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When there are multiple reference-style markdown links in the same deck with the same label, they will silently clash - i.e. one will overwrite the other. The problem can become very apparent when using many links like [see the docs][docs] in different slides, where [docs] points to a different URL each time. This commit adds a crude script to detect such duplicates and display them. This script was used to detect a bunch of duplicates and fix them (by making the label unique). There are still a few duplicates left but they point to the same places, so we decided to leave them as-is for now (but might change that later).
135 lines
3.8 KiB
Markdown
135 lines
3.8 KiB
Markdown
# Operators
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The Kubernetes documentation describes the [Operator pattern] as follows:
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*Operators are software extensions to Kubernetes that make use of custom resources to manage applications and their components. Operators follow Kubernetes principles, notably the control loop.*
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Another good definition from [CoreOS](https://coreos.com/blog/introducing-operators.html):
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*An operator represents **human operational knowledge in software,**
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<br/>
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to reliably manage an application.*
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There are many different use cases spanning different domains; but the general idea is:
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*Manage some resources (that reside inside our outside the cluster),
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<br/>
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using Kubernetes manifests and tooling.*
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[Operator pattern]: https://kubernetes.io/docs/concepts/extend-kubernetes/operator/
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---
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## Some uses cases
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- Managing external resources ([AWS], [GCP], [KubeVirt]...)
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- Setting up database replication or distributed systems
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<br/>
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(Cassandra, Consul, CouchDB, ElasticSearch, etcd, Kafka, MongoDB, MySQL, PostgreSQL, RabbitMQ, Redis, ZooKeeper...)
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- Running and configuring CI/CD
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<br/>
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([ArgoCD], [Flux]), backups ([Velero]), policies ([Gatekeeper], [Kyverno])...
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- Automating management of certificates and secrets
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<br/>
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([cert-manager]), secrets ([External Secrets Operator], [Sealed Secrets]...)
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- Configuration of cluster components ([Istio], [Prometheus])
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- etc.
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[ArgoCD]: https://argoproj.github.io/cd/
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[AWS]: https://aws-controllers-k8s.github.io/community/docs/community/services/
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[cert-manager]: https://cert-manager.io/
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[External Secrets Operator]: https://external-secrets.io/
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[Flux]: https://fluxcd.io/
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[Gatekeeper]: https://open-policy-agent.github.io/gatekeeper/website/docs/
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[GCP]: https://github.com/paulczar/gcp-cloud-compute-operator
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[Istio]: https://istio.io/latest/docs/setup/install/operator/
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[KubeVirt]: https://kubevirt.io/
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[Kyverno]: https://kyverno.io/
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[Prometheus]: https://prometheus-operator.dev/
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[Sealed Secrets]: https://github.com/bitnami-labs/sealed-secrets
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[Velero]: https://velero.io/
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---
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## What are they made from?
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- Operators combine two things:
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- Custom Resource Definitions
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- controller code watching the corresponding resources and acting upon them
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- A given operator can define one or multiple CRDs
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- The controller code (control loop) typically runs within the cluster
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(running as a Deployment with 1 replica is a common scenario)
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- But it could also run elsewhere
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(nothing mandates that the code run on the cluster, as long as it has API access)
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---
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## Operators for e.g. replicated databases
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- Kubernetes gives us Deployments, StatefulSets, Services ...
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- These mechanisms give us building blocks to deploy applications
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- They work great for services that are made of *N* identical containers
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(like stateless ones)
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- They also work great for some stateful applications like Consul, etcd ...
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(with the help of highly persistent volumes)
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- They're not enough for complex services:
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- where different containers have different roles
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- where extra steps have to be taken when scaling or replacing containers
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---
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## How operators work
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- An operator creates one or more CRDs
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(i.e., it creates new "Kinds" of resources on our cluster)
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- The operator also runs a *controller* that will watch its resources
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- Each time we create/update/delete a resource, the controller is notified
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(we could write our own cheap controller with `kubectl get --watch`)
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---
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## Operators are not magic
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- Look at this ElasticSearch resource definition:
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@@LINK[k8s/eck-elasticsearch.yaml]
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- What should happen if we flip the TLS flag? Twice?
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- What should happen if we add another group of nodes?
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- What if we want different images or parameters for the different nodes?
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*Operators can be very powerful.
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<br/>
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But we need to know exactly the scenarios that they can handle.*
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???
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:EN:- Kubernetes operators
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:FR:- Les opérateurs
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