Change the default of notification_format to unversioned

The default config `both` means that both the legacy and the versioned
notifications are emitted. This was selected as default in the past when
we thought that this will help the adoption of the versioned interface
while we worked on to make that new interface in feature parity with the
legacy. Even though the versioned notification interface is in feature
parity with the legacy interface since Stein the projects consuming nova
notifications do not have the resources to switch to the new interface.

In the other hand having `both` as a default in an environtment where
only the legacy notifications are consumed causes performance issues in
the message bus hence the bug #1805659.

The original plan was that we set the default to `versioned` when the
interface reaches feature parity but as major consumers are not ready
to switch we cannot do that.

So the only option left is to set the default to `unversioned`.

Related devstack patch: https://review.opendev.org/#/c/662849/

Closes-Bug: #1805659

Change-Id: I72faa356afffb7a079a9ce86fed1b463773a0507

Author: Balazs Gibizer

doc/source/reference/notifications.rst

@@ -113,9 +113,7 @@ notification payload:
 
 There is a Nova configuration parameter
 :oslo.config:option:`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``.
+that can be used to specify which notifications are emitted by Nova.
 
 The versioned notifications are emitted to a different topic than the legacy
 notifications. By default they are emitted to 'versioned_notifications' but it

nova/conf/notifications.py

@@ -57,7 +57,7 @@
         help="Default notification level for outgoing notifications."),
     cfg.StrOpt(
         'notification_format',
-        default='both',
+        default='unversioned',
         choices=[
             ('both', 'Both the legacy unversioned and the new versioned '
              'notifications are emitted'),
@@ -67,12 +67,14 @@
         ],
         deprecated_group='DEFAULT',
         help="""
-Specifies which notification format shall be used by nova.
+Specifies which notification format shall be emitted by nova.
 
-The default value is fine for most deployments and rarely needs to be changed.
-This value can be set to 'versioned' once the infrastructure moves closer to
-consuming the newer format of notifications. After this occurs, this option
-will be removed.
+The versioned notification interface are in feature parity with the legacy
+interface and the versioned interface is actively developed so new consumers
+should used the versioned interface.
+
+However, the legacy interface is heavily used by ceilometer and other mature
+OpenStack components so it remains the default.
 
 Note that notifications can be completely disabled by setting ``driver=noop``
 in the ``[oslo_messaging_notifications]`` group.

nova/tests/unit/conf_fixture.py

@@ -77,6 +77,10 @@ def setUp(self):
         # tests.
         self.conf.set_default('keep_alive', False, group="wsgi")
 
+        # many tests synchronizes on the reception of versioned notifications
+        self.conf.set_default(
+            'notification_format', "both", group="notifications")
+
         config.parse_args([], default_config_files=[], configure_db=False,
                           init_rpc=False)
         policy_opts.set_defaults(self.conf)

releasenotes/notes/unversioned-as-default-notification_format-f149db44b319aa07.yaml

@@ -0,0 +1,11 @@
+---
+upgrade:
+  - |
+    To resolve `bug 1805659`_ the default value of
+    ``[notifications]/notification_format`` is changed from ``both`` to
+    ``unversioned``. For more information see the `documentation of the config
+    option`_. If you are using versioned notifications, you will need to adjust
+    your config to ``versioned``"
+
+    .. _`bug 1805659`: https://bugs.launchpad.net/nova/+bug/1805659
+    .. _`documentation of the config option`: https://docs.openstack.org/nova/latest/configuration/config.html#notifications.notification_format