twisted: How to send and receive the same object with Perspective Broker?

Question

I have a simple 'echo' PB client and server where the client sends an object to the server which echo the same object back to the client:

The client:

from twisted.spread import pb
from twisted.internet import reactor
from twisted.python import util
from amodule import aClass

factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 8282, factory)
d = factory.getRootObject()

d.addCallback(lambda object: object.callRemote("echo", aClass()))
d.addCallback(lambda response: 'server echoed: '+response)
d.addErrback(lambda reason: 'error: '+str(reason.value))
d.addCallback(util.println)

d.addCallback(lambda _: reactor.stop())
reactor.run()

The server:

from twisted.application import internet, service
from twisted.internet import protocol
from twisted.spread import pb
from amodule import aClass

class RemoteClass(pb.RemoteCopy, aClass):
    pass
pb.setUnjellyableForClass(aClass, RemoteClass)

class PBServer(pb.Root):
    def remote_echo(self, a):
        return a

application = service.Application("Test app")

# Prepare managers
clientManager = internet.TCPServer(8282, pb.PBServerFactory(PBServer()));
clientManager.setServiceParent(application)

if __name__ == '__main__':
    print "Run with twistd"
    import sys
    sys.exit(1)

The aClass is a simple class implementing Copyable: from twisted.spread import pb

class aClass(pb.Copyable):
    pass

When i run the above code, i get this error: twisted.spread.jelly.InsecureJelly: Module builtin not allowed (in type builtin.RemoteClass).

In fact, the object is sent to the server without any problem since it was secured with pb.setUnjellyableForClass(aClass, RemoteClass) on the server side, but once it gets returned to the client, that error is raised.

Am looking for a way to get an easy way to send/receive my objects between two peers.

Answer 1

Perspective broker identifies classes by name when talking about them over the network. A class gets its name in part from the module in which it is defined. A tricky problem with defining classes in a file that you run from the command line (ie, your "main script") is that they may end up with a surprising name. When you do this:

python foo.py

The module name Python gives to the code in foo.py is not "foo" as one might expect. Instead it is something like "__main__" (which is why the if __name__ == "__main__": trick works).

However, if some other part of your application later tries to import something from foo.py, then Python re-evaluates its contents to create a new module named "foo".

Additionally, the classes defined in the "__main__" module of one process may have nothing to do with the classes defined in the "__main__" module of another process. This is the case in your example, where __main__.RemoteClass is defined in your server process but there is no RemoteClass in the __main__ module of your client process.

So, PB gets mixed up and can't complete the object transfer.

The solution is to keep the amount of code in your main script to a minimum, and in particular to never define things with names there (no classes, no function definitions).

However, another problem is the expectation that a RemoteCopy can be sent over PB without additional preparation. A Copyable can be sent, creating a RemoteCopy on the peer, but this is not a symmetric relationship. Your client also needs to allow this by making a similar (or different) pb.setUnjellyableForClass call.