A custom resource (CR) is an extension of the Kubernetes API that is not necessarily available in a default Kubernetes installation. It represents a customization of a particular Kubernetes installation
A Kubernetes Operator is an abstraction for deploying non-trivial applications on Kubernetes. It wraps the logic for deploying and operating an application using Kubernetes constructs. As an example, the Kafka operator provides a Kafka cluster as a first-class object.
Custom resource definitions (CRDs) extend the Kubernetes API, providing definitions to create and modify custom resources to a Kubernetes cluster. Custom resources are created as instances of CRDs.
CRDs require a one-time installation in a cluster. Depending on the cluster setup, installation typically requires cluster admin privileges.
CRDs and custom resources are defined as YAML files.
A CRD defines a new kind of resource, such as kind: Kafka, within a Kubernetes cluster.
Operators are implemented as a collection of controllers where each controller watches a specific resource type. When a relevant event occurs on a watched resource a reconcile cycle is started.
During the reconcile cycle, the controller has the responsibility to check that current state matches the desired state described by the watched resource.
Install the auxiliary Operator Lifecycle Manager (OLM), a tool to help manage the Operators running on your cluster.
$ curl -sL https://github.com/operator-framework/operator-lifecycle-manager/releases/download/0.14.1/install.sh | bash -s 0.14.1
customresourcedefinition.apiextensions.k8s.io/clusterserviceversions.operators.coreos.com created
customresourcedefinition.apiextensions.k8s.io/installplans.operators.coreos.com created
customresourcedefinition.apiextensions.k8s.io/subscriptions.operators.coreos.com created
customresourcedefinition.apiextensions.k8s.io/catalogsources.operators.coreos.com created
customresourcedefinition.apiextensions.k8s.io/operatorgroups.operators.coreos.com created
namespace/olm created
namespace/operators created
clusterrole.rbac.authorization.k8s.io/system:controller:operator-lifecycle-manager created
serviceaccount/olm-operator-serviceaccount created
clusterrolebinding.rbac.authorization.k8s.io/olm-operator-binding-olm created
deployment.apps/olm-operator created
deployment.apps/catalog-operator created
clusterrole.rbac.authorization.k8s.io/aggregate-olm-edit created
clusterrole.rbac.authorization.k8s.io/aggregate-olm-view created
operatorgroup.operators.coreos.com/global-operators created
operatorgroup.operators.coreos.com/olm-operators created
clusterserviceversion.operators.coreos.com/packageserver created
catalogsource.operators.coreos.com/operatorhubio-catalog created
Waiting for deployment "olm-operator" rollout to finish: 0 of 1 updated replicas are available...
deployment "olm-operator" successfully rolled out
deployment "catalog-operator" successfully rolled out
Package server phase: Installing
Package server phase: Succeeded
deployment "packageserver" successfully rolled outNext, install the Strimzi Kafka operator
$ kubectl create -f https://operatorhub.io/install/strimzi-kafka-operator.yaml
subscription.operators.coreos.com/my-strimzi-kafka-operator createdTo check if the operator has been installed successfully, wait until the PHASE changes into 'Succeeded' (it may take a while).
$ kubectl get csv -n operators
NAME DISPLAY VERSION REPLACES PHASE
strimzi-cluster-operator.v0.17.0 Strimzi 0.17.0 SucceededOne or more CRD/s should have been created by the operator
$ kubectl get crd
NAME CREATED AT
...
kafkabridges.kafka.strimzi.io 2020-01-16T22:50:13Z
kafkaconnects.kafka.strimzi.io 2020-01-16T22:50:13Z
kafkaconnects2is.kafka.strimzi.io 2020-01-16T22:50:13Z
kafkamirrormakers.kafka.strimzi.io 2020-01-16T22:50:13Z
kafkas.kafka.strimzi.io 2020-01-16T22:50:13Z
kafkatopics.kafka.strimzi.io 2020-01-16T22:50:14Z
kafkausers.kafka.strimzi.io 2020-01-16T22:50:14Z
...If you want to inspect the CRD kafkas.kafka.strimzi.io:
$ kubectl get crd kafkas.kafka.strimzi.io -o yaml | more
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
creationTimestamp: "2020-01-16T22:50:13Z"
generation: 1
labels:
app: strimzi
name: kafkas.kafka.strimzi.io
resourceVersion: "209301"
selfLink: /apis/apiextensions.k8s.io/v1beta1/customresourcedefinitions/kafkas.kafka.strimzi.io
uid: 95cf529d-ba41-45f3-a71c-5606c34ab3e9
spec:
additionalPrinterColumns:
- JSONPath: .spec.kafka.replicas
description: The desired number of Kafka replicas in the cluster
name: Desired Kafka replicas
type: integer
- JSONPath: .spec.zookeeper.replicas
description: The desired number of Zookeeper replicas in the cluster
name: Desired ZK replicas
type: integer
conversion:
strategy: None
group: kafka.strimzi.io
names:
kind: Kafka
listKind: KafkaList
plural: kafkas
shortNames:
- k
singular: kafka
preserveUnknownFields: true
scope: Namespaced
subresources:
:Now, we are going to implement the following architecture:
To test the operator, we are ready to create a Kafka resource, note the kind: Kafka
apiVersion: kafka.strimzi.io/v1beta1
kind: Kafka
metadata:
name: my-kafka-cluster-with-operator
spec:
kafka:
version: 2.3.0
replicas: 1
...$ kubectl apply -f kafka-cluster.yaml
kafka.kafka.strimzi.io/my-kafka-cluster-with-operator createdNote that since we installed the operator and its CRDs successfully, we can get kafka cluster using kubectl tool
$ kubectl get kafka
NAME DESIRED KAFKA REPLICAS DESIRED ZK REPLICAS
my-kafka-cluster-with-operator 1 1Get the Kafka pod list:
$ kubectl get po
NAME READY STATUS RESTARTS AGE
my-kafka-cluster-with-operator-entity-operator-64ff59fdf8-44dkp 3/3 Running 0 60s
my-kafka-cluster-with-operator-kafka-0 2/2 Running 0 90s
my-kafka-cluster-with-operator-zookeeper-0 2/2 Running 0 6m9sYou can delete the cluster now:
$ kubectl delete kafka my-kafka-cluster-with-operator
kafka.kafka.strimzi.io "my-kafka-cluster-with-operator" deleted