Getting started with the Operator SDK - Operator SDK | Operators

Architecture of the Operator SDK
Installing the Operator SDK CLI
Building a Go-based Memcached Operator using the Operator SDK
Managing a Memcached Operator using the Operator Lifecycle Manager
Additional resources

This guide outlines the basics of the Operator SDK and walks Operator authors with cluster administrator access to a Kubernetes-based cluster (such as OpenShift Container Platform) through an example of building a simple Go-based Memcached Operator and managing its lifecycle from installation to upgrade.

This is accomplished using two centerpieces of the Operator Framework: the Operator SDK (the operator-sdk CLI tool and controller-runtime library API) and the Operator Lifecycle Manager (OLM).

OpenShift Container Platform 4.3 supports Operator SDK v0.12.0 or later.

Architecture of 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. Operators take advantage of Kubernetes' extensibility to deliver the automation advantages of cloud services like provisioning, scaling, and backup and restore, while being able to run anywhere that Kubernetes can run.

Operators make it easy to manage complex, stateful applications on top of Kubernetes. However, writing an Operator today can be difficult because of challenges such as using low-level APIs, writing boilerplate, and a lack of modularity, which leads to duplication.

The Operator SDK is a framework designed to make writing Operators easier by providing:

High-level APIs and abstractions to write the operational logic more intuitively
Tools for scaffolding and code generation to quickly bootstrap a new project
Extensions to cover common Operator use cases

Workflow

The Operator SDK provides the following workflow to develop a new Operator:

Create a new Operator project using the Operator SDK command line interface (CLI).
Define new resource APIs by adding Custom Resource Definitions (CRDs).
Specify resources to watch using the Operator SDK API.
Define the Operator reconciling logic in a designated handler and use the Operator SDK API to interact with resources.
Use the Operator SDK CLI to build and generate the Operator deployment manifests.

Figure 1. Operator SDK workflow

At a high level, an Operator using the Operator SDK processes events for watched resources in an Operator author-defined handler and takes actions to reconcile the state of the application.

Manager file

The main program for the Operator is the manager file at cmd/manager/main.go. The manager automatically registers the scheme for all Custom Resources (CRs) defined under pkg/apis/ and runs all controllers under pkg/controller/.

The manager can restrict the namespace that all controllers watch for resources:

mgr, err := manager.New(cfg, manager.Options{Namespace: namespace})

By default, this is the namespace that the Operator is running in. To watch all namespaces, you can leave the namespace option empty:

mgr, err := manager.New(cfg, manager.Options{Namespace: ""})

Prometheus Operator support

Prometheus is an open-source systems monitoring and alerting toolkit. The Prometheus Operator creates, configures, and manages Prometheus clusters running on Kubernetes-based clusters, such as OpenShift Container Platform.

Helper functions exist in the Operator SDK by default to automatically set up metrics in any generated Go-based Operator for use on clusters where the Prometheus Operator is deployed.

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.

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

Installing from GitHub release

You can download and install a pre-built release binary of the SDK CLI from the project on GitHub.

Prerequisites

Go v1.13+
docker v17.03+, podman v1.2.0+, or buildah v1.7+
OpenShift CLI (oc) v4.3+ installed
Access to a cluster based on Kubernetes v1.12.0+
Access to a container registry

Procedure

Set the release version variable:
```
RELEASE_VERSION=v0.12.0
```

Download the release binary.

For Linux:

$ curl -OJL https://github.com/operator-framework/operator-sdk/releases/download/${RELEASE_VERSION}/operator-sdk-${RELEASE_VERSION}-x86_64-linux-gnu

For macOS:

$ curl -OJL https://github.com/operator-framework/operator-sdk/releases/download/${RELEASE_VERSION}/operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin

Verify the downloaded release binary.

Download the provided ASC file.

For Linux:

$ curl -OJL https://github.com/operator-framework/operator-sdk/releases/download/${RELEASE_VERSION}/operator-sdk-${RELEASE_VERSION}-x86_64-linux-gnu.asc

For macOS:

$ curl -OJL https://github.com/operator-framework/operator-sdk/releases/download/${RELEASE_VERSION}/operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin.asc

Place the binary and corresponding ASC file into the same directory and run the following command to verify the binary:

For Linux:

$ gpg --verify operator-sdk-${RELEASE_VERSION}-x86_64-linux-gnu.asc

For macOS:

$ gpg --verify operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin.asc

If you do not have the maintainer’s public key on your workstation, you will get the following error:

