RPC Messaging Callback System

Neutron already has a callback system for in-process resource callbacks where publishers and subscribers are able to publish and subscribe for resource events.

This system is different, and is intended to be used for inter-process callbacks, via the messaging fanout mechanisms.

In Neutron, agents may need to subscribe to specific resource details which may change over time. And the purpose of this messaging callback system is to allow agent subscription to those resources without the need to extend modify existing RPC calls, or creating new RPC messages.

A few resource which can benefit of this system:

  • QoS policies;

  • Security Groups.

Using a remote publisher/subscriber pattern, the information about such resources could be published using fanout messages to all interested nodes, minimizing messaging requests from agents to server since the agents get subscribed for their whole lifecycle (unless they unsubscribe).

Within an agent, there could be multiple subscriber callbacks to the same resource events, the resources updates would be dispatched to the subscriber callbacks from a single message. Any update would come in a single message, doing only a single oslo versioned objects deserialization on each receiving agent.

This publishing/subscription mechanism is highly dependent on the format of the resources passed around. This is why the library only allows versioned objects to be published and subscribed. Oslo versioned objects allow object version down/up conversion. [2] [3]

For the VO’s versioning schema look here: [4]

versioned_objects serialization/deserialization with the obj_to_primitive(target_version=..) and primitive_to_obj() [1] methods is used internally to convert/retrieve objects before/after messaging.

Serialized versioned objects look like:

{'versioned_object.version': '1.0',
 'versioned_object.name': 'QoSPolicy',
 'versioned_object.data': {'rules': [
                                     {'versioned_object.version': '1.0',
                                      'versioned_object.name': 'QoSBandwidthLimitRule',
                                      'versioned_object.data': {'name': u'a'},
                                      'versioned_object.namespace': 'versionedobjects'}
                           'uuid': u'abcde',
                           'name': u'aaa'},
 'versioned_object.namespace': 'versionedobjects'}

Rolling upgrades strategy

In this section we assume the standard Neutron upgrade process, which means upgrade the server first and then upgrade the agents:

More information about the upgrade strategy.

We provide an automatic method which avoids manual pinning and unpinning of versions by the administrator which could be prone to error.

Resource pull requests

Resource pull requests will always be ok because the underlying resource RPC does provide the version of the requested resource id / ids. The server will be upgraded first, so it will always be able to satisfy any version the agents request.

Resource push notifications

Agents will subscribe to the neutron-vo-<resource_type>-<version> fanout queue which carries updated objects for the version they know about. The versions they know about depend on the runtime Neutron versioned objects they started with.

When the server upgrades, it should be able to instantly calculate a census of agent versions per object (we will define a mechanism for this in a later section). It will use the census to send fanout messages on all the version span a resource type has.

For example, if neutron-server knew it has rpc-callback aware agents with versions 1.0, and versions 1.2 of resource type “A”, any update would be sent to neutron-vo-A_1.0 and neutron-vo-A_1.2.

TODO(mangelajo): Verify that after upgrade is finished any unused messaging resources (queues, exchanges, and so on) are released as older agents go away and neutron-server stops producing new message casts. Otherwise document the need for a neutron-server restart after rolling upgrade has finished if we want the queues cleaned up.

Leveraging agent state reports for object version discovery

We add a row to the agent db for tracking agent known objects and version numbers. This resembles the implementation of the configuration column.

Agents report at start not only their configuration now, but also their subscribed object type / version pairs, that are stored in the database and made available to any neutron-server requesting it:

'resource_versions': {'QosPolicy': '1.1',
                      'SecurityGroup': '1.0',
                      'Port': '1.0'}

There was a subset of Liberty agents depending on QosPolicy that required ‘QosPolicy’: ‘1.0’ if the qos plugin is installed. We were able to identify those by the binary name (included in the report):

  • ‘neutron-openvswitch-agent’

  • ‘neutron-sriov-nic-agent’

This transition was handled in the Mitaka version, but it’s not handled anymore in Newton, since only one major version step upgrades are supported.

Version discovery

With the above mechanism in place and considering the exception of neutron-openvswitch-agent and neutron-sriov-agent requiring QoSpolicy 1.0, we discover the subset of versions to be sent on every push notification.

Agents that are in down state are excluded from this calculation. We use an extended timeout for agents in this calculation to make sure we’re on the safe side, specially if deployer marked agents with low timeouts.

