This guide outlines Helm chart support in the Operator SDK and walks through an example of building and running an Nginx Operator with the operator-sdk CLI tool that uses an existing Helm chart.

Helm chart support in the Operator SDK

The Operator Framework is an open source toolkit to manage Kubernetes native applications, called Operators, in an effective, automated, and scalable way. This framework includes the Operator SDK, which assists developers in bootstrapping and building an Operator based on their expertise without requiring knowledge of Kubernetes API complexities.

One of the Operator SDK’s options for generating an Operator project includes leveraging an existing Helm chart to deploy Kubernetes resources as a unified application, without having to write any Go code. Such Helm-based Operators are designed to excel at stateless applications that require very little logic when rolled out, because changes should be applied to the Kubernetes objects that are generated as part of the chart. This may sound limiting, but can be sufficient for a surprising amount of use-cases as shown by the proliferation of Helm charts built by the Kubernetes community.

The main function of an Operator is to read from a custom object that represents your application instance and have its desired state match what is running. In the case of a Helm-based Operator, the object’s spec field is a list of configuration options that are typically described in Helm’s values.yaml file. Instead of setting these values with flags using the Helm CLI (e.g., helm install -f values.yaml), you can express them within a Custom Resource (CR), which, as a native Kubernetes object, enables the benefits of RBAC applied to it and an audit trail.

For an example of a simple CR called Tomcat:

apiVersion: apache.org/v1alpha1
kind: Tomcat
metadata:
  name: example-app
spec:
  replicaCount: 2

The replicaCount value, 2 in this case, is propagated into the chart’s templates where following is used:

{{ .Values.replicaCount }}

After an Operator is built and deployed, you can deploy a new instance of an app by creating a new instance of a CR, or list the different instances running in all environments using a kubectl or oc command:

$ kubectl get Tomcats --all-namespaces

There is no need to use the Helm CLI or install Tiller; Helm-based Operators import code from the Helm project. All you need to do is have an instance of the Operator running and register the CR with a Custom Resource Definition (CRD). And because it obeys RBAC, you can more easily prevent production changes.

See the following sections for instructions on installing the Operator SDK to build and run your own Helm-based Operator.

Installing the Operator SDK CLI

The Operator SDK has a CLI tool that assists developers in creating, building, and deploying a new Operator project. You can install the SDK CLI on your workstation so you are prepared to start authoring your own Operators.

Prerequisites
  • dep v0.5.0+

  • Git

  • Go v1.10+

  • Docker v17.03+

  • kubectl v1.11.3+

  • Access to a cluster based on Kubernetes v1.11.3+

  • Access to a container registry

This guide uses minikube v0.25.0+ as the local Kubernetes cluster and quay.io for the public registry.

Procedure
  1. Clone an operator-sdk repository:

    $ mkdir -p $GOPATH/src/github.com/operator-framework
    $ cd $GOPATH/src/github.com/operator-framework
    $ git clone https://github.com/operator-framework/operator-sdk
    $ cd operator-sdk
  2. Check out the desired release branch:

    $ git checkout master
  3. Install the SDK CLI tool:

    $ make dep
    $ make install

    This installs the CLI binary operator-sdk at $GOPATH/bin.

  4. Verify that the CLI tool was installed correctly:

    $ operator-sdk -h

Building an Operator with Helm charts using the Operator SDK

This procedure walks through an example of building a simple Nginx Operator powered by a Helm chart using tools and libraries provided by the Operator SDK.

It is best practice to build a new Operator for each chart. This can allow for more native-behaving Kubernetes APIs (e.g., oc get Nginx) and flexibility if you ever want to write a fully-fledged Operator in Go, migrating away from a Helm-based Operator.

Prerequisites
  • Operator SDK CLI installed on the development workstation

  • Access to a Kubernetes-based cluster v1.11.3+ (for example OKD 4.0) using an account with cluster-admin permissions

  • kubectl v1.11.3+ (can alternatively use oc)

Procedure
  1. Create a new project.

    To create a new Helm-based, namespace-scoped nginx-operator project, use the operator-sdk new command:

    $ operator-sdk new nginx-operator \
      --api-version=example.com/v1alpha1 --kind=Nginx --type=helm
    $ cd nginx-operator

    See Appendices to learn about the project directory structure created by the previous commands.

    This creates the nginx-operator project specifically for watching the Nginx resource with APIVersion example.com/v1apha1 and Kind Nginx.

    Operator scope

    A namespace-scoped Operator (the default) watches and manages resources in a single namespace, whereas a cluster-scoped operator watches and manages resources cluster-wide. Namespace-scoped operators are preferred because of their flexibility. They enable decoupled upgrades, namespace isolation for failures and monitoring, and differing API definitions.

