nova/doc/source/notifications.rst

Notifications in Nova

Similarly to other OpenStack services Nova emits notifications to the message bus with the Notifier class provided by oslo.messaging¹. From the notification consumer point of view a notification consists of two parts: an envelope with a fixed structure defined by oslo.messaging and a payload defined by the service emitting the notification. The envelope format is the following:

{
    "priority": <string, selected from a predefined list by the sender>,
    "event_type": <string, defined by the sender>,
    "timestamp": <string, the isotime of when the notification emitted>,
    "publisher_id": <string, defined by the sender>,
    "message_id": <uuid, generated by oslo>,
    "payload": <json serialized dict, defined by the sender>
}

There are two types of notifications in Nova: legacy notifications which have an unversioned payload and newer notifications which have a versioned payload.

Unversioned notifications

Nova code uses the nova.rpc.get_notifier call to get a configured oslo.messaging Notifier object and it uses the oslo provided functions on the Notifier object to emit notifications. The configuration of the returned Notifier object depends on the parameters of the get_notifier call and the value of the oslo.messaging configuration options driver and topics. There are notification configuration options in Nova which are specific for certain notification types like notifications.notify_on_state_change, notifications.notify_on_api_faults, notifications.default_level, etc.

The structure of the payload of the unversioned notifications is defined in the code that emits the notification and no documentation or enforced backward compatibility contract exists for that format.

Versioned notifications

The versioned notification concept is created to fix the shortcomings of the unversioned notifications. The envelope structure of the emitted notification is the same as in the unversioned notification case as it is provided by oslo.messaging. However the payload is not a free form dictionary but a serialized oslo versionedobject².

For example the wire format of the service.update notification looks like the following:

{
    "priority":"INFO",
    "payload":{
        "nova_object.namespace":"nova",
        "nova_object.name":"ServiceStatusPayload",
        "nova_object.version":"1.0",
        "nova_object.data":{
            "host":"host1",
            "disabled":false,
            "last_seen_up":null,
            "binary":"nova-compute",
            "topic":"compute",
            "disabled_reason":null,
            "report_count":1,
            "forced_down":false,
            "version":2
        }
    },
    "event_type":"service.update",
    "publisher_id":"nova-compute:host1"
}

The serialized oslo versionedobject as a payload provides a version number to the consumer so the consumer can detect if the structure of the payload is changed. Nova provides the following contract regarding the versioned notification payload:

• the payload version defined by the the nova_object.version field of the payload will be increased if and only if the syntax or the semantics of the nova_object.data field of the payload is changed.
• a minor version bump indicates a backward compatible change which means that only new fields are added to the payload so a well written consumer can still consume the new payload without any change.
• a major version bump indicates a backward incompatible change of the payload which can mean removed fields, type change, etc in the payload.

There is a Nova configuration parameter notifications.notification_format that can be used to specify which notifications are emitted by Nova. The possible values are unversioned, versioned, both and the default value is both.

The versioned notifications are emitted to a different topic than the legacy notifications. By default they are emitted to 'versioned_notifications' but it is configurable in the nova.conf with the versioned_notifications_topic config option.

How to add a new versioned notification

To support the above contract from the Nova code every versioned notification is modeled with oslo versionedobjects. Every versioned notification class shall inherit from the nova.notifications.objects.base.NotificationBase which already defines three mandatory fields of the notification event_type, publisher_id and priority. The new notification class shall add a new field payload with an appropriate payload type. The payload object of the notifications shall inherit from the nova.objects.notifications.base.NotificationPayloadBase class and shall define the fields of the payload as versionedobject fields. The base classes are described in the following section.

The nova.notifications.objects.base module

nova.notifications.objects.base

Please note that the notification objects shall not be registered to the NovaObjectRegistry to avoid mixing nova internal objects with the notification objects. Instead of that use the register_notification decorator on every concrete notification object.

The following code example defines the necessary model classes for a new notification `myobject.update`:

@notification.notification_sample('myobject-update.json')
@object_base.NovaObjectRegistry.register.register_notification
class MyObjectNotification(notification.NotificationBase):
    # Version 1.0: Initial version
    VERSION = '1.0'

    fields = {
        'payload': fields.ObjectField('MyObjectUpdatePayload')
    }


@object_base.NovaObjectRegistry.register.register_notification
class MyObjectUpdatePayload(notification.NotificationPayloadBase):
    # Version 1.0: Initial version
    VERSION = '1.0'
    fields = {
        'some_data': fields.StringField(),
        'another_data': fields.StringField(),
    }

After that the notification can be populated and emitted with the following code:

payload = MyObjectUpdatePayload(some_data="foo", another_data="bar")
MyObjectNotification(
    publisher=notification.NotificationPublisher.from_service_obj(
        <nova.objects.service.Service instance that emits the notification>),
    event_type=notification.EventType(
        object='myobject',
        action=fields.NotificationAction.UPDATE),
    priority=fields.NotificationPriority.INFO,
    payload=payload).emit(context)

