the code inside the `pb.connect()` function to implement both sides of the challenge-response sequence. When you (as the client) use `pb.connect()` to get to a given host/port, you end up talking to a single `Authorizer`. The username/password you give get matched against the `Identities` provided by that authorizer, and then the servicename/perspectivename you give are matched against the ones authorized by the `Identity` (in its `.keyring` attribute). You eventually get back a remote reference to a `Perspective` provided by the `Service` that you named.

Note:

Here is how the magic glue code works:

    app.listenTCP(8800, pb.BrokerFactory(pb.AuthRoot(auth)))

`pb.AuthRoot()` provides objects that are subclassed from `twisted.spread.pb.Root`, so as we saw in the first example, they can be served up by `pb.BrokerFactory()`. `AuthRoot` happens to use the `.rootObject` hook described earlier to serve up an `twisted.spread.pb.AuthServ` object, which wraps the `Authorizer` and offers a method called `.remote_username`, which is called by the client to declare which `Identity` it claims to be. That method starts the challenge-response sequence.

## Code Walkthrough: pb.connect()

So, now that you've seen the complete sequence, it's time for a code walkthrough. This will give you a chance to see the places where you might write subclasses to implement different behaviors. We will look at what happens when `pb6client1.py` meets `pb6server.py`. We tune in just as the client has run the `pb.connect()` call.

The client-side code can be summarized by the following sequence of function calls, all implemented in twisted/spread/pb.py . `pb.connect()` calls `getObjectAt()` directly, after that each step is executed as a callback when the previous step completes.

     getObjectAt(host,port,timeout)
     logIn(): authServRef.callRemote('username', username)
     _cbLogInRespond(): challenger.callRemote('respond', f[challenge,password])
     _cbLogInResponded(): identity.callRemote('attach', servicename,
                                              perspectivename, client)
     usercallback(perspective)

The client does `getObjectAt()` to connect to the given host and port, and retrieve the object named `root`. On the server side, the `BrokerFactory` accepts the connection, asks the `pb.AuthRoot` object for its `.rootObject()`, getting an `AuthServ` object (containing both the authorizer and the `twisted.spread.pb.Broker` protocol object). It gives a remote reference to that `AuthServ` out to the client.

Now the client invokes the '`.remote_username`' method on that `AuthServ`. The `AuthServ` asks the `Authorizer` to `.getIdentityRequest()`: this retrieves (or creates) the `Identity`. When that finishes, it asks the `Identity` to create a random challenge (usually just a random string). The client is given back both the challenge and a reference to a new `twisted.spread.pb.AuthChallenger` object which will only accept a response that matches that exact challenge.

The client does its part of the MD5 challenge-response protocol and sends the response to the `AuthChallenger`'s `.remote_response()` method. The `AuthChallenger` verifies the response: if it is valid then it gives back a reference to an `IdentityWrapper`, which contains an internal reference to the `Identity` that we now know matches the user at the other end of the connection.

The client then invokes the `.remote_attach` method on that `IdentityWrapper`, passing in a serviceName, perspectiveName, and remoteRef. The wrapper asks the `Identity` to get a perspective using `identity.requestPerspectiveForKey`, which does the is this user allowed to get this service/perspective check by looking at the tuples on its `.keyring`, and if that is allowed then it gets the `Service` (by giving serviceName to the authorizer), then asks the `Service` to provide the perspective (with `svc.getPerspectiveForIdentity`). The default `Service` will ignore the identity object and just look for `Perspective`s by perspectiveName. The `Service` looks up or creates the `Perspective` and returns it. The `.remote_attach` method runs the Perspective's `.attached` method (although there are some intermediate steps, in `IdentityWrapper._attached`, to make sure `.detached` will eventually be run, and the Perspective's `.brokerAttached` method is executed to give it a chance to return some other Perspective instead). Finally a remote reference to the `Perspective` is returned to the client.

The client gives the `Perspective` reference to the callback that was attached to the `Deferred` that `pb.connect()` returned, which brings us back up to the code visible in `pb6client1.py`.

## Viewable

Once you have `Perspective` objects to represent users, the `twisted.spread.flavors.Viewable` class can come into play. This class behaves a lot like `Referenceable`: it turns into a `RemoteReference` when sent over the wire, and certain methods can be invoked by the holder of that reference. However, the methods that can be called have names that start with `view_` instead of `remote_`, and those methods are always called with an extra `perspective` argument:

    class Foo(pb.Viewable):
        def view_doFoo(self, perspective, arg1, arg2):
            pass

This is useful if you want to let multiple clients share a reference to the same object. The `view_` methods can use the perspective argument to figure out which client is calling them. This gives them a way to do additional permission checks, do per-user accounting, etc.

## A Larger Example

Now it's time to look more closely at the Go server described before.

To simplify the example, we will build a server that handles just a single game. There are a variety of players who can participate in the game, named Alice, Bob, etc (the usual suspects). Two of them log in, choose sides, and begin to make moves.

We assume that the rules of the game are encapsulated into a `GoGame` object, so we can focus on the code that handles the remote players.

XXX: finish this section

That's the end of the tour. If you have any questions, the folks at the welcome office will be more than happy to help. Don't forget to stop at the gift store on your way out, and have a really nice day. Buh-bye now!

## Footnotes

1. There actually exists such a thing. It's called twisted.reality, and was the whole reason Twisted was created. I haven't played it yet: I'm too afraid.
2. Actually there is a perspective per user*service, but we'll get into that later
3. See twisted.enterprise.dbcred for a module that does exactly that.

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