Creating Helm-based Operators

Creating Helm-based Operators

This guide outlines Helm chart support in the Operator SDK and walks Operator authors 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 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 spec field of the object is a list of configuration options that are typically described in the Helm values.yaml file. Instead of setting these values with flags using the Helm CLI (for example, 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 template of the chart where the 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 the oc command:

$ oc get Tomcats --all-namespaces

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

Building a Helm-based Operator 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 (for example, 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 v0.19.4 CLI installed on the development workstation
Access to a Kubernetes-based cluster v1.11.3+ (for example OKD 4.6) using an account with cluster-admin permissions
OpenShift CLI (oc) v4.6+ installed

Procedure

Create a new Operator project. A namespace-scoped Operator watches and manages resources in a single namespace. Namespace-scoped Operators are preferred because of their flexibility. They enable decoupled upgrades, namespace isolation for failures and monitoring, and differing API definitions.

To create a new Helm-based, namespace-scoped nginx-operator project, use the following command:
```
$ operator-sdk new nginx-operator \
  --api-version=example.com/v1alpha1 \
  --kind=Nginx \
  --type=helm
```
```
$ cd nginx-operator
```
This creates the nginx-operator project specifically for watching the Nginx resource with API version example.com/v1apha1 and kind Nginx.
Customize the Operator logic.

For this example, the nginx-operator executes the following reconciliation logic for each Nginx custom resource (CR):
- Create an Nginx deployment if it does not exist.
- Create an Nginx service if it does not exist.
- Create an Nginx ingress if it is enabled and does not exist.
- Ensure that the deployment, service, and optional ingress match the desired configuration (for example, 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, review the Helm Chart developer documentation.
2. Understand the Nginx CR spec.
  
  Helm uses a concept called values to provide customizations to the defaults of a Helm chart, which are defined in the 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.com_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.com_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.

Deploy the CRD.

Before running the Operator, Kubernetes must know about the new custom resource definition (CRD) that the Operator will be watching. Deploy the following CRD:
```
$ oc create -f deploy/crds/example_v1alpha1_nginx_crd.yaml
```
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. Run as a pod inside a Kubernetes cluster. This is the preferred method for production use.
  1. Build the nginx-operator image and push it to a registry:
```
$ operator-sdk build quay.io/example/nginx-operator:v0.0.1
```
```
$ podman push quay.io/example/nginx-operator:v0.0.1
```
  2. 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
```
  3. Deploy the nginx-operator manifests:
```
$ oc create -f deploy/service_account.yaml
```
```
$ oc create -f deploy/role.yaml
```
```
$ oc create -f deploy/role_binding.yaml
```
```
$ oc create -f deploy/operator.yaml
```
  4. Verify that the nginx-operator deployment is up and running:
```
$ oc get deployment
```
    Example output
```
NAME                 DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
nginx-operator       1         1         1            1           1m
```
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 run --local command, the SDK looks in your local file system for this path.
  1. Create a symlink at this location to point to the path of your Helm chart:
```
$ sudo mkdir -p /opt/helm/helm-charts
```
```
$ sudo ln -s $PWD/helm-charts/nginx /opt/helm/helm-charts/nginx
```
  2. To run the Operator locally with the default Kubernetes configuration file present at $HOME/.kube/config:
```
$ operator-sdk run --local
```
    To run the Operator locally with a provided Kubernetes configuration file:
```
$ operator-sdk run --local --kubeconfig=<path_to_config>
```

Deploy the Nginx CR.

Apply the Nginx CR that you modified earlier:

$ oc apply -f deploy/crds/example.com_v1alpha1_nginx_cr.yaml

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

$ oc get deployment

Example output

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:

$ oc get pods

Example output

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:

$ oc get service

Example output

NAME                                      TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)    AGE
example-nginx-b9phnoz9spckcrua7ihrbkrt1   ClusterIP   10.96.26.3   <none>        8080/TCP   1m

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.com_v1alpha1_nginx_cr.yaml

Example output

apiVersion: "example.com/v1alpha1"
kind: "Nginx"
metadata:
  name: "example-nginx"
spec:
  replicaCount: 3

$ oc apply -f deploy/crds/example.com_v1alpha1_nginx_cr.yaml

Confirm that the Operator changes the deployment size:

$ oc get deployment

Example output

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:

$ oc get service

Example output

NAME                                      TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)  AGE
example-nginx-b9phnoz9spckcrua7ihrbkrt1   ClusterIP   10.96.26.3   <none>        80/TCP   1m

Clean up the resources:

$ oc delete -f deploy/crds/example.com_v1alpha1_nginx_cr.yaml

$ oc delete -f deploy/operator.yaml

$ oc delete -f deploy/role_binding.yaml

$ oc delete -f deploy/role.yaml

$ oc delete -f deploy/service_account.yaml

$ oc delete -f deploy/crds/example_v1alpha1_nginx_crd.yaml

Additional resources

See Appendices to learn about the project directory structures created by the Operator SDK.
Operator Development Guide for Red Hat Partners