Step 01 of 06
Learn the concept
A container image in a registry is an artifact, not a service. It does not restart itself, find a machine, receive traffic or explain why it failed. Kubernetes is the machinery that tries to make those promises true — and charges you in nouns.
The ideas this is made of
The API server is the source of truth
In Kubernetes, the object stored in the API server is the contract. A Deployment, Service or ConfigMap is not a script that runs once; it is desired state that remains available for every controller to read. kubectl apply writes that desired state. The API server validates it, stores it in etcd, and later records status reported by controllers and nodes.
Controllers make declarations true
A controller is a loop: list objects, watch for changes, compare desired state with observed state, then act. The Deployment controller creates ReplicaSets. The ReplicaSet controller creates Pods. The endpoint controller updates Service backends. This is reconciliation. It is deliberately boring, which is exactly what you want after midnight.
Declarative beats imperative when humans leave
An imperative command says, run this container now. A declarative object says, there should be three healthy copies of this container. The second form survives restarts, failed nodes and missed terminals. It also costs you ceremony: YAML, labels, status fields and controllers you must learn to interrogate.
Orchestration trades simplicity for repair
A single docker run is easier to understand than a cluster. It also has one process, one machine and one tired owner. Kubernetes buys placement, rollout, restart, service discovery and policy. The price is a new failure surface: unschedulable pods, broken selectors, bad probes, stale configuration and RBAC denial.
desired: 21 C
observed: 18 C
action: heat
observed: 21 C
action: stop heatingNobody holds a switch until the room is warm. The thermostat keeps comparing measured temperature with desired temperature. Kubernetes does the same with pods, endpoints and volumes, except the room sometimes says ImagePullBackOff.
Imperative and declarative operations
| Question | Imperative | Declarative |
|---|---|---|
What you say | Start this now | Keep this true |
Common tool |
|
|
After failure | Human repeats it | Controller repairs it |
Stored where | Shell history, maybe | Kubernetes API |
Debug focus | The command output | Object status and events |
What these are called on the job
Desired state — The object spec you submit to the Kubernetes API.
Observed state — What controllers and nodes report is actually happening.
Reconciliation — Repeatedly closing the gap between desired and observed state.
Controller — A process that watches the API and takes action for one kind of object.