A relation to memcached is erroneously required for HA

[Peter Matulis]

In terms of running nova-cloud-controller under HA, to make everything "green" a relation is needed between n-c-c and memcached:

$ juju add-relation memcached:cache nova-cloud-controller:memcache

IIUC, memcached is used for the sharing of auth tokens (among the n-c-c units) when n-c-c is in HA, and it does this via the nova-consoleauth daemon [0].

However it looks like these tokens are now being stored in the cloud database and nova-consoleauth is removed in Stein [1]. Consequently the latter package was removed from the n-c-c charm [2].

What am I missing? Why do we still need memcached? Does the n-c-c charm just "think" it needs it?

There may be a valid use case for keeping the interface around (as optional instead of mandatory as it is today). Clouds with large compute fleets could use memcached to alleviate pressure on the database [3].

[0]: https://docs.openstack.org/charm-guide/latest/release-notes/1904.html#nova-cloud-controller-single-nova-consoleauth
[1]: https://github.com/openstack/nova/blob/master/releasenotes/notes/deprecate-nova-consoleauth-ed6ccbc324a0fb10.yaml
[2]: https://bugs.launchpad.net/charm-nova-cloud-controller/+bug/1848478
[3]: https://docs.openstack.org/nova/xena/admin/service-groups.html