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      Licensed under the Apache License, Version 2.0 (the "License"); you may
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      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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      License for the specific language governing permissions and limitations
      under the License.

Notifications in Nova
=====================
Similarly to other OpenStack services Nova emits notifications to the message
bus with the Notifier class provided by oslo.messaging [1]_. 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 `notification_driver` and
`notification_topics`. There are notification configuration options in Nova
which are specific for certain notification types like
`notify_on_state_change`, `notify_api_faults`, `default_notification_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 versionedobject [2]_.

.. _service.update:

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 `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`.

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.objects.notification.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.notification.NotificationPayloadBase` class and shall define the
fields of the payload as versionedobject fields. The base classes are described
in [3]_.

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

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

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


    @base.NovaObjectRegistry.register
    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')
    @base.NovaObjectRegistry.register
    class ServiceStatusNotification(notification.NotificationBase):
        # Version 1.0: Initial version
        VERSION = '1.0'

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

    @base.NovaObjectRegistry.register
    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 :ref:`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.

Existing versioned notifications
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versioned_notifications::



.. [1] http://docs.openstack.org/developer/oslo.messaging/notifier.html
.. [2] http://docs.openstack.org/developer/oslo.versionedobjects
.. [3] http://docs.openstack.org/developer/nova/devref/api/nova.objects.notification.html
