Using Perspective Broker¶
The first example to look at is a complete (although somewhat trivial)
application. It uses
PBServerFactory() on the server side, and
PBClientFactory() on the client side.
# Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor class Echoer(pb.Root): def remote_echo(self, st): print 'echoing:', st return st if __name__ == '__main__': reactor.listenTCP(8789, pb.PBServerFactory(Echoer())) reactor.run()
# Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor from twisted.python import util factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8789, factory) d = factory.getRootObject() d.addCallback(lambda object: object.callRemote("echo", "hello network")) d.addCallback(lambda echo: 'server echoed: '+echo) d.addErrback(lambda reason: 'error: '+str(reason.value)) d.addCallback(util.println) d.addCallback(lambda _: reactor.stop()) reactor.run()
First we look at the server. This defines an Echoer class (derived from
pb.Root ), with a method called
pb.Root objects (because of
their inheritance of
pb.Referenceable , described
later) can define methods with names of the form
remote_* ; a
client which obtains a remote reference to that
pb.Root object will be able to
invoke those methods.
The pb.Root -ish object is
given to a pb.PBServerFactory
() . This is a
Factory object like
any other: the Protocol objects it creates for new
connections know how to speak the PB protocol. The object you give to
pb.PBServerFactory() becomes the “root object” , which
simply makes it available for the client to retrieve. The client may only
request references to the objects you want to provide it: this helps you
implement your security model. Because it is so common to export just a
single object (and because a
remote_* method on that one can
return a reference to any other object you might want to give out), the
simplest example is one where the PBServerFactory is given the root object, and
the client retrieves it.
The client side uses
pb.PBClientFactory to make a
connection to a given port. This is a two-step process involving opening
a TCP connection to a given host and port and requesting the root object
.getRootObject() has to wait until a network
connection has been made and exchange some data, it may take a while,
so it returns a Deferred, to which the gotObject() callback is
attached. (See the documentation on Deferring Execution for a complete explanation of Deferred s). If and when the
connection succeeds and a reference to the remote root object is
obtained, this callback is run. The first argument passed to the
callback is a remote reference to the distant root object. (you can
give other arguments to the callback too, see the other parameters for
The callback does:
object.callRemote("echo", "hello network")
which causes the server’s
.remote_echo() method to be invoked.
.callRemote("boom") would cause
.remote_boom() to be run, etc). Again because of the delay
callRemote() returns a
Deferred . Assuming the
remote method was run without causing an exception (including an attempt to
invoke an unknown method), the callback attached to that
Deferred will be
invoked with any objects that were returned by the remote method call.
In this example, the server’s
Echoer object has a method
invoked, exactly as if some code on the server side had done:
and from the definition of
remote_echo() we see that this just
returns the same string it was given: “hello network” .
From the client’s point of view, the remote call gets another Deferred object instead of
callRemote() always returns a Deferred . This is why PB is
described as a system for “translucent” remote method calls instead of “transparent” ones: you cannot pretend that the remote object is really
local. Trying to do so (as some other RPC mechanisms do, coughCORBAcough)
breaks down when faced with the asynchronous nature of the network. Using
Deferreds turns out to be a very clean way to deal with the whole thing.
The remote reference object (the one given to
getRootObject() ‘s success callback) is an instance the RemoteReference class. This means
you can use it to invoke methods on the remote object that it refers to. Only
instances of RemoteReference are eligible for
.callRemote() . The RemoteReference object is the one that lives
on the remote side (the client, in this case), not the local side (where the
actual object is defined).
In our example, the local object is that
which inherits from pb.Root ,
which inherits from
pb.Referenceable . It is that
Referenceable class that makes the object eligible to be available
for remote method calls  . If you have
an object that is Referenceable, then any client that manages to get a
reference to it can invoke any
remote_* methods they please.
The only thing they can do is invoke those
methods. In particular, they cannot access attributes. From a security point
of view, you control what they can do by limiting what the
remote_* methods can do.
Also note: the other classes like
Referenceable allow access to
other methods, in particular
may be accessed. Don’t write local-only methods with these names, because then
remote callers will be able to do more than you intended.
Also also note: the other classes like pb.Copyable do allow access to attributes, but you control which ones they can see.
