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, conn):
        # code that runs when a connection is created
        # (to init the service, if needed)
        pass

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

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

    exposed_the_real_answer_though = 43     # an exposed attribute

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

Note

The conn argument for on_connect and on_disconnect are added in rpyc 4.0. This is backwards incompatible with previous versions where instead the service constructor is called with a connection parameter and stores it into self._conn.

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.

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
<__main__.MyService object at 0x834e1ac>

This “root object” is a reference (netref) to the service instance living in the server process. It can be used access and invoke exposed attributes and methods:

>>> c.root.get_answer()
42
>>> c.root.the_real_answer_though
43

Meanwhile, the question is not exposed:

>>> c.root.get_question()
======= Remote traceback =======
...
  File "/home/tomer/workspace/rpyc/core/protocol.py", line 298, in sync_request
    raise obj
AttributeError: cannot access 'get_question'

Access policy

By default methods and attributes are only visible if they start with the exposed_ prefix. This also means that attributes of builtin objects such as lists or dicts are not accessible by default. If needed, you can configure this by passing appropriate options when creating the server. For example:

from rpyc.utils.server import ThreadedServer
server = ThreadedServer(MyService, port=18861, protocol_config={
    'allow_public_attrs': True,
})
server.start()

For a description of all available settings see the DEFAULT_CONFIG.

Shared service instance

Note that we have here passed the class MyService to the server with the effect that every incoming connection will use its own, independent MyService instance as root object.

If you pass in an instance instead, all incoming connections will use this instance as their shared root object, e.g.:

t = ThreadedServer(MyService(), port=18861)

Note the subtle difference (parentheses!) to the example above.

Note

Passing instances is supported starting with rpyc 4.0. In earlier versions, you can only pass a class of which every connection will receive a separate instance.

Passing arguments to the service

In the second case where you pass in a fully constructed service instance, it is trivial to pass additional arguments to the __init__ function. However, the situation is slightly more tricky if you want to pass arguments while separating the root objects for each connection. In this case, use classpartial() like so:

from rpyc.utils.helpers import classpartial

service = classpartial(MyService, 1, 2, pi=3)
t = ThreadedServer(service, port=18861)

Note

classpartial is added in version 4.0.

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 bin/rpyc_registry.py. This server listens on a broadcast UDP socket, and will answer to queries about which services are running where.

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. To find servers running a given service name:

>>> rpyc.discover("MY")
(('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: Callbacks and Symmetry