Add Kubetnetes domain specific database post

content/posts/2022-09-22-kubernetes-is-a-domain-specific-database.md

@@ -0,0 +1,41 @@
+---
+title: Kubernetes is a domain specific database
+date: "2022-09-22"
+tags: "note, 100DaysToOffload, infrastructure, kubernetes"
+---
+
+I just finished listening to [an
+episode](https://kubernetespodcast.com/episode/129-linkerd/) of the Kubernetes
+podcast. In it, Thomas Rampelberg makes an analogy that I think is worth
+sharing:
+
+> "[...] Kubernetes is really a domain-specific database. And you need to look at it
+that way. The YAML is literally writing a select statement or an insert
+statement for a database. That's what the YAML is. And it's awesome that it is
+already configured for how it is. And it's awesome that it's got a schema. But
+the YAML is you writing an insert statement into Kubernetes. [...]"
+
+The Kubernetes API abstracts two types of states: desired state and actual
+state. Whenever you apply a manifest, you update the *desired state* of the
+cluster, just like you do in a regular, non domain-specific database like
+PostgreSQL or Redis. Kubernetes then frequently compares the desired state with
+the *actual* state of the cluster. If they don't match, Kubernetes will do
+whatever it does to match these two states. Usually, this data is persisted
+using a key-value database like [etcd](https://etcd.io/) running in a cluster,
+though one could theoretically also hook up an external MySQL or Postgres
+database for this purpose.
+
+I found this great diagram by [Tim
+Downey](https://downey.io/blog/desired-state-vs-actual-state-in-kubernetes/),
+showing an oversimplified analogy of this pattern:
+
+![Thermostat
+Example](/assets/posts/2022-09-22-kubernetes-is-a-domain-specific-database/desired-state-hvac-diagram.png)
+
+You *insert* your desired state into the system, and the system adjusts the
+actual state to match the desired state. In the case of thermostats the state is
+a temperature. In Kubernetes, it's [resource
+objects](https://kubernetes.io/docs/concepts/overview/working-with-objects/kubernetes-objects/)
+
+This is post 037 of [#100DaysToOffload](https://100daystooffload.com/).
+