Chapter 1: The big picture: who does what

The big picture: who does what

Before the details, meet the crew behind the scenes. A has two halves: the control plane, an admiralty on the shore that gives the orders, and the Nodes, the ships that carry them out.

On the shore: the control plane

API server · the harbor master’s office

The only door into the cluster. Every command you type with kubectl arrives here, gets checked, and is written down.

etcd · the logbook

The single record of what you want: every Deployment, Service and setting. If it isn’t in the logbook, Kubernetes doesn’t know about it.

Scheduler · the helmsman

Picks a ship for every new Pod, based on room on board and any rules you set.

Controller manager · the clerks

Endlessly compare the logbook with reality and fix any difference, like replacing a missing Pod.

On every ship: the node parts

kubelet · the bosun aboard each ship

Lives on each ship, starts the Pods it was given, restarts a container that crashes, and keeps checking on them (more in chapter 6).

Container runtime · the deckhands

Actually pulls container images and runs them, for example containerd.

kube-proxy · the signalman

Sets up each ship’s networking so traffic for a Service reaches its Pods.

What happens when you type one command

You run kubectl create deployment shop --image=shop:1.0 --replicas=3. kubectl sends it to the API server.

The API server checks you're allowed, then writes the Deployment into etcd. That's all your command does: it records a wish.

A controller notices the new Deployment and creates a ReplicaSet, which creates 3 Pod records. They have no ship yet.

The scheduler spots Pods without a ship and assigns each one to a Node.

Each Node's kubelet sees Pods assigned to it and has the runtime start the containers. Then it reports back: Running.

Notice nobody shouts orders directly at a ship. Everyone reads and writes the logbook through the API server, and everyone keeps fixing differences on their own. That's why Kubernetes heals itself: there's always a clerk comparing want against have.