$ gpg --verify operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin.asc
$ gpg: assuming signed data in 'operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin'
$ gpg: Signature made Fri Apr  5 20:03:22 2019 CEST
$ gpg:                using RSA key <key_id> (1)
$ gpg: Can't check signature: No public key

1	RSA key string.

To download the key, run the following command, replacing <key_id> with the RSA key string provided in the output of the previous command:

$ gpg [--keyserver keys.gnupg.net] --recv-key "<key_id>" (1)

1	If you do not have a key server configured, specify one with the `--keyserver` option.

Install the release binary in your PATH:

For Linux:

$ chmod +x operator-sdk-${RELEASE_VERSION}-x86_64-linux-gnu
$ sudo cp operator-sdk-${RELEASE_VERSION}-x86_64-linux-gnu /usr/local/bin/operator-sdk
$ rm operator-sdk-${RELEASE_VERSION}-x86_64-linux-gnu

For macOS:

$ chmod +x operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin
$ sudo cp operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin /usr/local/bin/operator-sdk
$ rm operator-sdk-${RELEASE_VERSION}-x86_64-apple-darwin

Verify that the CLI tool was installed correctly:
```
$ operator-sdk version
```

Installing from Homebrew

You can install the SDK CLI using Homebrew.

Prerequisites

Homebrew
docker v17.03+, podman v1.2.0+, or buildah v1.7+
OpenShift CLI (oc) v4.3+ installed
Access to a cluster based on Kubernetes v1.12.0+
Access to a container registry

Procedure

Install the SDK CLI using the brew command:
```
$ brew install operator-sdk
```
Verify that the CLI tool was installed correctly:
```
$ operator-sdk version
```

Compiling and installing from source

You can obtain the Operator SDK source code to compile and install the SDK CLI.

Prerequisites

Git
Go v1.13+
docker v17.03+, podman v1.2.0+, or buildah v1.7+
OpenShift CLI (oc) v4.3+ installed
Access to a cluster based on Kubernetes v1.12.0+
Access to a container registry

Procedure

Clone the 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

Check out the desired release branch:
```
$ git checkout master
```
Compile and install the SDK CLI:
```
$ make dep
$ make install
```
This installs the CLI binary operator-sdk at $GOPATH/bin.
Verify that the CLI tool was installed correctly:
```
$ operator-sdk version
```

Building a Go-based Memcached Operator using the Operator SDK

The Operator SDK makes it easier to build Kubernetes native applications, a process that can require deep, application-specific operational knowledge. The SDK not only lowers that barrier, but it also helps reduce the amount of boilerplate code needed for many common management capabilities, such as metering or monitoring.

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

Prerequisites

Operator SDK CLI installed on the development workstation
Operator Lifecycle Manager (OLM) installed on a Kubernetes-based cluster (v1.8 or above to support the apps/v1beta2 API group), for example OpenShift Container Platform 4.3
Access to the cluster using an account with cluster-admin permissions
OpenShift CLI (oc) v4.1+ installed

Procedure

Create a new project.

Use the CLI to create a new memcached-operator project:

$ mkdir -p $GOPATH/src/github.com/example-inc/
$ cd $GOPATH/src/github.com/example-inc/
$ operator-sdk new memcached-operator
$ cd memcached-operator

Add a new Custom Resource Definition (CRD).
1. Use the CLI to add a new CRD API called Memcached, with APIVersion set to cache.example.com/v1apha1 and Kind set to Memcached:
  $ operator-sdk add api \ --api-version=cache.example.com/v1alpha1 \ --kind=Memcached
  This scaffolds the Memcached resource API under pkg/apis/cache/v1alpha1/.
2. Modify the spec and status of the Memcached Custom Resource (CR) at the pkg/apis/cache/v1alpha1/memcached_types.go file:
  type MemcachedSpec struct { // Size is the size of the memcached deployment Size int32 `json:"size"` } type MemcachedStatus struct { // Nodes are the names of the memcached pods Nodes []string `json:"nodes"` }
3. After modifying the *_types.go file, always run the following command to update the generated code for that resource type:
  $ operator-sdk generate k8s
Optional: Add custom validation to your CRD.

OpenAPI v3.0 schemas are added to CRD manifests in the spec.validation block when the manifests are generated. This validation block allows Kubernetes to validate the properties in a Memcached CR when it is created or updated.

Additionally, a pkg/apis/<group>/<version>/zz_generated.openapi.go file is generated. This file contains the Go representation of this validation block if the +k8s:openapi-gen=true annotation is present above the Kind type declaration, which is present by default. This auto-generated code is your Go Kind type’s OpenAPI model, from which you can create a full OpenAPI Specification and generate a client.

