Step 01 of 06
Learn the concept
Beacon asks the internet whether sites are healthy. Now the cluster asks Beacon the same kind of question. The punchline is that the cluster's question can kill the process, so it had better be a narrow one.
The ideas this is made of
A probe is the same idea as Course 1
Course 1 built a prober: ask an endpoint a question, classify the answer and record timing. Kubernetes probes do that inside the cluster. The kubelet asks /healthz or /readyz on a schedule. It does not understand your business; it only sees success, failure and thresholds. A probe is a small monitor with authority to change routing or restart a container.
Readiness removes traffic without killing the process
A readiness probe answers: should this Pod receive Service traffic now? If it fails, Kubernetes removes the Pod from Service endpoints. The container keeps running. That is perfect for dependency warmups, migrations or backpressure. It is not a punishment; it is a traffic gate. A failing readiness probe often appears as empty or reduced EndpointSlices.
Liveness restarts the container, so make it narrow
A liveness probe answers: is this process wedged beyond self-repair? If it fails enough times, kubelet restarts the container. A catastrophic beginner mistake is checking a database or external API in liveness. When the dependency fails, Kubernetes kills every otherwise healthy app instance, creating a restart storm on top of the outage.
Startup gives slow programs a grace period
A startup probe disables liveness and readiness checks until startup succeeds or its failure threshold is exhausted. It is useful for programs that need migrations, cache loading or slow first boot. Without it, an impatient liveness probe can kill the process before it has ever had a fair chance to become alive.
lights on: process alive
front door open: ready
safe jammed shut: liveness failure
coffee supplier late: not livenessA late supplier should not make the owner demolish the shop. That is what a liveness probe against a dependency does.
Liveness, readiness and startup
| Probe | Failure action | Good question |
|---|---|---|
startup | Keep waiting, then kill | Has boot completed? |
readiness | Remove from endpoints | Can I serve now? |
liveness | Restart container | Is the process stuck? |
dependency check | Usually readiness | Can required dependency be used? |
external internet | Never liveness | Outages are not deadlocks |
What these are called on the job
kubelet — The node agent that runs containers and executes probes for Pods on that node.
Liveness probe — A check whose repeated failure restarts the container.
Readiness probe — A check whose failure removes the Pod from Service endpoints.
Startup probe — A check that delays other probes while a container is still starting.