5 · Scale to zero

Watch ACME sleep, then wake it — and understand what the first request actually paid for.

This is the chapter worth doing slowly. Everything else is deployment mechanics; this is the behaviour you came for.

Watch it sleep

Leave ACME alone and watch its pods:

shell

kubectl get pods -l serving.knative.dev/service=acme -w

Give it about 90 seconds. Knative waits out a 60-second stable window, then a 30-second grace period, before terminating the pod. People watching for "about a minute" conclude it is broken and stop — this is the single most common false alarm.

Then the list empties. No pod. No container compute for ACME.

Your nodes keep running and billing. Scale-to-zero reclaims the pod, not the machine. On a managed cluster the nodes only go away if a cluster autoscaler independently reclaims them. The saving is real but it is per-app, not per-cluster.

Wake it

In another terminal:

shell

curl -sSf <the-URL-status-printed>/ > /dev/null && echo "awake"

Watch the pod reappear in the first terminal.

What happened: Knative's activator held your request, started a pod, waited for the readiness probe, and forwarded the request. You got a slower response rather than an error.

It is not a guarantee. A cold wake can still return 503 if the revision or the readiness probe does not come up in time — and the -sSf above will exit non-zero if that happens. Knowing that in advance is better than discovering it under load.

What the first request actually cost

That request paid a cold start, and how long depends on your cluster, node sizing, whether the image is already cached on the node, and the app itself.

knext publishes measured distributions rather than a headline number, because on a contended cluster the spread is wide enough that a median misleads. One measured example: on a two-node cluster, cold starts came out bimodal — a cluster of samples around ~2.5s and another around ~10.5s, with a clean gap between them. A single "average" of those two groups would describe neither.

See Scale to zero for the numbers and how they were taken.

The knob, and when to turn it

If cold starts matter more than idle cost for ACME:

kn-next.config.ts

scaling: { minScale: 1, maxScale: 10 }

One pod stays warm. No cold start, and you pay for it continuously. That is the whole trade, and it is one line either way.

There is a middle setting worth knowing about: keeping a warm floor only during hours you expect traffic. That is a scheduling question rather than a config flag — see Scale to zero.

Check your understanding

Why did the pod take ~90 seconds to disappear rather than immediately?
If ACME gets one request per hour, how many pods are running most of the time?
What would minScale: 1 change about the answer to the previous question — and about the bill?

Chapter 6: Add a database →