You don’t have to be a pb.Root to be remotely callable, but you do have to be pb.Referenceable . (Objects that inherit from pb.Referenceable but not from pb.Root can be remotely called, but only pb.Root -ish objects can be given to the PBServerFactory .)
Here is an example client and server which uses pb.Referenceable as a root object and as the result of a remotely exposed method. In each context, methods can be invoked on the exposed Referenceable instance. In this example, the initial root object has a method that returns a reference to the second object.
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb class Two(pb.Referenceable): def remote_three(self, arg): print "Two.three was given", arg class One(pb.Root): def remote_getTwo(self): two = Two() print "returning a Two called", two return two from twisted.internet import reactor reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run()
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor def main(): factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) def1 = factory.getRootObject() def1.addCallbacks(got_obj1, err_obj1) reactor.run() def err_obj1(reason): print "error getting first object", reason reactor.stop() def got_obj1(obj1): print "got first object:", obj1 print "asking it to getTwo" def2 = obj1.callRemote("getTwo") def2.addCallbacks(got_obj2) def got_obj2(obj2): print "got second object:", obj2 print "telling it to do three(12)" obj2.callRemote("three", 12) main()
handle all the details of waiting for the creation of a connection.
It returns a Deferred , which will have its
callback called when the reactor connects to the remote server and
pb.PBClientFactory gets the
root, and have its
errback called when the
object-connection fails for any reason, whether it was host lookup
failure, connection refusal, or some server-side error.
The root object has a method called
remote_getTwo , which
Two() instance. On the client end, the callback gets
a RemoteReference to that
instance. The client can then invoke two’s
objects have one method which is their purpose for being:
callRemote . This method allows you to call a
remote method on the object being referred to by the Reference. RemoteReference.callRemote , like pb.PBClientFactory.getRootObject , returns
a Deferred .
When a response to the method-call being sent arrives, the Deferred ‘s
will be made, depending on whether an error occurred in processing the
You can use this technique to provide access to arbitrary sets of objects.
Just remember that any object that might get passed “over the wire” must
inherit from Referenceable
(or one of the other flavors). If you try to pass a non-Referenceable object
(say, by returning one from a
remote_* method), you’ll get an
exception  .
References can come back to you¶
If your server gives a reference to a client, and then that client gives
the reference back to the server, the server will wind up with the same
object it gave out originally. The serialization layer watches for returning
reference identifiers and turns them into actual objects. You need to stay
aware of where the object lives: if it is on your side, you do actual method
calls. If it is on the other side, you do
.callRemote()  .
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor class Two(pb.Referenceable): def remote_print(self, arg): print "two.print was given", arg class One(pb.Root): def __init__(self, two): #pb.Root.__init__(self) # pb.Root doesn't implement __init__ self.two = two def remote_getTwo(self): print "One.getTwo(), returning my two called", self.two return self.two def remote_checkTwo(self, newtwo): print "One.checkTwo(): comparing my two", self.two print "One.checkTwo(): against your two", newtwo if self.two == newtwo: print "One.checkTwo(): our twos are the same" two = Two() root_obj = One(two) reactor.listenTCP(8800, pb.PBServerFactory(root_obj)) reactor.run()
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor def main(): foo = Foo() factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) factory.getRootObject().addCallback(foo.step1) reactor.run() # keeping globals around is starting to get ugly, so we use a simple class # instead. Instead of hooking one function to the next, we hook one method # to the next. class Foo: def __init__(self): self.oneRef = None def step1(self, obj): print "got one object:", obj self.oneRef = obj print "asking it to getTwo" self.oneRef.callRemote("getTwo").addCallback(self.step2) def step2(self, two): print "got two object:", two print "giving it back to one" print "one is", self.oneRef self.oneRef.callRemote("checkTwo", two) main()
The server gives a
Two() instance to the client, who then
returns the reference back to the server. The server compares the “two”
given with the “two” received and shows that they are the same, and that
both are real objects instead of remote references.
A few other techniques are demonstrated in
pb2client.py . One
is that the callbacks are added with
.addCallbacks . As you can tell from the Deferred documentation,
.addCallback is a
simplified form which only adds a success callback. The other is that to
keep track of state from one callback to the next (the remote reference to
the main One() object), we create a simple class, store the reference in an
instance thereof, and point the callbacks at a sequence of bound methods.