    However, there are use cases where a cluster-scoped operator may make sense. For example, the cert-manager operator is often deployed with cluster-scoped permissions and watches so that it can manage issuing certificates for an entire cluster.

    If you would like to create your nginx-operator project to be cluster-scoped, use the following operator-sdk new command instead:

    $ operator-sdk new nginx-operator \
        --cluster-scoped --api-version=example.com/v1alpha1 \
        --kind=Nginx --type=helm

    Using the --cluster-scoped flag will scaffold the new Operator with the following modifications:

    • deploy/operator.yaml: Set WATCH_NAMESPACE="" instead of setting it to the Pod’s namespace.

    • deploy/role.yaml: Use ClusterRole instead of Role.

    • deploy/role_binding.yaml:

      • Use ClusterRoleBinding instead of RoleBinding.

      • Set the subject namespace to REPLACE_NAMESPACE. This must be changed to the namespace in which the Operator is deployed.

  2. Customize the Operator logic.

    For this example, the nginx-operator executes the following reconciliation logic for each Nginx Custom Resource (CR):

    • Create a Nginx Deployment if it does not exist.

    • Create a Nginx Service if it does not exist.

    • Create a Nginx Ingress if it is enabled and does not exist.

    • Ensure that the Deployment, Service, and optional Ingress match the desired configuration (e.g., replica count, image, service type) as specified by the Nginx CR.

    By default, the nginx-operator watches Nginx resource events as shown in the watches.yaml file and executes Helm releases using the specified chart:

    ---
    - version: v1alpha1
      group: example.com
      kind: Nginx
      chart: /opt/helm/helm-charts/nginx
    1. Review the Nginx Helm chart.

      When a Helm Operator project is created, the Operator SDK creates an example Helm chart that contains a set of templates for a simple Nginx release.

      For this example, templates are available for Deployment, Service, and Ingress resources, along with a NOTES.txt template, which Helm chart developers use to convey helpful information about a release.

      If you are not already familiar with Helm Charts, take a moment to review the Helm Chart developer documentation.

    2. Understand the Nginx CR spec.

      Helm uses a concept called values to provide customizations to a Helm chart’s defaults, which are defined in the Helm chart’s values.yaml file.

      Override these defaults by setting the desired values in the CR spec. You can use the number of replicas as an example:

      1. First, inspect the helm-charts/nginx/values.yaml file to find that the chart has a value called replicaCount and it is set to 1 by default. To have 2 Nginx instances in your deployment, your CR spec must contain replicaCount: 2.

        Update the deploy/crds/example_v1alpha1_nginx_cr.yaml file to look like the following:

        apiVersion: example.com/v1alpha1
        kind: Nginx
        metadata:
          name: example-nginx
        spec:
          replicaCount: 2
      2. Similarly, the default service port is set to 80. To instead use 8080, update the deploy/crds/example_v1alpha1_nginx_cr.yaml file again by adding the service port override:

        apiVersion: example.com/v1alpha1
        kind: Nginx
        metadata:
          name: example-nginx
        spec:
          replicaCount: 2
          service:
            port: 8080

        The Helm Operator applies the entire spec as if it was the contents of a values file, just like the helm install -f ./overrides.yaml command works.

  3. Deploy the CRD.

    Before running the Operator, Kubernetes needs to know about the new custom resource definition (CRD) the operator will be watching. Deploy the following CRD:

    $ kubectl create -f deploy/crds/example_v1alpha1_nginx_crd.yaml
  4. Build and run the Operator.

    There are two ways to build and run the Operator:

    • As a Pod inside a Kubernetes cluster.

    • As a Go program outside the cluster using the operator-sdk up command.

    Choose one of the following methods.

    1. Option 1: Run as a Pod inside a Kubernetes cluster. This is the preferred method for production use.

      Build the nginx-operator image and push it to a registry:

      $ operator-sdk build quay.io/example/nginx-operator:v0.0.1
      $ docker push quay.io/example/nginx-operator:v0.0.1

      Kubernetes deployment manifests are generated in the deploy/operator.yaml file. The deployment image in this file needs to be modified from the placeholder REPLACE_IMAGE to the previous built image. To do this, run:

      $ sed -i 's|REPLACE_IMAGE|quay.io/example/nginx-operator:v0.0.1|g' deploy/operator.yaml

      If you created your Operator using the --cluster-scoped=true flag, update the service account namespace in the generated ClusterRoleBinding to match where you are deploying your Operator:

