An Overview Of Resources and Controllers.

We, as devs, can create definition files for an object, say a deployment. The deployment is a resource.
We, as devs, run kubectl create to tell kubectl to talk to the kubeapi-server to make the resource.
K8s creates the deployment and stored data about it in the etcd datastore.
We, as devs, can kuse kubectl against the kubeapi-server to do things like create, get, and delete the deployment, which will adjust the etcd data.
A controller in K8s does the work of monitoring the etcd data store, and updates the "real" cluster objects (deployments, pods, etc).

• An Overview Of Resources and Controllers.
  • An Example Mock Custom Resource, A Vacation Agreement Resource
    • Tell K8s To Learn A New Resource Type
    • Create One Of These Custom Resources
    • Create A Custom Controller To Work With The Resource
    • Use The Operator Framework to Deploy All Custom Resources In One

An Example Mock Custom Resource, A Vacation Agreement Resource

Tell Kubernetes that we want to make a new object type, a VacationSearch object through a new definition file, a custom resource definition (crd):

Tell K8s To Learn A New Resource Type

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: VacationSearch.vacations.com
spec:
  # this will be scoped to a namespace
  scope: Namespaced
  # matches the apiVersion in the VacationSearch object definition file, below
  group: vacations.com
  names:
    kind: VacationSearch
    singular: VacationSearch
    plural: VacationSearches
    shortNames:
      - vaca
  versions:
    - name: v1
      served: true
      storage: true
  # all the fields allowed in the object def spec section
  schema:
    openAPIV3Schema:
      type: object
      properties:
        spec:
          type: opbect
          properties:
            where:
              type: string
              required: true
            nightlyMaxRate:
              type: integer
            ammenities:
              type: ...array...?!
            nights:
              type: number
              required: true

Create One Of These Custom Resources

Here, a vaca-agreement resource to work with through this custom resource work.

apiVersion: vacationSearch.com/v1
kind: VacationSearch
metadata:
  name: cali-trip
spec:
  where: SanFrancisco
  nightlyMaxRate: 150
  ammenities: ['wifi', 'dishwasher']
  nights: 14

Say this resource type is allowed in K8s.
Say we can crud this through typical kubectl commands.
The ETCD Datastore would be updated with info about the resource by using kubectl <action> commands.
Say we also create a VacationSearchController.
This controller, this custom controller would...

• be written in something like nodejs
• watch VacationSearch resources in etcd
• would make api calls to rental sites, with params

Create A Custom Controller To Work With The Resource

The Controller will...

• monitor the etcd for VacationSearch objects, monitoring in a loop
• perform actions based on the VacationSearch objects
• be allowed to be in any code - python, node, go... K8s has a go client, and several other "supported" libraries

One example of a sample-controller is in the kubernetes github repo. To build this sample controller,

• install go on my machine
• clone the github repo with the sample controller
• cd into it
• customize controller.go to do the work!
• build the code with go, go build -o sample-controller .
• run the code! ./sample-controller -kubeconfig=$HOME/.kube/config
  • this tells k8s to use this new controller :)

This controller can be packed for distribution, by something like containerizing it.
The controller can even be deployed as a pod in the k8s world. Crazy!

Use The Operator Framework to Deploy All Custom Resources In One

The Operator Framework can be used to deply the custom resource definition file and the custom controller all-in-one. The Operator Framework is intended to do what human operators do.
There are many operators already built and available at OperatorHub.
Operators are complex: there's a lot going on - it is intended to do automation of automation.

The Operator Framework can do things like...

• Deal With Custom Resource Definitions
  • i.e the etcdCluster
  • etcdBackup - take backups
  • etcdRestore - restore backups
• Deal With Custom Controllers
  • i.e the etcdController
  • backup operator
  • restoreOperator