As an Operator author, you can use Kubebuilder markers (annotations) to configure custom validations for your API. These markers must always have a +kubebuilder:validation prefix. For example, adding an enum-type specification can be done by adding the following marker:
```
// +kubebuilder:validation:Enum=Lion;Wolf;Dragon
type Alias string
```
Usage of markers in API code is discussed in the Kubebuilder Generating CRDs and Markers for Config/Code Generation documentation. A full list of OpenAPIv3 validation markers is also available in the Kubebuilder CRD Validation documentation.

If you add any custom validations, run the following command to update the OpenAPI validation section in the CRD’s deploy/crds/cache.example.com_memcacheds_crd.yaml file:
```
$ operator-sdk generate openapi
```
Example generated YAML
```
spec:
  validation:
    openAPIV3Schema:
      properties:
        spec:
          properties:
            size:
              format: int32
              type: integer
```
Add a new Controller.
1. Add a new Controller to the project to watch and reconcile the Memcached resource:
  $ operator-sdk add controller \ --api-version=cache.example.com/v1alpha1 \ --kind=Memcached
  This scaffolds a new Controller implementation under pkg/controller/memcached/.
2. For this example, replace the generated controller file pkg/controller/memcached/memcached_controller.go with the example implementation.
  
  The example controller executes the following reconciliation logic for each Memcached CR:
  Create a Memcached deployment if it does not exist.
  
  Ensure that the deployment size is the same as specified by the Memcached CR spec.
  
  Update the Memcached CR status with the names of the Memcached Pods.
  The next two sub-steps inspect how the Controller watches resources and how the reconcile loop is triggered. You can skip these steps to go directly to building and running the Operator.
3. Inspect the Controller implementation at the pkg/controller/memcached/memcached_controller.go file to see how the Controller watches resources.
  
  The first watch is for the Memcached type as the primary resource. For each Add, Update, or Delete event, the reconcile loop is sent a reconcile Request (a <namespace>:<name> key) for that Memcached object:
  err := c.Watch( &source.Kind{Type: &cachev1alpha1.Memcached{}}, &handler.EnqueueRequestForObject{})
  The next watch is for deployments, but the event handler maps each event to a reconcile Request for the owner of the deployment. In this case, this is the Memcached object for which the deployment was created. This allows the controller to watch deployments as a secondary resource:
  err := c.Watch(&source.Kind{Type: &appsv1.deployment{}}, &handler.EnqueueRequestForOwner{ IsController: true, OwnerType: &cachev1alpha1.Memcached{}, })
4. Every Controller has a Reconciler object with a Reconcile() method that implements the reconcile loop. The reconcile loop is passed the Request argument which is a <namespace>:<name> key used to lookup the primary resource object, Memcached, from the cache:
  func (r *ReconcileMemcached) Reconcile(request reconcile.Request) (reconcile.Result, error) { // Lookup the Memcached instance for this reconcile request memcached := &cachev1alpha1.Memcached{} err := r.client.Get(context.TODO(), request.NamespacedName, memcached) ... }
  Based on the return value of Reconcile() the reconcile Request may be requeued and the loop may be triggered again:
  // Reconcile successful - don't requeue return reconcile.Result{}, nil // Reconcile failed due to error - requeue return reconcile.Result{}, err // Requeue for any reason other than error return reconcile.Result{Requeue: true}, nil
Build and run the Operator.
1. Before running the Operator, the CRD must be registered with the Kubernetes API server:
  $ oc create \ -f deploy/crds/cache_v1alpha1_memcached_crd.yaml
2. After registering the CRD, there are two options for running the Operator:
  As a deployment inside a Kubernetes cluster
  
  As Go program outside a cluster
  Choose one of the following methods.
  1. Option A: Running as a deployment inside the cluster.
    
    Build the memcached-operator image and push it to a registry:
    
    $ operator-sdk build quay.io/example/memcached-operator:v0.0.1
    
    The deployment manifest is generated at deploy/operator.yaml. Update the deployment image as follows since the default is just a placeholder:
    
    $ sed -i 's|REPLACE_IMAGE|quay.io/example/memcached-operator:v0.0.1|g' deploy/operator.yaml
    
    Ensure you have an account on Quay.io for the next step, or substitute your preferred container registry. On the registry, create a new public image repository named memcached-operator.
    
