..
      Licensed under the Apache License, Version 2.0 (the "License"); you may
      not use this file except in compliance with the License. You may obtain
      a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
      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>
    }

Notifications can be completely disabled by setting the following in
your nova configuration file:

.. code-block:: ini

  [oslo_messaging_notifications]
  driver = noop

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.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 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 an additional field 'nova_object.name' for every payload besides
  'nova_object.data' and 'nova_object.version'. This field contains the name of
  the nova internal representation of the payload type. Client code should not
  depend on this name.

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
..........................................
.. automodule:: nova.notifications.objects.base
    :noindex:
    :members:
    :show-inheritance:

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 :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.

Notification payload classes can use inheritance to avoid duplicating common
payload fragments in nova code. However the leaf classes used directly in a
notification should be created with care to avoid future needs of adding extra
level of inheritance that changes the name of the leaf class as that name is
present in the payload class. If this cannot be avoided and the only change is
the renaming then the version of the new payload shall be the same as the old
payload was before the rename. See [3]_ as an example. If the renaming
involves any other changes on the payload (e.g. adding new fields) then the
version of the new payload shall be higher than the old payload was. See [4]_
as an example.

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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. note:: Versioned notifications are added in each release, so the samples
  represented below may not necessarily be in an older version of nova. Ensure
  you are looking at the correct version of the documentation for the release
  you are using.

.. This is a reference anchor used in the main index page.
.. _versioned_notification_samples:

.. versioned_notifications::



.. [1] http://docs.openstack.org/developer/oslo.messaging/notifier.html
.. [2] http://docs.openstack.org/developer/oslo.versionedobjects
.. [3] https://review.openstack.org/#/c/463001/
.. [4] https://review.openstack.org/#/c/453077/
