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Operator overview

Kubernetes/Openshift Operators are software extensions They automate the management of complex stateful applications

They are meant to encapsulate the operational knowledge required to run these applications

Fundamental components

Operators rely on 3 primary components:

Custom Resource Definitions (CRDs)

CRDs extend the Kubernetes API by introducing new resource types specific to the app.

They define the schema and structure for the custom resource, which can be managed with kubectl

Custom Controllers

A piece of software that monitors and manages the state of custom resources

It implements the control loop, that continuously reconciles the desired state with the state of the system

They can perform a number of tasks such as:

  • creating
  • updating
  • deleting
  • managing upgrades
  • health checks
  • etc

Operator Lifecycle Manager (OLM) -> Optional

A kubernetes extension that helps manage the lifecycle of Operators.

That may include:

  • Installation
  • Upgrade
  • Uninstallation

The fundamental components are outlined in 3 specific locations in the kubebuilder file structure

.
├── api        <-- This directory contains the API definition for your custom resources
│   └── v1alpha1
│       ├── groupversion_info.go
│       ├── testresource_types.go   <-- 
│       └── zz_generated.deepcopy.go
├── bin
│   └── controller-gen
├── config
│   ├── crd    <-- This directory contains the CRD manifests generated from API definition rules
│   │   ├── bases
│   │   │   └── testing.test.com_testresources.yaml  <-- Generated CRD Manifest
│   │   ├── kustomization.yaml
│   │   ├── kustomizeconfig.yaml
│   │   └── patches
│   │       ├── cainjection_in_testresources.yaml
│   │       └── webhook_in_testresources.yaml
│   ├── default
│   │   ├── kustomization.yaml
│   │   ├── manager_auth_proxy_patch.yaml
│   │   └── manager_config_patch.yaml
│   ├── manager
│   │   ├── kustomization.yaml
│   │   └── manager.yaml
│   ├── prometheus
│   │   ├── kustomization.yaml
│   │   └── monitor.yaml
│   ├── rbac
│   │   ├── auth_proxy_client_clusterrole.yaml
│   │   ├── auth_proxy_role_binding.yaml
│   │   ├── auth_proxy_role.yaml
│   │   ├── auth_proxy_service.yaml
│   │   ├── kustomization.yaml
│   │   ├── leader_election_role_binding.yaml
│   │   ├── leader_election_role.yaml
│   │   ├── role_binding.yaml
│   │   ├── role.yaml
│   │   ├── service_account.yaml
│   │   ├── testresource_editor_role.yaml
│   │   └── testresource_viewer_role.yaml
│   └── samples
│       └── testing_v1alpha1_testresource.yaml
├── controllers    <-- This directory contains the controller implementation
│   ├── suite_test.go
│   └── testresource_controller.go  <-- Implements the custom controller + reconcile logic
├── Dockerfile
├── go.mod
├── go.sum
├── hack
│   └── boilerplate.go.txt
├── main.go
├── Makefile
├── PROJECT
└── README.md

Libraries

Go will be using 3rd party in order to operate

sigs.k8s.io/controller-runtime

This is the primary library used with kubebuilder.

It provides a higher-level API than client-go, contains tools and abstractions for building custom controllers

sigs.k8s.io/controller-runtime/pkg/client

Provides high level client for interacting with the kubernetes API

Including:

  • creating resources
  • updating resources
  • deleting resources

sigs.k8s.io/controller-runtime/pkg/controller

Offers tools for building and managing controllers

sigs.k8s.io/pkg/reconcile

Contains the reconciler interface, in which the reconcile logic is implemented

sigs.k8s.io/controller-runtime/pkg/log

Contains logging utils built on top of the logr interface

k8s.io/apimachinery

Contains various packages packages for working with the Kubernetes API

This includes:

  • types
  • serializers
  • utility functions

k8s.io/apimachinery/pkg/apis/meta/v1

Contains the API metadata types

  • ObjectMeta
  • TypeMeta
  • Status

k8s.io/apimachinery/pkg/runtime

Provides the runtime infra for working with the Kubernetes API

k8s.io/api

This library contains the typed Go structs for Kubernetes objects

  • Deployments
  • Services
  • ConfigMaps
  • etc

k8s.io/client-go

Not used too often in favor of the controller runtime client.

Still encountered in some cases.

go-logr/logr

The logging interface for Go