kubectl prints a warning about spec.SessionAffinity being ignored when creating a headless service

[simonpasquier]

What happened?

When creating a headless service (see manifest in the reproducer section) without session affinity defined, kubectl (version v1.34.1) always prints a warning:

Warning: spec.SessionAffinity is ignored for headless services

As far as I can tell, session affinity when unset defaults to None so there's no way for the validation to determine whether or not a value was defined by the client.

What did you expect to happen?

No warning.

How can we reproduce it (as minimally and precisely as possible)?

Run this command

kubectl create -f -<<EOF
apiVersion: v1
kind: Service
metadata:
  name: headless
spec:
  clusterIP: None
  ports:
  - name: web
    port: 9090
    protocol: TCP
    targetPort: web
  selector:
    app.kubernetes.io/name: prometheus
  type: ClusterIP
EOF

Anything else we need to know?

The change seems to be introduced by #132214

Kubernetes version

$ kubectl version
Client Version: v1.34.1
Kustomize Version: v5.7.1
Server Version: v1.34.0

Cloud provider

kind

OS version

NAME="Fedora Linux"
VERSION="42 (Workstation Edition)"
RELEASE_TYPE=stable
ID=fedora
VERSION_ID=42
VERSION_CODENAME=""
PLATFORM_ID="platform:f42"
PRETTY_NAME="Fedora Linux 42 (Workstation Edition)"
ANSI_COLOR="0;38;2;60;110;180"
LOGO=fedora-logo-icon
CPE_NAME="cpe:/o:fedoraproject:fedora:42"
DEFAULT_HOSTNAME="fedora"
HOME_URL="https://fedoraproject.org/"
DOCUMENTATION_URL="https://docs.fedoraproject.org/en-US/fedora/f42/"
SUPPORT_URL="https://ask.fedoraproject.org/"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Fedora"
REDHAT_BUGZILLA_PRODUCT_VERSION=42
REDHAT_SUPPORT_PRODUCT="Fedora"
REDHAT_SUPPORT_PRODUCT_VERSION=42
SUPPORT_END=2026-05-13
VARIANT="Workstation Edition"
VARIANT_ID=workstation

Install tools

Container runtime (CRI) and version (if applicable)

Related plugins (CNI, CSI, ...) and versions (if applicable)

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[rxinui]

Hi, I'm not sure to understand the purpose of removing the warning. Is it noise to the end-user or non-sense?

[simonpasquier]

if I understand correctly, it's non-sense because as shown in the example, the warning gets printed even though the headless service doesn't define any session affinity value.

[rxinui]

I see. I think this is triggered by the fact that clusterIP: None is enabled.

I guess this is a general warning.

[Peac36]

@simonpasquier - thanks for reporting this. It's indeed a bug in my implementation. The warning is printed whenever SessionAffinity is not an empty string or not provided in resource data -

kubernetes/pkg/api/service/warnings.go

Line 51 in 43706d6

if service.Spec.SessionAffinity != "" {

However, I've just realized that this field has a default value that is used every time the SessionAffinity is not specified -

kubernetes/pkg/apis/core/v1/defaults.go

Lines 107 to 109 in 845e94d

	if obj.Spec.SessionAffinity == "" {
	obj.Spec.SessionAffinity = v1.ServiceAffinityNone
	}

This means that we'll print this warning whenever a headless service is created or updated, which is wrong.
I'll prepare a fix for that that will print this error only when sessionAffinity is ClientIP

[rxinui]

@Peac36 I was preparing a quick fix locally but better have it from the original commiter 👍

I reckon this falls under sig-apps @soltysh

[Peac36]

/assign