Neutron RPC API Layer

Neutron uses the oslo.messaging library to provide an internal communication channel between Neutron services. This communication is typically done via AMQP, but those details are mostly hidden by the use of oslo.messaging and it could be some other protocol in the future.

RPC APIs are defined in Neutron in two parts: client side and server side.

Client Side

Here is an example of an rpc client definition:

import oslo_messaging

from neutron_lib import rpc as n_rpc


class ClientAPI(object):
    """Client side RPC interface definition.

    API version history:
        1.0 - Initial version
        1.1 - Added my_remote_method_2
    """

    def __init__(self, topic):
        target = oslo_messaging.Target(topic=topic, version='1.0')
        self.client = n_rpc.get_client(target)

    def my_remote_method(self, context, arg1, arg2):
        cctxt = self.client.prepare()
        return cctxt.call(context, 'my_remote_method', arg1=arg1, arg2=arg2)

    def my_remote_method_2(self, context, arg1):
        cctxt = self.client.prepare(version='1.1')
        return cctxt.call(context, 'my_remote_method_2', arg1=arg1)

This class defines the client side interface for an rpc API. The interface has 2 methods. The first method existed in version 1.0 of the interface. The second method was added in version 1.1. When the newer method is called, it specifies that the remote side must implement at least version 1.1 to handle this request.

Server Side

The server side of an rpc interface looks like this:

import oslo_messaging


class ServerAPI(object):

    target = oslo_messaging.Target(version='1.1')

    def my_remote_method(self, context, arg1, arg2):
        return 'foo'

    def my_remote_method_2(self, context, arg1):
        return 'bar'

This class implements the server side of the interface. The oslo_messaging.Target() defined says that this class currently implements version 1.1 of the interface.

Versioning

Note that changes to rpc interfaces must always be done in a backwards compatible way. The server side should always be able to handle older clients (within the same major version series, such as 1.X).

It is possible to bump the major version number and drop some code only needed for backwards compatibility. For more information about how to do that, see https://wiki.openstack.org/wiki/RpcMajorVersionUpdates.

Example Change

As an example minor API change, let’s assume we want to add a new parameter to my_remote_method_2. First, we add the argument on the server side. To be backwards compatible, the new argument must have a default value set so that the interface will still work even if the argument is not supplied. Also, the interface’s minor version number must be incremented. So, the new server side code would look like this:

import oslo_messaging


class ServerAPI(object):

    target = oslo_messaging.Target(version='1.2')

    def my_remote_method(self, context, arg1, arg2):
        return 'foo'

    def my_remote_method_2(self, context, arg1, arg2=None):
        if not arg2:
            # Deal with the fact that arg2 was not specified if needed.
        return 'bar'

We can now update the client side to pass the new argument. The client must also specify that version ‘1.2’ is required for this method call to be successful. The updated client side would look like this:

import oslo_messaging

from neutron.common import rpc as n_rpc


class ClientAPI(object):
    """Client side RPC interface definition.

    API version history:
        1.0 - Initial version
        1.1 - Added my_remote_method_2
        1.2 - Added arg2 to my_remote_method_2
    """

    def __init__(self, topic):
        target = oslo_messaging.Target(topic=topic, version='1.0')
        self.client = n_rpc.get_client(target)

    def my_remote_method(self, context, arg1, arg2):
        cctxt = self.client.prepare()
        return cctxt.call(context, 'my_remote_method', arg1=arg1, arg2=arg2)

    def my_remote_method_2(self, context, arg1, arg2):
        cctxt = self.client.prepare(version='1.2')
        return cctxt.call(context, 'my_remote_method_2',
                          arg1=arg1, arg2=arg2)

More Info

For more information, see the oslo.messaging documentation: https://docs.openstack.org/oslo.messaging/latest/.