The above code will generate the following notification on the wire:

{
    "priority":"INFO",
    "payload":{
        "nova_object.namespace":"nova",
        "nova_object.name":"MyObjectUpdatePayload",
        "nova_object.version":"1.0",
        "nova_object.data":{
            "some_data":"foo",
            "another_data":"bar",
        }
    },
    "event_type":"myobject.update",
    "publisher_id":"<the name of the service>:<the host where the service runs>"
}

There is a possibility to reuse an existing versionedobject as notification payload by adding a SCHEMA field for the payload class that defines a mapping between the fields of existing objects and the fields of the new payload object. For example the service.status notification reuses the existing nova.objects.service.Service object when defines the notification's payload:

@notification.notification_sample('service-update.json')
@object_base.NovaObjectRegistry.register.register_notification
class ServiceStatusNotification(notification.NotificationBase):
    # Version 1.0: Initial version
    VERSION = '1.0'

    fields = {
        'payload': fields.ObjectField('ServiceStatusPayload')
    }

@object_base.NovaObjectRegistry.register.register_notification
class ServiceStatusPayload(notification.NotificationPayloadBase):
    SCHEMA = {
        'host': ('service', 'host'),
        'binary': ('service', 'binary'),
        'topic': ('service', 'topic'),
        'report_count': ('service', 'report_count'),
        'disabled': ('service', 'disabled'),
        'disabled_reason': ('service', 'disabled_reason'),
        'availability_zone': ('service', 'availability_zone'),
        'last_seen_up': ('service', 'last_seen_up'),
        'forced_down': ('service', 'forced_down'),
        'version': ('service', 'version')
    }
    # Version 1.0: Initial version
    VERSION = '1.0'
    fields = {
        'host': fields.StringField(nullable=True),
        'binary': fields.StringField(nullable=True),
        'topic': fields.StringField(nullable=True),
        'report_count': fields.IntegerField(),
        'disabled': fields.BooleanField(),
        'disabled_reason': fields.StringField(nullable=True),
        'availability_zone': fields.StringField(nullable=True),
        'last_seen_up': fields.DateTimeField(nullable=True),
        'forced_down': fields.BooleanField(),
        'version': fields.IntegerField(),
    }

    def populate_schema(self, service):
        super(ServiceStatusPayload, self).populate_schema(service=service)

If the SCHEMA field is defined then the payload object needs to be populated with the populate_schema call before it can be emitted:

payload = ServiceStatusPayload()
payload.populate_schema(service=<nova.object.service.Service object>)
ServiceStatusNotification(
    publisher=notification.NotificationPublisher.from_service_obj(
        <nova.object.service.Service object>),
    event_type=notification.EventType(
        object='service',
        action=fields.NotificationAction.UPDATE),
    priority=fields.NotificationPriority.INFO,
    payload=payload).emit(context)

The above code will emit the already shown notification<service.update> on the wire.

Every item in the SCHEMA has the syntax of:

<payload field name which needs to be filled>:
    (<name of the parameter of the populate_schema call>,
     <the name of a field of the parameter object>)

The mapping defined in the SCHEMA field has the following semantics. When the populate_schema function is called the content of the SCHEMA field is enumerated and the value of the field of the pointed parameter object is copied to the requested payload field. So in the above example the host field of the payload object is populated from the value of the host field of the service object that is passed as a parameter to the populate_schema call.

A notification payload object can reuse fields from multiple existing objects. Also a notification can have both new and reused fields in its payload.

Note that the notification's publisher instance can be created two different ways. It can be created by instantiating the NotificationPublisher object with a host and a binary string parameter or it can be generated from a Service object by calling NotificationPublisher.from_service_obj function.

Versioned notifications shall have a sample file stored under doc/sample_notifications directory and the notification object shall be decorated with the notification_sample decorator. For example the service.update notification has a sample file stored in doc/sample_notifications/service-update.json and the ServiceUpdateNotification class is decorated accordingly.

What should be in the notification payload

This is just a guideline. You should always consider the actual use case that requires the notification.

• Always include the identifier (e.g. uuid) of the entity that can be used to query the whole entity over the REST API so that the consumer can get more information about the entity.
• You should consider including those fields that are related to the event you are sending the notification about. For example if a change of a field of the entity triggers an update notification then you should include the field to the payload.
• An update notification should contain information about what part of the entity is changed. Either by filling the nova_object.changes part of the payload (note that it is not supported by the notification framework currently) or sending both the old state and the new state of the entity in the payload.
• You should never include a nova internal object in the payload. Create a new object and use the SCHEMA field to map the internal object to the notification payload. This way the evolution of the internal object model can be decoupled from the evolution of the notification payload.
• The delete notification should contain the same information as the create or update notifications. This makes it possible for the consumer to listen only to the delete notifications but still filter on some fields of the entity (e.g. project_id).

Existing versioned notifications

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1. http://docs.openstack.org/developer/oslo.messaging/notifier.html↩︎

2. http://docs.openstack.org/developer/oslo.versionedobjects↩︎