kubebuilder - group, domain, kind, version

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+.idea

content/posts/k8s-operator-kubebuilder-group-version-domain-kind.md

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+---
+title: "k8s operator with kubebuilder - domain, group, kind, version"
+date: 2023-06-07T15:05+01:00
+draft: false
+---
+
+While trying to close my k8s knowledge gaps, I've followed [titorual](https://github.com/syntasso/instruqt-kubecon)
+on how to create k8s operator with kubebuilder. After executing few magic commands, 
+like
+
+```
+kubebuilder init --domain tutorial.kubebuilder.io --repo tutorial.kubebuilder.io/project
+kubebuilder create api --group batch --version v1 --kind CronJob
+```
+
+I've keen to know more - what are domain, group, kind, why they needed at all and can we omit them?
+
+There is some information on that topic in kubebuilder tutorial - [Groups and Versions and Kinds, oh my!](https://book.kubebuilder.io/cronjob-tutorial/gvks.html#groups-and-versions-and-kinds-oh-my)
+Also, there is a [question on stackoverflow](https://stackoverflow.com/questions/72337882/what-is-domain-in-kubebuilder-scaffolding)
+
+It turns out that those abstractions serve namespacing purposes, and you cannot omit domain.
+Below are few quotes from StackOverflow
+
+
+A `group` along with a `version` and a `kind` uniquely identify a type of resource
+that the Kubernetes API server knows about.
+
+Functionally, the `domain/group` along with the version form the `apiVersion` field
+of any resources you provision against
+
+`group` is a required field on the `CustomResourceDefinition`
+
+Also, `group` can be used to have single objects with the same `Kind: Bucket` but different
+implementations
+
+```
+apiVersion: aws.amazon.com/v1
+kind: Bucket
+```
+
+is different from
+
+```
+apiVersion: gcp.google.com/v1
+kind: Bucket
+```
+
+Those CRDS are provisioned in fresh k3d k8s cluster:
+
+```
+$ kubectl get crds
+NAME                                     CREATED AT
+addons.k3s.cattle.io                     2023-06-13T22:08:22Z
+helmcharts.helm.cattle.io                2023-06-13T22:08:23Z
+helmchartconfigs.helm.cattle.io          2023-06-13T22:08:23Z
+middlewaretcps.traefik.containo.us       2023-06-13T22:08:52Z
+serverstransports.traefik.containo.us    2023-06-13T22:08:52Z
+ingressrouteudps.traefik.containo.us     2023-06-13T22:08:52Z
+ingressroutetcps.traefik.containo.us     2023-06-13T22:08:52Z
+ingressroutes.traefik.containo.us        2023-06-13T22:08:52Z
+tlsstores.traefik.containo.us            2023-06-13T22:08:52Z
+tlsoptions.traefik.containo.us           2023-06-13T22:08:52Z
+middlewares.traefik.containo.us          2023-06-13T22:08:52Z
+traefikservices.traefik.containo.us      2023-06-13T22:08:52Z
+```
+
+Let's see what is inside last one - `traefikservices.traefik.containo.us`,
+some lines were stripped for clarity
+
+It means, k8s API server have CRD with `group: traefik.containo.us`
+
+```
+kubectl get crd traefikservices.traefik.containo.us -o yaml|kubectl neat
+
+apiVersion: apiextensions.k8s.io/v1
+kind: CustomResourceDefinition
+metadata:
+  name: traefikservices.traefik.containo.us
+spec:
+  group: traefik.containo.us
+  names:
+    kind: TraefikService
+  versions:
+  - name: v1alpha1
+    schema:
+      openAPIV3Schema:
+      [skip]    
+ ```
+
+Quote from [k8s docs on CRD](https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#create-a-customresourcedefinition)
+
+For example, if you save the following CustomResourceDefinition to `resourcedefinition.yaml`
+
+```
+apiVersion: apiextensions.k8s.io/v1
+kind: CustomResourceDefinition
+metadata:
+  # name must match the spec fields below, and be in the form: <plural>.<group>
+  name: crontabs.stable.example.com
+spec:
+  # group name to use for REST API: /apis/<group>/<version>
+  group: stable.example.com
+  # list of versions supported by this CustomResourceDefinition
+  versions:
+    - name: v1
+      # Each version can be enabled/disabled by Served flag.
+      served: true
+      # One and only one version must be marked as the storage version.
+      storage: true
+      schema:
+        openAPIV3Schema:
+          type: object
+          properties:
+            spec:
+              type: object
+              properties:
+                cronSpec:
+                  type: string
+                image:
+                  type: string
+                replicas:
+                  type: integer
+  # either Namespaced or Cluster
+  scope: Namespaced
+  names:
+    # plural name to be used in the URL: /apis/<group>/<version>/<plural>
+    plural: crontabs
+    # singular name to be used as an alias on the CLI and for display
+    singular: crontab
+    # kind is normally the CamelCased singular type. Your resource manifests use this.
+    kind: CronTab
+    # shortNames allow shorter string to match your resource on the CLI
+    shortNames:
+    - ct
+ ```
+
+and create it with
+
+```
+kubectl apply -f resourcedefinition.yaml
+```
+
+Then a new namespaced RESTful API endpoint is created at:
+
+```
+/apis/stable.example.com/v1/namespaces/*/crontabs/...
+```
+
+This endpoint URL can then be used to create and manage custom objects.
+The kind of these objects will be CronTab from the spec of the CustomResourceDefinition
+object you created above.
+
+Hope, this helps with the understanding what are `domain`, `group`, `kind` and `version` for CRDs.