# Neutron Callback System

In Neutron, core and service components may need to cooperate during the execution of certain operations, or they may need to react upon the occurrence of certain events. For instance, when a Neutron resource is associated to multiple services, the components in charge of these services may need to play an active role in determining what the right state of the resource needs to be.

The cooperation may be achieved by making each object aware of each other, but this leads to tight coupling, or alternatively it can be achieved by using a callback-based system, where the same objects are allowed to cooperate in a loose manner.

This is particularly important since the spin off of the advanced services like VPN and Firewall, where each service’s codebase lives independently from the core and from one another. This means that the tight coupling is no longer a practical solution for object cooperation. In addition to this, if more services are developed independently, there is no viable integration between them and the Neutron core. A callback system, and its registry, tries to address these issues.

In object-oriented software systems, method invocation is also known as message passing: an object passes a message to another object, and it may or may not expect a message back. This point-to-point interaction can take place between the parties directly involved in the communication, or it can happen via an intermediary. The intermediary is then in charge of keeping track of who is interested in the messages and in delivering the messages forth and back, when required. As mentioned earlier, the use of an intermediary has the benefit of decoupling the parties involved in the communications, as now they only need to know about the intermediary; the other benefit is that the use of an intermediary opens up the possibility of multiple party communication: more than one object can express interest in receiving the same message, and the same message can be delivered to more than one object. To this aim, the intermediary is the entity that exists throughout the system lifecycle, as it needs to be able to track whose interest is associated to what message.

In a design for a system that enables callback-based communication, the following aspects need to be taken into account:

- how to become consumer of messages (i.e. how to be on the receiving end of the message);

- how to become producer of messages (i.e. how to be on the sending end of the message);

- how to consume/produce messages selectively;

Translate and narrow this down to Neutron needs, and this means the design of a callback system where messages are about lifecycle events (e.g. before creation, before deletion, etc.) of Neutron resources (e.g. networks, routers, ports, etc.), where the various parties can express interest in knowing when these events for a specific resources take place.

Rather than keeping the conversation abstract, let us delve into some examples, that would help understand better some of the principles behind the provided mechanism.

## Event payloads

The event payloads are defined in `neutron_lib.callbacks.events` and define a set of payload objects based on consumption pattern. The following event objects are defined today:

- `EventPayload`: Base object for all other payloads and define the common set of attributes used by events. The `EventPayload` can also be used directly for basic payloads that don’t need to transport additional values.

- `DBEventPayload`: Payloads pertaining to database callbacks. These objects capture both the pre and post state (among other things) for database changes.

- `APIEventPayload`: Payloads pertaining to API callbacks. These objects capture details relating to an API event; such as the method name and API action.

Each event object is described in greater detail in its own subsection below.

### Event objects: EventPayload

The `EventPayload` object is the parent class of all other payload objects and defines the common set of attributes applicable to most events. For example, the `EventPayload` contains the `context`, `request_body`, etc. In addition, a `metadata` attribute is available to transport event data that’s not yet standardized. While the `metadata` attribute is there for use, it should only be used in special cases like phasing in new payload attributes.

Payload objects also transport resource state via the `states` attribute. This collection of resource objects tracks the state changes for the respective resource related to the event. For example database changes might have a pre and post updated resource that’s used as `states`. Tracking states allows consumers to inspect the various changes in the resource and take action as needed; for example checking the pre and post object to determine the delta. State object types are event specific; API events may use python `dicts` as state objects whereas database events use resource/OVO model objects.

Note that states as well as any other event payload attributes are not copied; subscribers obtain a direct reference to event payload objects (states, metadata, etc.) and should not be modified by subscribers.

### Event objects: DBEventPayload

For datastore/database events, `DBEventPayload` can be used as the payload event object. In addition to the attributes inherited from `EventPayload`, database payloads also contain an additional `desired_state`. The desired state is intended for use with pre create/commit scenarios where the publisher has a resource object (yet to be persisted) that’s used in the event payload.

These event objects are suitable for the standard before/after database events we have today as well as any that might arise in the future.

