Updating replicas does not update the ring builder files before triggering a rebalance

[James Hebden]

When updating the replicas setting, the ring builder files are not changed, and as such, the rebalance which is triggered by the config-changed hook reports that a rebalance is not required.

Reviewing the hook code, it seems that only changes to devices and/or nodes will instigate a change to the builder files, and that replicas is only considered when first initialising the rings.

[Edward Hope-Morley (hopem)]

Please provide the following:

  * your swift-proxy charm config

  * /var/log/juju/unit-swift-proxy-<id>.log from your leader unit (dump in a pastebin if possible)

  * output of 'swift-ring-builder /etc/swift/object.ring.gz'

[Edward Hope-Morley (hopem)]

Ok I've had a look at the swift-proxy charm code to refresh my memory on how this is currently working and can confirm that replicas config is only used by the charm when the builders are first created so modifying this config option post-deployment will not have any effect. This is in fact the intended behaviour i.e. we just didn't add support for this to be changed. I assume that right now, users of this charm are manually updating the builders and re-spinning and syncing the rings manually. As previously mentioned this is not advisable since it will eventually conflict with the charms handling ring maintenance the next time it tries to update the rings in any way. So in my view the way forward is to extend what we have to be able to support updating this setting post-deployment while ensuring that we are able to do this without putting the cluster at-risk. I can see how this might seem suitable as a charm action but since the code that currently handles rings and builders is not controlled by actions (it actually pre-dates actions) and relies on charm config that actions can't set, it probably going to make most sense to extend the existing hook code. I intend to work on a patch to do this and document how it should be done so will post back once i have something.