World K2: Trays & Orders

ReplicaSets keep the count

You will learn: how a ReplicaSet keeps the right number of pods, and how it counts them.

Checked with Kubernetes 1.37 on 31 Oct 2026

The problem

Lunch time. You want 3 trays out, always. One tray slips and falls. Who notices, and who makes a new one?

Watch

Want3written on the order for trays

See33 ready

Do All goodGaps fixed: 0

dev-workerOpen

trays-7tmqp
trays-nzvvz

dev-worker2Open

trays-cg4fl

A counts trays, again and again. Want 3, see 2? It makes 1 more. Tap delete on a tray and watch the board.

Try it

A ReplicaSet has 3 parts: how many (replicas), which trays count (selector), how to make one (template). Here the labels do not match. Fix it.

Fix the manifest, then press Apply.

trays.yaml18 lines

Try it

The rush is coming. Ask the ReplicaSet for 5 trays.

You are here: cluster kind-dev namespace default

dev-control-planeOpenManager

No trays here.

dev-workerOpen

dev-worker2Open

Time: +0s (sped up)

Hint

kubectl scale rs trays --replicas=5 changes the count on the order.

Predict

The ReplicaSet wants 3. You start one more pod by hand, with the label app=tray. What happens?

Check

You are here: cluster kind-dev namespace default

~/shop $ kubectl run extra --image=nginx:1.29-alpine --labels=app=traypod/extra created~/shop $ kubectl get podsNAME          READY   STATUS    RESTARTS   AGEtrays-7tmqp   1/1     Running   0          16strays-cg4fl   1/1     Running   0          16strays-nzvvz   1/1     Running   0          16s

extra was created, then deleted at once. It matched app=tray, so it made 4. Labels decide what counts.

You did it!

You can now explain how a ReplicaSet keeps the count.

Go deeper (optional)

You will rarely write a ReplicaSet yourself. A Deployment makes them for you, and adds safe updates. That is the next lesson.

Real rep (optional): on a throwaway cluster, apply a ReplicaSet, delete one pod, then run kubectl describe rs trays. Its Events show each new pod.