Starting at Mitaka, any agent interested in versioned objects via this API should report their resource/version tuples of interest (the resource type/ version pairs they’re subscribed to).

The plugins interested in this RPC mechanism must inherit AgentDbMixin, since this mechanism is only intended to be used from agents at the moment, while it could be extended to be consumed from other components if necessary.

The AgentDbMixin provides:

def get_agents_resource_versions(self, tracker):
Caching mechanism

The version subset per object is cached to avoid DB requests on every push given that we assume that all old agents are already registered at the time of upgrade.

Cached subset is re-evaluated (to cut down the version sets as agents upgrade) after neutron.api.rpc.callbacks.version_manager.VERSIONS_TTL.

As a fast path to update this cache on all neutron-servers when upgraded agents come up (or old agents revive after a long timeout or even a downgrade) the server registering the new status update notifies the other servers about the new consumer resource versions via cast.

All notifications for all calculated version sets must be sent, as non-upgraded agents would otherwise not receive them.

It is safe to send notifications to any fanout queue as they will be discarded if no agent is listening.

Topic names for every resource type RPC endpoint


In the future, we may want to get oslo messaging to support subscribing topics dynamically, then we may want to use:

neutron-vo-<resource_class_name>-<resource_id>-<version> instead,

or something equivalent which would allow fine granularity for the receivers to only get interesting information to them.

Subscribing to resources

Imagine that you have agent A, which just got to handle a new port, which has an associated security group, and QoS policy.

The agent code processing port updates may look like:

from neutron.api.rpc.callbacks.consumer import registry
from neutron.api.rpc.callbacks import events
from neutron.api.rpc.callbacks import resources

def process_resource_updates(context, resource_type, resource_list, event_type):

    # send to the right handler which will update any control plane
    # details related to the updated resources...

def subscribe_resources():
    registry.register(process_resource_updates, resources.SEC_GROUP)

    registry.register(process_resource_updates, resources.QOS_POLICY)

def port_update(port):

    # here we extract sg_id and qos_policy_id from port..

    sec_group = registry.pull(resources.SEC_GROUP, sg_id)
    qos_policy = registry.pull(resources.QOS_POLICY, qos_policy_id)

The relevant function is:

  • register(callback, resource_type): subscribes callback to a resource type.

The callback function will receive the following arguments:

  • context: the neutron context that triggered the notification.

  • resource_type: the type of resource which is receiving the update.

  • resource_list: list of resources which have been pushed by server.

  • event_type: will be one of CREATED, UPDATED, or DELETED, see neutron.api.rpc.callbacks.events for details.

With the underlying oslo_messaging support for dynamic topics on the receiver we cannot implement a per “resource type + resource id” topic, rabbitmq seems to handle 10000’s of topics without suffering, but creating 100’s of oslo_messaging receivers on different topics seems to crash.

We may want to look into that later, to avoid agents receiving resource updates which are uninteresting to them.

Unsubscribing from resources

To unsubscribe registered callbacks:

  • unsubscribe(callback, resource_type): unsubscribe from specific resource type.

  • unsubscribe_all(): unsubscribe from all resources.

Sending resource events

On the server side, resource updates could come from anywhere, a service plugin, an extension, anything that updates, creates, or destroys the resources and that is of any interest to subscribed agents.

A callback is expected to receive a list of resources. When resources in the list belong to the same resource type, a single push RPC message is sent; if the list contains objects of different resource types, resources of each type are grouped and sent separately, one push RPC message per type. On the receiver side, resources in a list always belong to the same type. In other words, a server-side push of a list of heterogeneous objects will result into N messages on bus and N client-side callback invocations, where N is the number of unique resource types in the given list, e.g. L(A, A, B, C, C, C) would be fragmented into L1(A, A), L2(B), L3(C, C, C), and each list pushed separately.

Note: there is no guarantee in terms of order in which separate resource lists will be delivered to consumers.

The server/publisher side may look like:

from neutron.api.rpc.callbacks.producer import registry
from neutron.api.rpc.callbacks import events

def create_qos_policy(...):
    policy = fetch_policy(...)
    registry.push([policy], events.CREATED)

def update_qos_policy(...):
    policy = fetch_policy(...)
    registry.push([policy], events.UPDATED)

def delete_qos_policy(...):
    policy = fetch_policy(...)
    registry.push([policy], events.DELETED)