Example usage:

    # BEFORE_CREATE:
    DBEventPayload(context,
                   request_body=params_of_create_request,
                   resource_id=id_of_resource_if_avail,
                   desired_state=db_resource_to_commit)

    # AFTER_CREATE:
    DBEventPayload(context,
                   request_body=params_of_create_request,
                   states=[my_new_copy_after_create],
                   resource_id=id_of_resource)

    # PRECOMMIT_CREATE:
    DBEventPayload(context,
                   request_body=params_of_create_request,
                   resource_id=id_of_resource_if_avail,
                   desired_state=db_resource_to_commit)

    # BEFORE_DELETE:
    DBEventPayload(context,
                   states=[resource_to_delete],
                   resource_id=id_of_resource)

    # AFTER_DELETE:
    DBEventPayload(context,
                   states=[copy_of_deleted_resource],
                   resource_id=id_of_resource)

    # BEFORE_UPDATE:
    DBEventPayload(context,
                   request_body=body_of_update_request,
                   states=[original_db_resource],
                   resource_id=id_of_resource
                   desired_state=updated_db_resource_to_commit)

    # AFTER_UPDATE:
    DBEventPayload(context,
                   request_body=body_of_update_request,
                   states=[original_db_resource, updated_db_resource],
                   resource_id=id_of_resource)

### Event objects: APIEventPayload

For API related callbacks, the `APIEventPayload` object can be used to transport callback payloads. For example, the REST API resource controller can use API events for pre/post operation callbacks.

In addition to transporting all the attributes of `EventPayload`, the `APIEventPayload` object also includes the `action`, `method_name` and `collection_name` payload attributes permitting API components to pass along API controller specifics.

Sample usage:

    # BEFORE_RESPONSE for create:
    APIEventPayload(context, method_name, action,
             request_body=req_body,
             states=[create_result],
             collection_name=self._collection_name)

    # BEFORE_RESPONSE for delete:
    APIEventPayload(context, method_name, action,
             states=[copy_of_deleted_resource],
             collection_name=self._collection_name)

    # BEFORE_RESPONSE for update:
    APIEventPayload(context, method_name, action,
             states=[original, updated],
             collection_name=self._collection_name)

## Subscribing to events

Imagine that you have entity A, B, and C that have some common business over router creation. A wants to tell B and C that the router has been created and that they need to get on and do whatever they are supposed to do. In a callback-less world this would work like so:

    # A is done creating the resource
    # A gets hold of the references of B and C
    # A calls B
    # A calls C
    B->my_random_method_for_knowing_about_router_created()
    C->my_random_very_difficult_to_remember_method_about_router_created()

If B and/or C change, things become sour. In a callback-based world, things become a lot more uniform and straightforward:

    # B and C ask I to be notified when A is done creating the resource
    # Suppose D another entity want subscription with higher priority
    # notification
    # ...
    # A is done creating the resource
    # A gets hold of the reference to the intermediary I
    # A calls I
    I->publish()

Since B and C will have expressed interest in knowing about A’s business, and D also subscribed for router creation with higher priority, ‘I’ will deliver the messages to D first and then to B and C in any order. If B, C and D change, A and ‘I’ do not need to change.

In practical terms this scenario would be translated in the code below:

    from neutron_lib.callbacks import events
    from neutron_lib.callbacks import resources
    from neutron_lib.callbacks import registry

    def callback1(resource, event, trigger, payload):
        print('Callback1 called by trigger: ', trigger)
        print('payload: ', payload)

    def callback2(resource, event, trigger, payload):
        print('Callback2 called by trigger: ', trigger)
        print('payload: ', payload)

    def callbackhighpriority(resource, event, trigger, payload):
        print("Prepared data for entities")

    # A is using event in case for some callback or internal operations
    registry.subscribe(callbackhighpriority, resources.ROUTER,
                       events.BEFORE_CREATE, priority=0)

    # B and C express interest with I
    registry.subscribe(callback1, resources.ROUTER, events.BEFORE_CREATE)
    registry.subscribe(callback2, resources.ROUTER, events.BEFORE_CREATE)
    print('Subscribed')

