YAML Anchor support

[george-angel]

Starting from kustomize/v4.0.0 - there is no way for us to build manifests that contain yaml anchors. Previously we were able to workaround by setting --enable_kyaml=false.

Example:

proximo.yaml

apiVersion: v1
kind: Service
metadata:
  annotations:
    prometheus.io/path: /__/metrics
    prometheus.io/port: "8080"
    prometheus.io/scrape: "true"
  labels:
    app: &app proximo
    uw.systems/mirror: "true"
  name: *app
spec:
  ports:
  - name: ops
    port: 8080
    protocol: TCP
  - name: http
    port: 6868
    protocol: TCP
  selector:
    app: *app

kustomization.yaml

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
  - proximo.yaml

Trying to do a build:

$ kustomize build .
Error: considering field 'metadata/name' of object
apiVersion: v1
kind: Service
metadata:
  annotations:
    prometheus.io/path: /__/metrics
    prometheus.io/port: "8080"
    prometheus.io/scrape: "true"
  labels:
    app: &app proximo
    uw.systems/mirror: "true"
  name: *app
spec:
  ports:
  - name: ops
    port: 8080
    protocol: TCP
  - name: http
    port: 6868
    protocol: TCP
  selector:
    app: *app
: wrong Node Kind for metadata.name expected: ScalarNode was AliasNode: value: {*app}

Thank you.

[eddiezane]

/label anchors

[k8s-ci-robot]

@eddiezane: The label(s) /label anchors cannot be applied. These labels are supported: api-review, tide/merge-method-merge, tide/merge-method-rebase, tide/merge-method-squash, team/katacoda

In response to this:

/label anchors

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[eddiezane]

@KnVerey to reply

[KnVerey]

Several issues have been opened about this YAML anchor regression, which affects v4+ universally and v3+ if kyaml is enabled. Although they don't all result in the same error message, it seems to me that they are all related to the switch from apimachinery to kyaml. To that end, I'm going to close the various other issues and consolidate on this one.

If a community member would be interested in diving in to restore YAML anchor support, we would be happy to accept the changes. That said, given the effort involved, it would be helpful to have more information about the purpose of the feature from those who have historically been using it. YAML anchors provide a way to reuse configuration snippets, but in the examples I've seen so far, the same result could be achieved with Kustomize-native mechanisms (patches, commonLabels, etc.). This is even more true now that the extremely powerful ReplacementsTranformer has merged. It would be helpful to have more information as to why and in which cases YAML anchors are being preferred.

If there isn't interest in contributing support, and effective results can be achieve within Kustomize's own mechanisms, we should instead consider simplifying by removing the feature entirely (as some other projects have chosen to do) and providing clear error messages to that effect.

If you use YAML anchors with Kustomize, please comment with more context to help us understand why this feature is important to you.

/triage under-consideration