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kubernetes
`kubectl --sort-by=.metadata.uid` should not use natural sort
What happened?
After doing kubectl get pod -A --sort-by=.metadata.uid I realized the pods were not ordered by uid
What did you expect to happen?
All the pods of the cluster being ordered by uid
How can we reproduce it (as minimally and precisely as possible)?
The following screenshot shows the result of the commands trying to order the pods:
Anything else we need to know?
No response
Kubernetes version
$ kubectl version
Client Version: v1.29.2
Kustomize Version: v5.0.4-0.20230601165947-6ce0bf390ce3
Server Version: v1.29.2
$ kubeadm version
kubeadm version: &version.Info{Major:"1", Minor:"29", GitVersion:"v1.29.2", GitCommit:"4b8e819355d791d96b7e9d9efe4cbafae2311c88", GitTreeState:"clean", BuildDate:"2024-02-14T10:39:04Z", GoVersion:"go1.21.7", Compiler:"gc", Platform:"linux/amd64"}
$ kubelet --version
Kubernetes v1.29.2
Cloud provider
No cloud provider. Installation done in two virtual machines (i.e., control plane and worker) using virtualbox
OS version
# On Linux:
$ cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.6 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.6 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
$ uname -a
Linux control-plane-server 5.4.0-173-generic kubernetes/kubernetes#191-Ubuntu SMP Fri Feb 2 13:55:07 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
Install tools
Container runtime (CRI) and version (if applicable)
Related plugins (CNI, CSI, ...) and versions (if applicable)
This issue is currently awaiting triage.
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I did some research on this and found something interesting.
Kubectl uses something called "Natural Sorting" to sort strings.
https://github.com/kubernetes/kubernetes/blob/f4e246bc93ffb68b33ed67c7896c379efa4207e7/staging/src/k8s.io/kubectl/pkg/cmd/get/sorter.go#L272-L284
And this results in some unexpected results when sorting strings that contain both digits and alpha characters.
In 2021, a change was made to the code to sort Quantity strings (100m, 8Gi, etc), but this is handling only a special case.
The general problem is that strings are sorted in an unexpected way, even for things like name, so it is not limited to just UID... it's just more obvious due to the alphanumeric format of UIDs.
Example:
Create two pods, named a12 and a9a:
kubectl apply -f - << EOF
apiVersion: v1
kind: Namespace
metadata:
name: sorttest
---
apiVersion: v1
kind: Pod
metadata:
name: a12
namespace: sorttest
spec:
containers:
- image: registry.k8s.io/pause
name: pause
---
apiVersion: v1
kind: Pod
metadata:
name: a9a
namespace: sorttest
spec:
containers:
- image: registry.k8s.io/pause
name: pause
EOF
Then get the pods, sorted by name:
kubectl get pod -n sorttest --sort-by={.metadata.name}
NAME READY STATUS RESTARTS AGE
a9a 1/1 Running 0 101s
a12 1/1 Running 0 101s
Why is a9a sorted before a12?
I believe it is because "Natural Sorting" is attempting to group all the digits together for comparison separately from alpha characters, so it is comparing 9 to 12, not 9 to 1.
I don't know the use case for wanting to sort strings in this way.
I can't imagine this would be the way most people would expect the output to be sorted, and when it does, it just makes it seem like a bug, whereas clearly this is intentional, according to the code.
@eddiezane @mpuckett159 (and anyone else): What would the impact be of changing the method kubectl uses for sorting to use a simple string comparison instead of "Natural Sorting"? I guess it would be considered a breaking change, but is there a way we could work toward changing the sort via some kind of flag and deprecation period?
Related issues:
- https://github.com/kubernetes/kubectl/issues/1517 where the pod names get misinterpreted as Quantity in some cases (if it is parsed successfully as a quantity).
Hi @brianpursley
That is a very detailed explanation. Thank you very much!
My two cents, this is a bug and should be fixed. I am sure that people will say that because this changes user observable behavior we should not implement a fix because it could break downstream tooling, however I can think of no reasonable use case where this would be considered "expected behavior" and would definitely like to push forward a more correct sorting method for strings. I'm not sure the best method to differentiate between fields that would benefit from natural sorting and which it would detract from, but we should really figure out a way to handle that. This has long been something I haven't really understood but noticed also. Glad to finally have an explanation for this.
My initial investigation which is linked in @brianpursley 's post above definitely suggests that values are all sorted using the natural sort found in the apiMachinery section. I think it would be quite a big change likely involving some sort of internal catalog that indicates which fields are plain strings requiring regular string sort, and which are numbers (potentially with SI units making JSON regard them as strings), which would require natural sort.
But I'd really like to see it addressed! The issues I raised both here and in apiMachinery are going stale!
/retitle kubectl --sort-by=.metadata.uid should not use natural sort
/kind feature
/remove-kind bug
To be clear on this, only properties that contain some kind of numeric quantity (all resources, including custom) should pass through the natural sort algorithm - else it should be a plain string sort.
I think there is another one as a duplicate of this https://github.com/kubernetes/kubectl/issues/1517
I think there is another one as a duplicate of this https://github.com/kubernetes/kubectl/issues/1517