Part 3: Services and New Style RPyC

So far we have covered the features of classic RPyC. However, the new model of RPyC programming (starting with RPyC 3.00), is based on services. As you might have noticed in the classic mode, the client basically gets full control over the server, which is why we (used to) call RPyC servers slaves. Luckily, this is no longer the case. The new model is service oriented: services provide a way to expose a well-defined set of capabilities to the other party, which makes RPyC a generic RPC platform. In fact, the classic RPyC that you’ve seen so far, is simply “yet another” service.

Services are quite simple really. To prove that, the SlaveService (the service that implements classic RPyC) is only 30 lines long, including comments ;). Basically, a service has the following boilerplate:

import rpyc

class MyService(rpyc.Service):
    def on_connect(self):
        # code that runs when a connection is created
        # (to init the serivce, if needed)
        pass

    def on_disconnect(self):
        # code that runs when the connection has already closed
        # (to finalize the service, if needed)
        pass

    def exposed_get_answer(self): # this is an exposed method
        return 42

    def get_question(self):  # while this method is not exposed
        return "what is the airspeed velocity of an unladen swallow?"

As you can see, apart from the special initialization/finalization methods, you are free to define the class like any other class. Unlike regular classes, however, you can choose which attributes will be exposed to the other party: if the name starts with exposed_, the attribute will be remotely accessible, otherwise it is only locally accessible. In this example, clients will be able to call get_answer, but not get_question, as we’ll see in a moment.

Note

The server creates a Service object for every instance. This means that instance variables are not shared between different clients. If you do need shared state, you have to use globals or subclass the Server class to pass additional variables to new Service objects.

To expose your service to the world, however, you will need to start a server. There are many ways to do that, but the simplest is

# ... continuing the code snippet from above ...

if __name__ == "__main__":
    from rpyc.utils.server import ThreadedServer
    t = ThreadedServer(MyService, port = 18861)
    t.start()

To the remote party, the service is exposed as the root object of the connection, e.g., conn.root. Now you know all you need to understand this short demo:

>>> import rpyc
>>> c = rpyc.connect("localhost", 18861)
>>> c.root     # this is the "root object", i.e., the service that is exposed to this client
<__main__.MyService object at 0x834e1ac>
>>>
>>> c.root.get_answer
<bound method MyService.exposed_get_answer of <__main__.MyService object at 0x834e1ac>>
>>> c.root.get_answer()
42
>>> c.root.exposed_get_answer()    # it can be accessed with the 'exposed_' prefix as well
42
>>> c.root.get_question()   # but "get_question" is not exposed!
======= Remote traceback =======
...
  File "/home/tomer/workspace/rpyc/core/protocol.py", line 298, in sync_request
    raise obj
AttributeError: cannot access 'get_question'

But Wait, There’s More!

All services have a //name//, which is normally the name of the class, minus the "Service" suffix. In our case, the service name is "MY" (service names are case-insensitive). If you wish to define a custom name, or multiple names (aliases), you can do so by setting the ALIASES list. The first alias is considered to be the “formal name”, while the rest are aliases:

class SomeOtherService(rpyc.Service):
    ALIASES = ["floop", "bloop"]
    ...

In the original code snippet, this is what the client gets:

>>> c.root.get_service_name()
'MY'
>>> c.root.get_service_aliases()
('MY',)

The reason services have names is for the service registry: normally, a server will broadcast its details to a nearby registry server for discovery. To use service discovery, a make sure you start the registry_server.py that comes in the rpyc/scripts directory. This server listens on a broadcast UDP socket, and will answer to queries about which services are running where.

../_images/registry_server.png

Once a registry server is running somewhere “broadcastable” on your network, and the servers are configured to auto-register with it (the default), clients can discover services automagically:

>>> rpyc.discover("MY")      # to find servers running a given service name
(('192.168.1.101', 18861),)

# and if you don't care to which you server you connect, you use connect_by_service:
>>> c2 = rpyc.connect_by_service("MY")
>>> c2.root.get_answer()
42

Decoupled Services

So far we’ve discussed only about the service that the server exposes, but what about the client? Does the client expose a service too? After all, RPyC is a symmetric protocol – there’s no difference between the client and the server. Well, as you might have guessed, the answer is yes: both client and server expose services. However, the services exposed by the two parties need not be the same – they are decoupled.

By default, clients (using one of the connect() functions to connect to a server) expose the VoidService. As the name suggests, this service exposes no functionality to the other party, meaning the server can’t make requests to the client (except for explicitly passed capabilities, like function callbacks). You can set the service exposed by the client by passing the service = parameter to one of the connect() functions.

The fact that the services on both ends of the connection are decoupled, does not mean they can be arbitrary. For instance, “service A” might expect to be connected to “service B” – and runtime errors (mostly AttributeError) will ensue if this not the case. Many times the services on both ends can be different, but do keep it in mind that if you need interaction between the parties, both services must be “compatible”.

Note

Classic mode: when using any of the connect() functions, the client-side service is set to SlaveService as well (being identical to the server).

Continue to part 4...