Add kustomize migration path

[michaellzc]

This migration guide is nowhere near ready

Known Limitation

All deploy and sts has to be recreated.

In helm, we introduced two new labels to all deploy and sts at .spec.selector.matchLabels and .spec.template.metadata.labels

deploy-sourcegraph-helm/charts/sourcegraph/templates/frontend/sourcegraph-frontend.Deployment.yaml

Lines 18 to 21 in 9116a74

	selector:
	matchLabels:
	{{- include "sourcegraph.selectorLabels" . | nindent 6 }}
	app: sourcegraph-frontend

https://github.com/sourcegraph/deploy-sourcegraph/blob/f629296cddc1ec84cb31b95e7c874160d05b6bc5/base/frontend/sourcegraph-frontend.Deployment.yaml#L16-L18

app.kubernetes.io/name: {{ include "sourcegraph.name" . }}
app.kubernetes.io/instance: {{ .Release.Name }}

Pod annotations are mutable but deploy's label selector is immutable

Unable to use kubeclt diff to easily "diff" renderer manifest and cluster state

The same deal. It just errored out and complained label selector is immutable.

helm template -n sourcegraph -f override.yaml sourcegraph charts/sourcegraph/ | kubectl diff -f -

Alternative

• Deploy a fresh deployment with helm and attach previously deployed PV? (still, it'll cause some downtime)
• Remove the added label? I know they're good conventions and can be used to run multiple releases of the same chart under the same namespace. However, we're not namespacing (not k8s namespace, but prefixing resources name with the helm release name) resources in the chart.
• We should also take into consideration that not all users (or maybe most?) won't be using helm to managed deployment lifecycle, instead they will just take the rendered manifest and plug back into their own pipeline.

close https://github.com/sourcegraph/sourcegraph/issues/31967

[michaellzc]

Current dependencies on/for this PR:

• main
  • PR Add kustomize migration path #54 (this PR)

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