    # A notifies
    def do_notify():
        registry.publish(resources.ROUTER, events.BEFORE_CREATE,
                         do_notify, events.EventPayload(None))

    print('Notifying...')
    do_notify()

The output is:

    > Subscribed
    > Notifying...
    > Prepared data for entities
    > Callback1 called by trigger:  <function do_notify at 0x7f73166ae620>
    > payload:  <neutron_lib.callbacks.events.EventPayload object at 0x7f731bc8fb70>
    > Callback2 called by trigger:  <function do_notify at 0x7f73166ae620>
    > payload:  <neutron_lib.callbacks.events.EventPayload object at 0x7f731bc8fb70>

Thanks to the intermediary existence throughout the life of the system, A, B, C and D are flexible to evolve their internals, dynamics, and lifecycles.

Since different entities can subscribe to same events of a resource, the callback priority mechanism is in place to guarantee the order of execution for callbacks, entities have to subscribe events with a priority number of Integer type, lower the priority number is higher would be priority of callback. The following adds more details:

- Priorities for callbacks should be coded in neutron_lib/callbacks/priority_group.py

- If no priority is assigned during subscription then a default value will be used.

- For callbacks having same priority, the execution order will be arbitary.

## Subscribing and aborting events

Interestingly in Neutron, certain events may need to be forbidden from happening due to the nature of the resources involved. To this aim, the callback-based mechanism has been designed to support a use case where, when callbacks subscribe to specific events, the action that results from it, may lead to the propagation of a message back to the sender, so that it itself can be alerted and stop the execution of the activity that led to the message dispatch in the first place.

The typical example is where a resource, like a router, is used by one or more high-level service(s), like a VPN or a Firewall, and actions like interface removal or router destruction cannot not take place, because the resource is shared.

To address this scenario, special events are introduced, ‘BEFORE_*’ events, to which callbacks can subscribe and have the opportunity to ‘abort’, by raising an exception when notified.

Since multiple callbacks may express an interest in the same event for a particular resource, and since callbacks are executed independently from one another, this may lead to situations where notifications that occurred before the exception must be aborted. To this aim, when an exception occurs during the notification process, an abort_* event is propagated immediately after. It is up to the callback developer to determine whether subscribing to an abort notification is required in order to revert the actions performed during the initial execution of the callback (when the BEFORE_* event was fired). Exceptions caused by callbacks registered to abort events are ignored. The snippet below shows this in action:

    from neutron_lib.callbacks import events
    from neutron_lib.callbacks import exceptions
    from neutron_lib.callbacks import resources
    from neutron_lib.callbacks import registry

    def callback1(resource, event, trigger, payload=None):
        raise Exception('I am failing!')

    def callback2(resource, event, trigger, payload=None):
        print('Callback2 called by %s on event  %s' % (trigger, event))

    registry.subscribe(callback1, resources.ROUTER, events.BEFORE_CREATE)
    registry.subscribe(callback2, resources.ROUTER, events.BEFORE_CREATE)
    registry.subscribe(callback2, resources.ROUTER, events.ABORT_CREATE)
    print('Subscribed')

    def do_notify():
        registry.publish(resources.ROUTER, events.BEFORE_CREATE, do_notify)

    print('Notifying...')
    try:
        do_notify()
    except exceptions.CallbackFailure as e:
        print("Error: %s" % e)

The output is:

    > Subscribed
    > Notifying...
    > Callback2 called by <function do_notify at 0x7f3194c7f410> on event  before_create
    > Callback2 called by <function do_notify at 0x7f3194c7f410> on event  abort_create
    > Error:  Callback __main__.callback1 failed with "I am failing!"

In this case, upon the notification of the BEFORE_CREATE event, Callback1 triggers an exception that can be used to stop the action from taking place in do_notify(). On the other end, Callback2 will be executing twice, once for dealing with the BEFORE_CREATE event, and once to undo the actions during the ABORT_CREATE event. It is worth noting that it is not mandatory to have the same callback register to both BEFORE_* and the respective ABORT_* event; as a matter of fact, it is best to make use of different callbacks to keep the two logic separate.

