rabbit_hosts and rabbit_host options are deprecated for removal: [DEFAULT]/transport_url is needed

[Dmitrii Shcherbakov]

In short: we need to use [DEFAULT]/transport_url for both RPC and notifications and have an option to specify a separate transport_url for notifications. Not only will it solve the problem with future removal of old options but will also allow us to easily deploy two rabbitmq clusters separating out RPC and notifications (telemetry in busy deployments).

Both classic and reactive openstack charms are at risk if they keep using parts/section-rabbitmq-oslo in its current form.

https://github.com/openstack/charm-layer-openstack/blob/master/templates/parts/section-rabbitmq-oslo
https://github.com/juju/charm-helpers/blob/master/charmhelpers/contrib/openstack/templates/section-rabbitmq-oslo

Options like rabbit_host and rabbit_hosts are deprecated for removal.

https://docs.openstack.org/oslo.messaging/ocata/opts.html#oslo_messaging_rabbit.rabbit_hosts
https://docs.openstack.org/oslo.messaging/ocata/opts.html#oslo_messaging_rabbit.rabbit_host

https://github.com/openstack/oslo.messaging/commit/2f0d53ba6

     cfg.StrOpt('rabbit_host',
                default='localhost',
                deprecated_group='DEFAULT',
+ deprecated_for_removal=True,
+ deprecated_reason="Replaced by [DEFAULT]/transport_url",

With classic charms transport_url is rendered only in section-oslo-notifications (which is not present in this layer btw).

https://github.com/juju/charm-helpers/blob/master/charmhelpers/contrib/openstack/templates/section-oslo-notifications

https://docs.openstack.org/oslo.messaging/latest/configuration/opts.html#oslo_messaging_notifications.transport_url

Which is different from [DEFAULT]/transport_url which should be used.

https://docs.openstack.org/oslo.messaging/latest/configuration/opts.html#DEFAULT.transport_ur

https://github.com/openstack/oslo.messaging/blob/master/doc/source/admin/AMQP1.0.rst#transport-url-enable
"In oslo.messaging, the transport_url parameters define the OpenStack service backends for RPC and Notify."

"Note, that if a 'transport_url' parameter is not specified in the [NOTIFICATIONS] section, the [DEFAULT] transport_url will be used for both RPC and Notify backends."

transport_url option has been there for years. https://github.com/openstack/oslo.messaging/commit/20c3674021680ed85173db1f7b62acde35ffd231#diff-e9102dfd7aea40ca2249e2336d05785aR27

transport_url for notifications specifically has been added later deprecating notification_transport_url.