.. 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": , "event_type": , "timestamp": , "publisher_id": , "message_id": , "payload": } 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 `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( ), 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":":" } 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=) ServiceStatusNotification( publisher=notification.NotificationPublisher.from_service_obj( ), 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` on the wire. Every item in the `SCHEMA` has the syntax of:: : (, ) 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