This is a convenient way to encapsulate a state machine. Each response kicks
off the next method, and any data that needs to be carried from one state to
the next can simply be saved as an attribute of the object.
Remember that the client can give you back any remote reference you’ve given them. Don’t base your zillion-dollar stock-trading clearinghouse server on the idea that you trust the client to give you back the right reference. The security model inherent in PB means that they can only give you back a reference that you’ve given them for the current connection (not one you’ve given to someone else instead, nor one you gave them last time before the TCP session went down, nor one you haven’t yet given to the client), but just like with URLs and HTTP cookies, the particular reference they give you is entirely under their control.
References to client-side objects¶
Anything that’s Referenceable can get passed across the wire, in either direction . The “client” can give a reference to the “server” , and then the server can use .callRemote() to invoke methods on the client end. This fuzzes the distinction between “client” and “server” : the only real difference is who initiates the original TCP connection; after that it’s all symmetric.
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor class One(pb.Root): def remote_takeTwo(self, two): print "received a Two called", two print "telling it to print(12)" two.callRemote("print", 12) reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run()
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor class Two(pb.Referenceable): def remote_print(self, arg): print "Two.print() called with", arg def main(): two = Two() factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) def1 = factory.getRootObject() def1.addCallback(got_obj, two) # hands our 'two' to the callback reactor.run() def got_obj(obj, two): print "got One:", obj print "giving it our two" obj.callRemote("takeTwo", two) main()
In this example, the client gives a reference to its own object to the server. The server then invokes a remote method on the client-side object.
Raising Remote Exceptions¶
Everything so far has covered what happens when things go right. What about when they go wrong? The Python Way is to raise an exception of some sort. The Twisted Way is the same.
The only special thing you do is to define your
subclass by deriving it from pb.Error . When any remotely-invokable method
perspective_* ) raises a
pb.Error -derived exception, a serialized form of that Exception
object will be sent back over the wire  . The other side (which
callRemote ) will have the “
callback run with a Failure object that contains a copy of
the exception object. This
Failure object can be queried to
retrieve the error message and a stack traceback.
Failure is a
special class, defined in
twisted/python/failure.py , created to
make it easier to handle asynchronous exceptions. Just as exception handlers
can be nested,
errback functions can be chained. If one errback
can’t handle the particular type of failure, it can be “passed along” to a
errback handler further down the chain.
For simple purposes, think of the
Failure as just a container
Exception objects. To extract the string that
was put into the exception, use its
To get the type of the exception (as a string), look at its
.type attribute. The stack traceback is available too. The
intent is to let the errback function get just as much information about the
exception as Python’s normal
try: clauses do, even though the
exception occurred in somebody else’s memory space at some unknown time in
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor class MyError(pb.Error): """This is an Expected Exception. Something bad happened.""" pass class MyError2(Exception): """This is an Unexpected Exception. Something really bad happened.""" pass class One(pb.Root): def remote_broken(self): msg = "fall down go boom" print "raising a MyError exception with data '%s'" % msg raise MyError(msg) def remote_broken2(self): msg = "hadda owie" print "raising a MyError2 exception with data '%s'" % msg raise MyError2(msg) def main(): reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run() if __name__ == '__main__': main()
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor def main(): factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) d = factory.getRootObject() d.addCallbacks(got_obj) reactor.run() def got_obj(obj): # change "broken" into "broken2" to demonstrate an unhandled exception d2 = obj.callRemote("broken") d2.addCallback(working) d2.addErrback(broken) def working(): print "erm, it wasn't *supposed* to work.." def broken(reason): print "got remote Exception" # reason should be a Failure (or subclass) holding the MyError exception print " .__class__ =", reason.__class__ print " .getErrorMessage() =", reason.getErrorMessage() print " .type =", reason.type reactor.stop() main()
$ ./exc_client.py got remote Exception .__class__ = twisted.spread.pb.CopiedFailure .getErrorMessage() = fall down go boom .type = __main__.MyError Main loop terminated.
