OVN upgrade order does not match upstream

[Trent Lloyd]

According to the upstream OVN documentation:
https://docs.ovn.org/en/latest/intro/install/ovn-upgrades.html

The upgrade order should be:
- ovn-controller (charm: ovn-chassis, a subordinate of nova-compute or ovn-dedicated-chassis)
- ovn-northd, ovn databases (charm: ovn-central)

However currently in the upgrade guide the order is incorrect:
https://docs.openstack.org/charm-guide/latest/admin/upgrades/openstack.html
16 neutron-api
17 ovn-dedicated-chassis [this is OK]
18 ovn-central [should be after nova-compute]
21 nova-compute

The charm upgrade guide also states that:

The general order is:
- all principal charms
- all subordinate charms

And the openstack upgrade guide also says:
Services that are associated with subordinate charms are upgradable but only indirectly. They get upgraded along with their parent principal application.

I'm not sure if that is going to always be true. Something has to trigger the upgrade of the OVN packages specifically. Currently that is done by the charm-upgrade hook. And we now upgrade charms themselves to get new versions of OVN, which is not something the guide really covers atm as it did not used to be that way. Previously we always used the managed upgrade methods and updated the "source" or "openstack-origin" but now we potentially need new charms at the same time and upgrading the charm often implicitly upgrades the software.

For ovn-chassis the apt repo comes from nova-compute, so there is also an issue here that if we wanted to upgrade OVN before nova-compute itself we would need to update the apt source but not upgrade nova itself yet, using the nova-compute charm. That is sometimes easier said than done.

We may need to split the above issues out into separate bugs but I wanted to mention them for completeness. In any case, the service upgrade order appears to be incorrect at a minimum.

-----------------------------------
Release: 0.0.1.dev656 on 2022-08-31 17:46:35
SHA: 4faf6b24cb34eb503481ff0ec492c0a265bac18e
Source: https://opendev.org/openstack/charm-guide/src/doc/source/admin/upgrades/charms.rst
URL: https://docs.openstack.org/charm-guide/latest/admin/upgrades/charms.html

[Frode Nordahl (fnordahl)]

Thank you for reporting this bug. It is true that the current upstream documentation suggests the ordering you point out, however that order does not work for upgrades from OVN 20.03 to 22.03.

This is detailed in bug 1940043 and there is also a update pending to the upstream documentation [0].

The documentation should probably be extended to cover the two upgrade procedures separately.

0: https://mail.openvswitch.org/pipermail/ovs-dev/2022-December/400132.html