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The Ptemplate-operator

  1. Introduction

  2. The Ptemplate-operator example

  3. Important Files

    3.1. api/v1alpha1/ptemplate_types.go

    3.2. controllers/ptemplate_controller.go

    3.3. config/rbac/role.yaml

    3.4. Makefile

  4. Installing

    4.1. Building And Deploying

    4.2. Creating An Instance

  5. Uninstalling

    5.1. Uninstall CRDs

    5.2. Undeploy controller

  6. Links

Introduction

An Operator is an extension to Kubernetes and OpenShift that adds a Custom Resource type that provides higher level functionality.

The Ptemplate-operator example

Our example ptemplate-operator implements the Ptemplate resource type (or Kind). When a resource of this Kind is created Kubernetes notifies the ptemplate-operator instance which runs its Reconcile() function which then creates a number of other resources needed to run our example. It does this by creating a Module resource that KMM picks up and processes, and a DaemonSet resource for or consumer pods that is picked up by core Kubernetes

e.g (See here)

apiVersion: ptemplates.pt.example.com/v1alpha
kind: Ptemplate
metadata:
  name: consumer-pod
spec:
  maxdev: 5
  defaultmsg: ptemplate
  consumerimage: quay.io/chrisp262/pt-device-plugin:consumer-latest
  requiredDevices: 2
  imageRepoSecret:
    name: pt-char-pull-secret
  selector:
    node-role.kubernetes.io/worker: worker

Implementing an operator requires a large amount of boilerplate code. The easiest way to create this code is to use the operator-sdk. It's QuickStart and tutorial walks you through generating the boilerplate code for the operator and adding an API to it (which defines the yaml structure used to create the resource instance).

Important Files

Once its generated all the code, a minimum the following files need customising to implement the functionality that this Operator requires.

api/v1alpha1/ptemplate_types.go

This contains the data types that define the Custom Resources specification as a series of nested golang structures. Each field has a json: tag that is used to define the associated field in the yaml (or json) used to create the resource e.g.

ConsumerImage   string                   `json:"consumer"`

means that if a consumer field is defined in the yaml that value is used to populate the ConsumerImage field in the relevant golang structure

controllers/ptemplate_controller.go

The heart of the operator is the Reconcile() function in this file which implements the actual functionality.

This function is run whenever an object of a type owned by the operator (listed in the SetupWithManager() function), so it needs to be idempotent. In the ptemplate-operator we own a DaemonSet so reconcile is run for each ptemplate resource, whenever any DaemonSet in the cluster is changed. It needs to get the configuration for the relevant ptemplate resource, check the sub-resources that should exist (in this case a KMM Module, and a DaemonSet) and reconcile them, creating them if they do not exist, correcting them if they differ from the ptemplate.Spec , update the ptemplate.Status if needed, and most importantly of all, do nothing if nothing has changed.

For production purposes it would make sense to break this file into separate files for the Module and consumer DaemonSet, remove some of the logging, handle errors better, and generally do all the things that turn example code into production quality. But that would also reduce its clarity!

config/rbac/role.yaml

By default the operator only has permissions on its own CRD, to be able to access any other resource in the cluster (e.g. to list, and create DaemonSets or Pods) permissions need to be added to this file. These then get deployed as part of the operator-manager-role e.g.

kubectl get clusterrole ptemplate-operator-manager-role

If the operator logs show permissions errors this is probably the file that needs changing.

Makefile

The variable IMAGE_TAG_BASE was set to have the required image name and registry, for convenience.

Installing

Building And Deploying

Building the operator is as easy as running make build this will also regenerate any of the boilerplate code to reflect changes in the API (api/v1alpha1/ptemplate_types.go).

Once the code is building successfully you can build the operator image running the docker-build target

make docker-build

Will build the code and create a docker image with it named for the IMAGE_TAG_BASE variable, which can then be pushed to a registry.

make deploy

Will deploy the image as a pod. It will also create all the other required resource such as the CRD itself, serviceaccounts, clusterroles and rolebindings etc. These are initially created by operator-sdk, but can be changed if needed.

(During testing I had a strange error where I was writing to the ptemplate.status.consumers field to report the status but it wasn't getting changed, and no errors were being thrown, eventually I realised I had updated the CRD, and the config/crd/bases/pt.example.com_ptemplates.yaml file had been updated by Make, but I hadn't re-run make deploy so Kubernetes wasn't aware of my changes. Once I'd done that everything worked correctly)

Creating An Instance

Once the operator is installed and running successfully you can create a ptemplate resource from the example yaml

kubectl apply -f example.yaml

This should create the module resource and a DaemonSet for the consumer in the current namespace. The next time the kmm operator reconciles it will see the Module and load the kmod.

Any errors during the resource creation process can be seen in the controllers log. The controller is a pod with a name that starts ptemplate-operator-controller-manager- in the ptemplate-operator-system namespace:

e.g.

kubectl logs -n ptemplate-operator-system ptemplate-operator-controller-manager-5d4795f966-4cb98

Uninstalling

Uninstall CRDs

To delete the CRDs from the cluster:

make uninstall

Undeploy controller

Undeploy the controller from the cluster:

make undeploy

Links