I'm not sure about this one. It looks to me like the node name is only lower-cased when no cloud provider is set.

Here's where the hostname gets lower-cased:

Here's what kubelet does with it:

In short: if there's a cloud provider, kubelet will get the node name from that instead of from GetHostname.

For the AWS cloud provider, I don't see it being lower-cased:

I don't actually know if it's possible for AWS's PrivateDnsName to contain uppercase characters, but I would like to err on the side of caution and assume it might.

@dshcherb can we reduce the scope of this change to only apply when no cloud-provider is set? I would feel safest with something like this instead:

if cloud_provider == '':
    return gethostname().lower()
elif cloud_provider == 'aws':
    return getfqdn()
else:
    return gethostname()

The K8s docs seem to be pretty clear on conventions:

https://kubernetes.io/docs/concepts/overview/working-with-objects/names/
"By convention, the names of Kubernetes resources should be up to maximum length of 253 characters and consist of lower case alphanumeric characters, -, and ., but certain resources have more specific restrictions."
https://github.com/kubernetes/community/blob/master/contributors/design-proposals/architecture/identifiers.md - this has a bit vague references to resources specifically but mentions DNS labels and lowercase nevertheless.

I have also followed some code paths (from "node create" to the request validation) - looks like an uppercase node (resource) name would not pass ValidateNodeName check as it requires a node name to be have the following regex:
const dns1035LabelFmt string = "[a-z]([-a-z0-9]*[a-z0-9])?"

https://gist.github.com/dshcherb/1ba9b9cb62f8753de210bc459115c07a

I could reduce the scope but I don't think this is necessary due to the above.

Thoughts?

Thanks @dshcherb. You're right - it looks like the apiserver does adhere strictly to that convention:

$ cat test-node.yaml 
apiVersion: v1
kind: Node
metadata:
  name: TEST-NODE

$ kubectl create -f test-node.yaml 
The Node "TEST-NODE" is invalid: metadata.name: Invalid value: "TEST-NODE": a DNS-1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')

I'm +1 to this as-is then - the charm should always convert it to lower case. Thanks for setting me straight.