      $ export OPERATOR_NAMESPACE=$(kubectl config view --minify -o jsonpath='{.contexts[0].context.namespace}')
      $ sed -i "s|REPLACE_NAMESPACE|$OPERATOR_NAMESPACE|g" deploy/role_binding.yaml

      If you are performing these steps on OSX, use the following commands instead:

      $ sed -i "" 's|REPLACE_IMAGE|quay.io/example/nginx-operator:v0.0.1|g' deploy/operator.yaml
      $ sed -i "" "s|REPLACE_NAMESPACE|$OPERATOR_NAMESPACE|g" deploy/role_binding.yaml

      Deploy the nginx-operator:

      $ kubectl create -f deploy/service_account.yaml
      $ kubectl create -f deploy/role.yaml
      $ kubectl create -f deploy/role_binding.yaml
      $ kubectl create -f deploy/operator.yaml

      Verify that the nginx-operator is up and running:

      $ kubectl get deployment
      NAME                 DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
      nginx-operator       1         1         1            1           1m
    2. Option 2: Run outside the cluster. This method is preferred during the development cycle to speed up deployment and testing.

      It is important that the chart path referenced in the watches.yaml file exists on your machine. By default, the watches.yaml file is scaffolded to work with an Operator image built with the operator-sdk build command. When developing and testing your operator with the operator-sdk up local command, the SDK looks in your local file system for this path.

      It is recommend to create a symlink at this location to point to your Helm chart’s path:

      $ sudo mkdir -p /opt/helm/helm-charts
      $ sudo ln -s $PWD/helm-charts/nginx /opt/helm/helm-charts/nginx

      To run the Operator locally with the default Kubernetes configuration file present at $HOME/.kube/config:

      $ operator-sdk up local
      INFO[0000] Go Version: go1.10.3
      INFO[0000] Go OS/Arch: linux/amd64
      INFO[0000] operator-sdk Version: v0.3.0+git

      To run the Operator locally with a provided Kubernetes configuration file:

      $ operator-sdk up local --kubeconfig=<path_to_config>
      INFO[0000] Go Version: go1.10.3
      INFO[0000] Go OS/Arch: linux/amd64
      INFO[0000] operator-sdk Version: v0.3.0+git
  5. Deploy the Nginx custom resource.

    Apply the Nginx CR that you modified earlier:

    $ kubectl apply -f deploy/crds/example_v1alpha1_nginx_cr.yaml

    Ensure that the nginx-operator creates the Deployment for the CR:

    $ kubectl get deployment
    NAME                                           DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
    example-nginx-b9phnoz9spckcrua7ihrbkrt1        2         2         2            2           1m

    Check the Pods to confirm two replicas were created:

    $ kubectl get pods
    NAME                                                      READY     STATUS    RESTARTS   AGE
    example-nginx-b9phnoz9spckcrua7ihrbkrt1-f8f9c875d-fjcr9   1/1       Running   0          1m
    example-nginx-b9phnoz9spckcrua7ihrbkrt1-f8f9c875d-ljbzl   1/1       Running   0          1m

    Check that the Service port is set to 8080:

    $ kubectl get service
    NAME                                      TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)    AGE
    example-nginx-b9phnoz9spckcrua7ihrbkrt1   ClusterIP   10.96.26.3   <none>        8080/TCP   1m
  6. Update the replicaCount and remove the port.

    Change the spec.replicaCount field from 2 to 3, remove the spec.service field, and apply the change:

    $ cat deploy/crds/example_v1alpha1_nginx_cr.yaml
    apiVersion: "example.com/v1alpha1"
    kind: "Nginx"
    metadata:
      name: "example-nginx"
    spec:
      replicaCount: 3
    
    $ kubectl apply -f deploy/crds/example_v1alpha1_nginx_cr.yaml

    Confirm that the Operator changes the Deployment size:

    $ kubectl get deployment
    NAME                                           DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
    example-nginx-b9phnoz9spckcrua7ihrbkrt1        3         3         3            3           1m

    Check that the Service port is set to the default 80:

    $ kubectl get service
    NAME                                      TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)  AGE
    example-nginx-b9phnoz9spckcrua7ihrbkrt1   ClusterIP   10.96.26.3   <none>        80/TCP   1m
  7. Clean up the resources:

    $ kubectl delete -f deploy/crds/example_v1alpha1_nginx_cr.yaml
    $ kubectl delete -f deploy/operator.yaml
    $ kubectl delete -f deploy/role_binding.yaml
    $ kubectl delete -f deploy/role.yaml
    $ kubectl delete -f deploy/service_account.yaml
    $ kubectl delete -f deploy/crds/example_v1alpha1_nginx_cr.yaml