Oh, and what happens if you raise some other kind of exception? Something
that isn’t subclassed from
pb.Error ? Well, those are
called “unexpected exceptions” , which make Twisted think that something
has really gone wrong. These will raise an exception on the
server side. This won’t break the connection (the exception is
trapped, just like most exceptions that occur in response to network
traffic), but it will print out an unsightly stack trace on the server’s
stderr with a message that says “Peer Will Receive PB Traceback” , just
as if the exception had happened outside a remotely-invokable method. (This
message will go the current log target, if log.startLogging was used to redirect it). The
client will get the same
Failure object in either case, but
subclassing your exception from
pb.Error is the way to tell
Twisted that you expect this sort of exception, and that it is ok to just
let the client handle it instead of also asking the server to complain. Look
exc_client.py and change it to invoke
broken() to see the change in the server’s
If you don’t add an
errback function to the Deferred , then a remote
exception will still send a
Failure object back over, but it
will get lodged in the
Deferred with nowhere to go. When that
Deferred finally goes out of scope, the side that did
callRemote will emit a message about an “Unhandled error in Deferred” , along with an ugly stack trace. It can’t raise an exception at
that point (after all, the
callRemote that triggered the
problem is long gone), but it will emit a traceback. So be a good programmer
and always add
errback handlers, even if they are just
calls to log.err .
Try/Except blocks and Failure.trap¶
To implement the equivalent of the Python try/except blocks (which can
trap particular kinds of exceptions and pass others “up” to
try/except blocks), you can use the
.trap() method in conjunction with multiple
errback handlers on the
Deferred . Re-raising an
exception in an
errback handler serves to pass that new
exception to the next handler in the chain. The
trap method is
given a list of exceptions to look for, and will re-raise anything that
isn’t on the list. Instead of passing unhandled exceptions “up” to an
try block, this has the effect of passing the
exception “off” to later
errback handlers on the same
Deferred . The
trap calls are used in chained
errbacks to test for each kind of exception in sequence.
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import reactor from twisted.spread import pb class MyException(pb.Error): pass class One(pb.Root): def remote_fooMethod(self, arg): if arg == "panic!": raise MyException return "response" def remote_shutdown(self): reactor.stop() reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run()
#!/usr/bin/env python # Copyright (c) Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb, jelly from twisted.python import log from twisted.internet import reactor class MyException(pb.Error): pass class MyOtherException(pb.Error): pass class ScaryObject: # not safe for serialization pass def worksLike(obj): # the callback/errback sequence in class One works just like an # asynchronous version of the following: try: response = obj.callMethod(name, arg) except pb.DeadReferenceError: print " stale reference: the client disconnected or crashed" except jelly.InsecureJelly: print " InsecureJelly: you tried to send something unsafe to them" except (MyException, MyOtherException): print " remote raised a MyException" # or MyOtherException except: print " something else happened" else: print " method successful, response:", response class One: def worked(self, response): print " method successful, response:", response def check_InsecureJelly(self, failure): failure.trap(jelly.InsecureJelly) print " InsecureJelly: you tried to send something unsafe to them" return None def check_MyException(self, failure): which = failure.trap(MyException, MyOtherException) if which == MyException: print " remote raised a MyException" else: print " remote raised a MyOtherException" return None def catch_everythingElse(self, failure): print " something else happened" log.err(failure) return None def doCall(self, explanation, arg): print explanation try: deferred = self.remote.callRemote("fooMethod", arg) deferred.addCallback(self.worked) deferred.addErrback(self.check_InsecureJelly) deferred.addErrback(self.check_MyException) deferred.addErrback(self.catch_everythingElse) except pb.DeadReferenceError: print " stale reference: the client disconnected or crashed" def callOne(self): self.doCall("callOne: call with safe object", "safe string") def callTwo(self): self.doCall("callTwo: call with dangerous object", ScaryObject()) def callThree(self): self.doCall("callThree: call that raises remote exception", "panic!") def callShutdown(self): print "telling them to shut down" self.remote.callRemote("shutdown") def callFour(self): self.doCall("callFour: call on stale reference", "dummy") def got_obj(self, obj): self.remote = obj reactor.callLater(1, self.callOne) reactor.callLater(2, self.callTwo) reactor.callLater(3, self.callThree) reactor.callLater(4, self.callShutdown) reactor.callLater(5, self.callFour) reactor.callLater(6, reactor.stop) factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) deferred = factory.getRootObject() deferred.addCallback(One().got_obj) reactor.run()
$ ./trap_client.py callOne: call with safe object method successful, response: response callTwo: call with dangerous object InsecureJelly: you tried to send something unsafe to them callThree: call that raises remote exception remote raised a MyException telling them to shut down callFour: call on stale reference stale reference: the client disconnected or crashed
In this example,
callTwo tries to send an instance of a
locally-defined class through
callRemote . The default security
model implemented by jelly
on the remote end will not allow unknown classes to be unserialized (i.e.