    Push the image to the registry:
    
    $ podman push quay.io/example/memcached-operator:v0.0.1
    
    Setup RBAC and deploy memcached-operator:
    
    $ oc create -f deploy/role.yaml $ oc create -f deploy/role_binding.yaml $ oc create -f deploy/service_account.yaml $ oc create -f deploy/operator.yaml
    
    Verify that memcached-operator is up and running:
    
    $ oc get deployment NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE memcached-operator 1 1 1 1 1m
  2. Option B: Running locally outside the cluster.
    
    This method is preferred during development cycle to deploy and test faster.
    
    Run the Operator locally with the default Kubernetes configuration file present at $HOME/.kube/config:
    
    $ operator-sdk up local --namespace=default
    
    You can use a specific kubeconfig using the flag --kubeconfig=<path/to/kubeconfig>.

Verify that the Operator can deploy a Memcached application by creating a Memcached CR.

Create the example Memcached CR that was generated at deploy/crds/cache_v1alpha1_memcached_cr.yaml:

$ cat deploy/crds/cache_v1alpha1_memcached_cr.yaml
apiVersion: "cache.example.com/v1alpha1"
kind: "Memcached"
metadata:
  name: "example-memcached"
spec:
  size: 3

$ oc apply -f deploy/crds/cache_v1alpha1_memcached_cr.yaml

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

$ oc get deployment
NAME                     DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
memcached-operator       1         1         1            1           2m
example-memcached        3         3         3            3           1m

Check the Pods and CR status to confirm the status is updated with the memcached Pod names:

$ oc get pods
NAME                                  READY     STATUS    RESTARTS   AGE
example-memcached-6fd7c98d8-7dqdr     1/1       Running   0          1m
example-memcached-6fd7c98d8-g5k7v     1/1       Running   0          1m
example-memcached-6fd7c98d8-m7vn7     1/1       Running   0          1m
memcached-operator-7cc7cfdf86-vvjqk   1/1       Running   0          2m

$ oc get memcached/example-memcached -o yaml
apiVersion: cache.example.com/v1alpha1
kind: Memcached
metadata:
  clusterName: ""
  creationTimestamp: 2018-03-31T22:51:08Z
  generation: 0
  name: example-memcached
  namespace: default
  resourceVersion: "245453"
  selfLink: /apis/cache.example.com/v1alpha1/namespaces/default/memcacheds/example-memcached
  uid: 0026cc97-3536-11e8-bd83-0800274106a1
spec:
  size: 3
status:
  nodes:
  - example-memcached-6fd7c98d8-7dqdr
  - example-memcached-6fd7c98d8-g5k7v
  - example-memcached-6fd7c98d8-m7vn7

Verify that the Operator can manage a deployed Memcached application by updating the size of the deployment.

Change the spec.size field in the memcached CR from 3 to 4:

$ cat deploy/crds/cache_v1alpha1_memcached_cr.yaml
apiVersion: "cache.example.com/v1alpha1"
kind: "Memcached"
metadata:
  name: "example-memcached"
spec:
  size: 4

Apply the change:

$ oc apply -f deploy/crds/cache_v1alpha1_memcached_cr.yaml

Confirm that the Operator changes the deployment size:

$ oc get deployment
NAME                 DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
example-memcached    4         4         4            4           5m

Clean up the resources:

$ oc delete -f deploy/crds/cache_v1alpha1_memcached_cr.yaml
$ oc delete -f deploy/crds/cache_v1alpha1_memcached_crd.yaml
$ oc delete -f deploy/operator.yaml
$ oc delete -f deploy/role.yaml
$ oc delete -f deploy/role_binding.yaml
$ oc delete -f deploy/service_account.yaml

Additional resources

For more information about OpenAPI v3.0 validation schemas in CRDs, refer to the Kubernetes documentation.

Managing a Memcached Operator using the Operator Lifecycle Manager

The previous section has covered manually running an Operator. In the next sections, we will explore using the Operator Lifecycle Manager (OLM), which is what enables a more robust deployment model for Operators being run in production environments.

The OLM helps you to install, update, and generally manage the lifecycle of all of the Operators (and their associated services) on a Kubernetes cluster. It runs as an Kubernetes extension and lets you use oc for all the lifecycle management functions without any additional tools.

Prerequisites

OLM installed on a Kubernetes-based cluster (v1.8 or above to support the apps/v1beta2 API group), for example OpenShift Container Platform 4.3 Preview OLM enabled
Memcached Operator built

Procedure

Generate an Operator manifest.

An Operator manifest describes how to display, create, and manage the application, in this case Memcached, as a whole. It is defined by a ClusterServiceVersion (CSV) object and is required for the OLM to function.

