World K2: Trays & Orders

StatefulSets: names that stay

You will learn: how a StatefulSet gives pods names that stay, started in order.

Checked with Kubernetes 1.37 on 31 Oct 2026

The problem

Your database runs 3 copies. Copy 0 is the boss; the others follow it. After a crash, the boss must come back as the boss. Random names make that hard. What do you need?

Watch

You are here: cluster kind-dev namespace default

~/shop $ kubectl apply -f web.yamlservice/web createdstatefulset.apps/web created~/shop $ kubectl rollout status statefulset/web --timeout=180sWaiting for 3 pods to be ready...Waiting for 2 pods to be ready...Waiting for 1 pods to be ready...partitioned roll out complete: 3 new pods have been updated...~/shop $ kubectl get podsNAME    READY   STATUS    RESTARTS   AGEweb-0   1/1     Running   0          9sweb-1   1/1     Running   0          5sweb-2   1/1     Running   0          1s

A gives each tray a number that stays. The trays start in order: web-0 first, then web-1, then web-2.

Try it

How does web start? Put the steps in order.

First step at the top. Put these in order. Use the arrows, or drag.

web-0 starts
web-1 starts
web-0 is Ready
web-2 starts
web-1 is Ready

Predict

You delete web-1. What comes back?

Check

You are here: cluster kind-dev namespace default

~/shop $ kubectl delete pod web-1pod "web-1" deleted from default namespace~/shop $ kubectl wait --for=condition=Ready pod/web-1 --timeout=90spod/web-1 condition met~/shop $ kubectl get podsNAME    READY   STATUS    RESTARTS   AGEweb-0   1/1     Running   0          24sweb-1   1/1     Running   0          1sweb-2   1/1     Running   0          16s

Look at AGE. web-1 is brand new, but it kept its name. Same seat, new tray.

Try it

Your turn. Add a fourth copy. Guess its name before you look.

You are here: cluster kind-dev namespace default

web3/3 ready

dev-control-planeOpenManager

No trays here.

dev-workerOpen

web-0
web-2

dev-worker2Open

web-1

Time: +0s (sped up)

Hint

kubectl scale statefulset web --replicas=4, then kubectl get pods.

Check

Deployment or StatefulSet? Sort each one.

Drag a card into a box. Or tap the card, then tap the box.

You did it!

You can now run pods with names that stay, in order.

Go deeper (optional)

A StatefulSet needs a headless Service for steady addresses like web-0.web (World K3). It can give each tray its own storage with volumeClaimTemplates (World K4). Running a database yourself is real work. Many teams use a managed one.

Real rep (optional): on a throwaway cluster, scale web down to 1. Watch which pods leave first.