taken off the wire as a stream of bytes and turned back into an object: a
living, breathing instance of some class): one reason is that it does not
know which local class ought to be used to create an instance that
corresponds to the remote object  .
The receiving end of the connection gets to decide what to accept and what
to reject. It indicates its disapproval by raising a jelly.InsecureJelly exception. Because it occurs
at the remote end, the exception is returned to the caller asynchronously,
errback handler for the associated
is run. That errback receives a
Failure which wraps the
trap re-raises exceptions that it wasn’t asked
to look for. You can only check for one set of exceptions per errback
handler: all others must be checked in a subsequent handler.
check_MyException shows how multiple kinds of exceptions can be
checked in a single errback: give a list of exception types to
trap , and it will return the matching member. In this case, the
kinds of exceptions we are checking for (
MyOtherException ) may be raised by the remote end: they inherit
from pb.Error .
The handler can return
None to terminate processing of the
errback chain (to be precise, it switches to the callback that follows the
errback; if there is no callback then processing terminates). It is a good
idea to put an errback that will catch everything (no
tests, no possible chance of raising more exceptions, always returns
None ) at the end of the chain. Just as with regular
try: except: handlers, you need to think carefully about ways in which
your errback handlers could themselves raise exceptions. The extra
importance in an asynchronous environment is that an exception that falls
off the end of the
Deferred will not be signalled until that
Deferred goes out of scope, and at that point may only cause a
log message (which could even be thrown away if log.startLogging is not used to point it at
stdout or a log file). In contrast, a synchronous exception that is not
handled by any other
except: block will very visibly terminate
the program immediately with a noisy stack trace.
callFour shows another kind of exception that can occur
callRemote : pb.DeadReferenceError . This one occurs when the
remote end has disconnected or crashed, leaving the local side with a stale
reference. This kind of exception happens to be reported right away (XXX: is
this guaranteed? probably not), so must be caught in a traditional
try: except pb.DeadReferenceError block.
Yet another kind that can occur is a pb.PBConnectionLost exception. This occurs
(asynchronously) if the connection was lost while you were waiting for a
callRemote call to complete. When the line goes dead, all
pending requests are terminated with this exception. Note that you have no
way of knowing whether the request made it to the other end or not, nor how
far along in processing it they had managed before the connection was
lost. XXX: explain transaction semantics, find a decent reference.
|||There are a few other classes that can bestow this ability, but pb.Referenceable is the easiest to understand; see ‘flavors’ below for details on the others.|
|||This can be overridden, by subclassing one of the Serializable flavors and defining custom serialization code for your class. See Passing Complex Types for details.|
|||The binary nature of this local vs. remote scheme works because you cannot give RemoteReferences to a third party. If you could, then your object A could go to B, B could give it to C, C might give it back to you, and you would be hard pressed to tell if the object lived in C’s memory space, in B’s, or if it was really your own object, tarnished and sullied after being handed down like a really ugly picture that your great aunt owned and which nobody wants but which nobody can bear to throw out. Ok, not really like that, but you get the idea.|
|||To be precise, the Failure will be sent if any exception is raised, not just pb.Error-derived ones. But the server will print ugly error messages if you raise ones that aren’t derived from pb.Error.|
The naive approach
of simply doing
exception arises because the class being sent over the wire has not been
registered with the serialization layer (known as jelly ). The easiest way to make it possible to
copy entire class instances over the wire is to have them inherit from pb.Copyable , and then to use