From the memcached-operator/ directory that was created when you built the Memcached Operator, generate the CSV manifest:
```
$ operator-sdk olm-catalog gen-csv --csv-version 0.0.1
```
See Building a CSV for the Operator Framework for more information on manually defining a manifest file.
Create an OperatorGroup that specifies the namespaces that the Operator will target. Create the following OperatorGroup in the namespace where you will create the CSV. In this example, the default namespace is used:
```
apiVersion: operators.coreos.com/v1
kind: OperatorGroup
metadata:
  name: memcached-operator-group
  namespace: default
spec:
  targetNamespaces:
  - default
```
Deploy the Operator. Use the files that were generated into the deploy/ directory by the Operator SDK when you built the Memcached Operator.
1. Edit the generated CSV manifest file by adding description and displayName fields for each custom resource definition (CRD) kind in the spec.customresourcedefinitions.owned section:
  deploy/olm-catalog/memcached-operator/0.0.1/memcached-operator.v0.0.1.clusterserviceversion.yaml file
  ... spec: customresourcedefinitions: owned: - kind: Memcached name: memcacheds.cache.example.com version: v1alpha1 description: <crd_description> (1) displayName: Memcached (2) ...
  1 Specify a description for the CRD.
  
  2 Specify a display name for the CRD.
2. Apply the CSV manifest to the specified namespace in the cluster:
  $ oc apply -f deploy/olm-catalog/memcached-operator/0.0.1/memcached-operator.v0.0.1.clusterserviceversion.yaml
  When you apply this manifest, the cluster does not immediately update because it does not yet meet the requirements specified in the manifest.
3. Create the role, role binding, and service account to grant resource permissions to the Operator, and the Custom Resource Definition (CRD) to create the Memcached type that the Operator manages:
  $ oc create -f deploy/crds/cache.example.com_memcacheds_crd.yaml $ oc create -f deploy/service_account.yaml $ oc create -f deploy/role.yaml $ oc create -f deploy/role_binding.yaml
  Because the OLM creates Operators in a particular namespace when a manifest is applied, administrators can leverage the native Kubernetes RBAC permission model to restrict which users are allowed to install Operators.

Create an application instance.

The Memcached Operator is now running in the default namespace. Users interact with Operators via instances of CustomResources; in this case, the resource has the kind Memcached. Native Kubernetes RBAC also applies to CustomResources, providing administrators control over who can interact with each Operator.

Creating instances of Memcached in this namespace will now trigger the Memcached Operator to instantiate pods running the memcached server that are managed by the Operator. The more CustomResources you create, the more unique instances of Memcached are managed by the Memcached Operator running in this namespace.

$ cat <<EOF | oc apply -f -
apiVersion: "cache.example.com/v1alpha1"
kind: "Memcached"
metadata:
  name: "memcached-for-wordpress"
spec:
  size: 1
EOF

$ cat <<EOF | oc apply -f -
apiVersion: "cache.example.com/v1alpha1"
kind: "Memcached"
metadata:
  name: "memcached-for-drupal"
spec:
  size: 1
EOF

$ oc get Memcached
NAME                      AGE
memcached-for-drupal      22s
memcached-for-wordpress   27s

$ oc get pods
NAME                                       READY     STATUS    RESTARTS   AGE
memcached-app-operator-66b5777b79-pnsfj    1/1       Running   0          14m
memcached-for-drupal-5476487c46-qbd66      1/1       Running   0          3s
memcached-for-wordpress-65b75fd8c9-7b9x7   1/1       Running   0          8s

Update an application.

Manually apply an update to the Operator by creating a new Operator manifest with a replaces field that references the old Operator manifest. The OLM ensures that all resources being managed by the old Operator have their ownership moved to the new Operator without fear of any programs stopping execution. It is up to the Operators themselves to execute any data migrations required to upgrade resources to run under a new version of the Operator.

The following command demonstrates applying a new Operator manifest file using a new version of the Operator and shows that the pods remain executing:
```
$ curl -Lo memcachedoperator.0.0.2.csv.yaml https://raw.githubusercontent.com/operator-framework/getting-started/master/memcachedoperator.0.0.2.csv.yaml
$ oc apply -f memcachedoperator.0.0.2.csv.yaml
$ oc get pods
NAME                                       READY     STATUS    RESTARTS   AGE
memcached-app-operator-66b5777b79-pnsfj    1/1       Running   0          3s
memcached-for-drupal-5476487c46-qbd66      1/1       Running   0          14m
memcached-for-wordpress-65b75fd8c9-7b9x7   1/1       Running   0          14m
```

Additional resources

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

1	Specify a description for the CRD.
2	Specify a display name for the CRD.