## Unsubscribing to events

There are a few options to unsubscribe registered callbacks:

- clear(): it unsubscribes all subscribed callbacks: this can be useful especially when winding down the system, and notifications shall no longer be triggered.

- unsubscribe(): it selectively unsubscribes a callback for a specific resource’s event. Say callback C has subscribed to event A for resource R, any notification of event A for resource R will no longer be handed over to C, after the unsubscribe() invocation.

- unsubscribe_by_resource(): say that callback C has subscribed to event A, B, and C for resource R, any notification of events related to resource R will no longer be handed over to C, after the unsubscribe_by_resource() invocation.

- unsubscribe_all(): say that callback C has subscribed to events A, B for resource R1, and events C, D for resource R2, any notification of events pertaining resources R1 and R2 will no longer be handed over to C, after the unsubscribe_all() invocation.

The snippet below shows these concepts in action:

    from neutron_lib.callbacks import events
    from neutron_lib.callbacks import exceptions
    from neutron_lib.callbacks import resources
    from neutron_lib.callbacks import registry

    def callback1(resource, event, trigger, payload=None):
        print('Callback1 called by %s on event %s for resource %s' % (trigger, event, resource))

    def callback2(resource, event, trigger, payload=None):
        print('Callback2 called by %s on event %s for resource %s' % (trigger, event, resource))

    registry.subscribe(callback1, resources.ROUTER, events.BEFORE_READ)
    registry.subscribe(callback1, resources.ROUTER, events.BEFORE_CREATE)
    registry.subscribe(callback1, resources.ROUTER, events.AFTER_DELETE)
    registry.subscribe(callback1, resources.PORT, events.BEFORE_UPDATE)
    registry.subscribe(callback2, resources.ROUTER_GATEWAY, events.BEFORE_UPDATE)
    print('Subscribed')

    def do_notify():
        print('Notifying...')
        registry.publish(resources.ROUTER, events.BEFORE_READ, do_notify)
        registry.publish(resources.ROUTER, events.BEFORE_CREATE, do_notify)
        registry.publish(resources.ROUTER, events.AFTER_DELETE, do_notify)
        registry.publish(resources.PORT, events.BEFORE_UPDATE, do_notify)
        registry.publish(resources.ROUTER_GATEWAY, events.BEFORE_UPDATE, do_notify)

    do_notify()
    registry.unsubscribe(callback1, resources.ROUTER, events.BEFORE_READ)
    do_notify()
    registry.unsubscribe_by_resource(callback1, resources.PORT)
    do_notify()
    registry.unsubscribe_all(callback1)
    do_notify()
    registry.clear()
    do_notify()

The output is:

    Subscribed
    Notifying...
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event before_read for resource router
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event before_create for resource router
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event after_delete for resource router
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event before_update for resource port
    Callback2 called by <function do_notify at 0x7f062c8f67d0> on event before_update for resource router_gateway
    Notifying...
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event before_create for resource router
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event after_delete for resource router
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event before_update for resource port
    Callback2 called by <function do_notify at 0x7f062c8f67d0> on event before_update for resource router_gateway
    Notifying...
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event before_create for resource router
    Callback1 called by <function do_notify at 0x7f062c8f67d0> on event after_delete for resource router
    Callback2 called by <function do_notify at 0x7f062c8f67d0> on event before_update for resource router_gateway
    Notifying...
    Callback2 called by <function do_notify at 0x7f062c8f67d0> on event before_update for resource router_gateway
    Notifying...

## Subscribing events using registry decorator

Now neutron-lib supports using registry decorators to subscribe events. There are two decorators `has_registry_receivers`, which sets up the class `__new__` method to subscribe the bound method in the callback registry after object instantiation. `receives` use to decorate callback method which must defines the resource and events. Any class use `receives` must be decorated with `has_registry_receivers`.

