# Kubebuilder

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### Content

- CRDs
- Controller / Operator
- Kubebuilder
- Live Coding
- Real Life Use Cases

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## CRDs

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## Controller

> A **control loop** that watches the state [...] and makes changes attempting to move the **current state towards the desired state**.

- [Kubernetes Glossary](https://kubernetes.io/docs/reference/glossary/?fundamental=true#term-controller)

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## Controller

- integrated in `kube-apiserver` or `kube-controller-manager`
- watch builtin k8s objects (`core`, `apps`, `batch`)
- logic ontop of data
- create sub-resources

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## Controller

### deployment controller

- watches `Deployments`
- creates and deletes `ReplicaSets`
- scales `ReplicaSets` for rolling deployments

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## Controller

### kubelet

- watches `PodSpecs`
- starts/stops containers

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## Operator

- term coined by CoreOS
- same as controller but for CRDs
- integrate operational knowledge
- provide usability features
  - updates
  - scaling
  - backups
  - integration with k8s

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## Kubebuilder

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## Live Coding

Check out the source at  
https://github.com/apricote/kubebuilder-poke-sync

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## Real Life Use Cases

### Databases

- scaling
- configuration
- disaster recovery
- e.g. [zalando/postgres-operator](https://github.com/zalando/postgres-operator)
- e.g. [mongodb-enterprise-kubernetes](https://github.com/mongodb/mongodb-enterprise-kubernetes)

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## Real Life Use Cases

### Provisioning external resources

- call external APIs
- expose external state in cluster
- e.g. [jetstack/cert-manager](https://github.com/jetstack/cert-manager)
- e.g. [awslabs/aws-service-operator](https://github.com/awslabs/aws-service-operator)

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## Real Life Use Cases

### Application Lifecycle Managment

- deployment
- supervised updates
- health checking + alerts
- Helm Chart++
- e.g. [gitlab-operator](https://gitlab.com/charts/components/gitlab-operator)
- e.g. [Tenant Operator](https://blog.kolide.com/using-a-kubernetes-operator-to-manage-tenancy-in-a-b2b-saas-app-250f1c9416ce)