What are Kubernetes Operators and Custom Resources?
Chi-Sheng Liu
Posted on August 28, 2024
Kubernetes API Resources
I assume that you have a basic understanding of Kubernetes and know how to use kubectl.
Pod, ReplicaSet, Service, and even Namespace are actually types of API Resources.
You can use kubectl api-resources to view all the API Resources currently available in your Kubernetes cluster.
$ kubectl api-resources
NAME SHORTNAMES APIVERSION NAMESPACED KIND
bindings v1 true Binding
componentstatuses cs v1 false ComponentStatus
configmaps cm v1 true ConfigMap
endpoints ep v1 true Endpoints
events ev v1 true Event
limitranges limits v1 true LimitRange
namespaces ns v1 false Namespace
nodes no v1 false Node
persistentvolumeclaims pvc v1 true PersistentVolumeClaim
persistentvolumes pv v1 false PersistentVolume
pods po v1 true Pod
podtemplates v1 true PodTemplate
replicationcontrollers rc v1 true ReplicationController
resourcequotas quota v1 true ResourceQuota
secrets v1 true Secret
serviceaccounts sa v1 true ServiceAccount
services svc v1 true Service
...
Kubernetes Custom Resources
Custom Resources, as the name suggests, refer to custom API Resources that we can create and install in Kubernetes to extend its functionality.
According to the official documentation, there are two ways to create Custom Resources, but the most common method is using Custom Resource Definitions (CRD), so we will focus on CRDs here.
In fact, CRD itself is a type of API Resource, and this can be clearly seen with the following command:
$ kubectl api-resources | grep -i custom
customresourcedefinitions crd,crds apiextensions.k8s.io/v1 false CustomResourceDefinition
Think about how we usually create Resources; typically, we write a YAML file and then create it with kubectl apply. The steps to create a CRD are the same.
First, create a file called chishengliu-crd.yaml with the following content:
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
name: chishenglius.chishengliu.com
spec:
group: chishengliu.com
scope: Namespaced
names:
plural: chishenglius
singular: chishengliu
kind: ChiShengLiu
versions:
- name: v1
served: true
storage: true
schema:
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
mood:
type: string
Then run the following command:
$ kubectl apply -f chishengliu-crd.yaml
customresourcedefinition.apiextensions.k8s.io/chishenglius.chishengliu.com created
$ kubectl api-resources | grep -i chishengliu
chishenglius chishengliu.com/v1 true ChiShengLiu
You will notice that there is now a new Resource in the cluster called ChiShengLiu.
To create this kind of ChiShengLiu Resource, you also write a YAML file and then use kubectl apply. Create a file called happy-chishengliu.yaml with the following content:
apiVersion: chishengliu.com/v1
kind: ChiShengLiu
metadata:
name: happy-chishengliu
spec:
mood: "happy"
Then run the following command:
$ kubectl apply -f happy-chishengliu.yaml
chishengliu.chishengliu.com/happy-chishengliu created
$ kubectl describe chishengliu happy-chishengliu
Name: happy-chishengliu
Namespace: default
Labels: <none>
Annotations: <none>
API Version: chishengliu.com/v1
Kind: ChiShengLiu
Metadata:
Creation Timestamp: 2024-08-28T17:37:57Z
Generation: 1
Resource Version: 1609
UID: 3cbc6964-2c7b-4e60-b113-4fa5f4dd0b3c
Spec:
Mood: happy
Events: <none>
You can see that we have created a ChiShengLiu Resource named happy-chishengliu.
Kubernetes Operator
Having a Custom Resource alone is not very useful. You might notice that after creating this ChiShengLiu Resource, nothing changes in the Kubernetes cluster, unlike with built-in Resources.
Built-in Resources have functionality because there are corresponding Controllers running in the cluster. The Controller's job is to monitor certain Resources in the cluster and adjust the cluster to the state declared in the Resource's spec. For example, the Replication Controller monitors all ReplicaSets in the cluster, and when any ReplicaSet is created, updated, or deleted, it adjusts the cluster to the corresponding state, which means creating the correct number of Pods.
Therefore, if we want the ChiShengLiu Resource we created earlier to have an effect, we need to install a Controller for ChiShengLiu in the cluster. This Controller is not built-in and needs to be written by ourselves, so it's called a Custom Controller, also known as a Kubernetes Operator. The official documentation refers to this pattern as the Operator Pattern.
The most common use case for Kubernetes Operators is to encapsulate underlying logic, especially for framework or tool developers, allowing users to install everything by simply applying a YAML file with kubectl apply, with the Operator handling the rest.
So, how do we write a Kubernetes Operator? Stay tuned for future updates in this series.
References
Posted on August 28, 2024
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