## Testing with callbacks

A python fixture is provided for implementations that need to unit test and mock the callback registry. This can be used for example, when your code publishes callback events that you need to verify. Consumers can use `neutron_lib.tests.unit.callbacks.base.CallbackRegistryFixture` in their unit test classes with the `useFixture()` method passing along a `CallbackRegistryFixture` instance. If mocking of the actual singleton callback manager is necessary, consumers can pass a value to with the `callback_manager` kwarg. For example:

    def setUp(self):
        super(MyTestClass, self).setUp()
        self.registry_fixture = callback_base.CallbackRegistryFixture()
        self.useFixture(self.registry_fixture)
        # each test now uses an isolated callback manager

## FAQ

Can I use the callbacks registry to subscribe and notify non-core resources and events?

> Short answer is yes. The callbacks module defines literals for what are considered core Neutron resources and events. However, the ability to subscribe/notify is not limited to these as you can use your own defined resources and/or events. Just make sure you use string literals, as typos are common, and the registry does not provide any runtime validation. Therefore, make sure you test your code!

What is the relationship between Callbacks and Taskflow?

> There is no overlap between Callbacks and Taskflow or mutual exclusion; as matter of fact they can be combined; You could have a callback that goes on and trigger a taskflow. It is a nice way of separating implementation from abstraction, because you can keep the callback in place and change Taskflow with something else.

Is there any ordering guarantee during notifications?

> Depends, if the prorities are defined or passed during subscription, then yes callbacks will be executed in order, meaning the callback having the lowest integer value for priority will be executed first and so on. When priorities are not explicitly defined during subscription, all the callbacks will have default priority and will be executed in an arbitary order.

How is the notifying object expected to interact with the subscribing objects?

> The `publish` method implements a one-way communication paradigm: the publisher sends a message without expecting a response back (in other words it fires and forget). However, due to the nature of Python, the payload can be mutated by the subscribing objects, and this can lead to unexpected behavior of your code, if you assume that this is the intentional design. Bear in mind, that passing-by-value using deepcopy was not chosen for efficiency reasons. Having said that, if you intend for the publisher object to expect a response, then the publisher itself would need to act as a subscriber.

Is the registry thread-safe?

> Short answer is no: it is not safe to make mutations while callbacks are being called (more details as to why can be found line 937 of dictobject). A mutation could happen if a ‘subscribe’/’unsubscribe’ operation interleaves with the execution of the publish loop. Albeit there is a possibility that things may end up in a bad state, the registry works correctly under the assumption that subscriptions happen at the very beginning of the life of the process and that the unsubscriptions (if any) take place at the very end. In this case, chances that things do go badly may be pretty slim. Making the registry thread-safe will be considered as a future improvement.

What kind of function can be a callback?

> Anything you fancy: lambdas, ‘closures’, class, object or module methods. For instance:

    from neutron_lib.callbacks import events
    from neutron_lib.callbacks import resources
    from neutron_lib.callbacks import registry

    def callback1(resource, event, trigger, payload):
        print('module callback')

    class MyCallback(object):

        def callback2(self, resource, event, trigger, payload):
            print('object callback')

        @classmethod
        def callback3(cls, resource, event, trigger, payload):
            print('class callback')

    c = MyCallback()
    registry.subscribe(callback1, resources.ROUTER, events.BEFORE_CREATE)
    registry.subscribe(c.callback2, resources.ROUTER, events.BEFORE_CREATE)
    registry.subscribe(MyCallback.callback3, resources.ROUTER, events.BEFORE_CREATE)

    def do_notify():
        def nested_subscribe(resource, event, trigger, payload):
            print('nested callback')

        registry.subscribe(nested_subscribe, resources.ROUTER, events.BEFORE_CREATE)

        registry.publish(resources.ROUTER, events.BEFORE_CREATE,
                         do_notify, events.EventPayload(None))

    print('Notifying...')
    do_notify()

And the output is going to be:

    Notifying...
    module callback
    object callback
    class callback
    nested callback

this page last updated: 2026-03-26 12:58:28

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