Liveness probe concept in Kubernetes - Step-by-Step Execution

Process Flow - Liveness probe concept

Pod starts running

↓

Kubelet sends liveness probe

↓

Probe checks container health

↓

Healthy?

Yes→Wait for next probe

No↓

Kubelet kills container

↓

Container restarts

↩Back to sending liveness probe

The liveness probe checks if a container is alive. If not, Kubernetes restarts it to keep the app healthy.

Execution Sample

Kubernetes

livenessProbe:
  httpGet:
    path: /healthz
    port: 8080
  initialDelaySeconds: 5
  periodSeconds: 10

This YAML config tells Kubernetes to check the /healthz URL on port 8080 every 10 seconds after waiting 5 seconds initially.

Process Table

Step	Action	Probe Result	Kubelet Decision	Container State
1	Pod starts, wait 5 seconds	No probe yet	Wait	Running
2	Send liveness probe (GET /healthz)	Success (200 OK)	Do nothing	Running
3	Wait 10 seconds	No probe	Wait	Running
4	Send liveness probe (GET /healthz)	Failure (timeout)	Kill container	Terminating
5	Container restarts	No probe yet	Wait	Starting
6	Send liveness probe (GET /healthz)	Success (200 OK)	Do nothing	Running

💡 Container is healthy again, so liveness probes continue periodically.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	After Step 6
Container State	Running	Running	Terminating	Starting	Running
Probe Result	None	Success	Failure	None	Success

Key Moments - 3 Insights

Why does Kubernetes kill the container after a failed liveness probe?

Because the liveness probe failure means the container is unhealthy or stuck, so Kubernetes restarts it to fix the problem (see execution_table step 4).

What happens during the initialDelaySeconds period?

Kubernetes waits before sending the first liveness probe to give the container time to start up (see execution_table step 1).

Does a successful liveness probe restart the container?

No, a successful probe means the container is healthy, so Kubernetes does nothing and continues monitoring (see execution_table step 2 and 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the container state after step 4?

A Running

B Terminating

C Starting

D Stopped

💡 Hint

Check the 'Container State' column at step 4 in the execution_table.

At which step does Kubernetes restart the container?

A Step 4

B Step 3

C Step 5

D Step 2

💡 Hint

Look for the step where the container state changes to 'Starting' in the execution_table.

If initialDelaySeconds was set to 0, what would change in the execution table?

A Step 1 would send a probe immediately

B Container would never restart

C Probe results would always be failure

D Container state would start as Terminating

💡 Hint

Refer to the description of initialDelaySeconds and step 1 in the execution_table.

Concept Snapshot

Liveness Probe in Kubernetes:
- Checks if container is alive
- Configured with httpGet, tcpSocket, or exec
- initialDelaySeconds waits before first check
- periodSeconds sets check frequency
- Failure causes container restart
- Keeps app healthy automatically

Full Transcript

A liveness probe is a health check Kubernetes uses to see if a container is alive. When a pod starts, Kubernetes waits for initialDelaySeconds before sending the first probe. It then sends probes every periodSeconds. If the probe succeeds, Kubernetes does nothing and waits for the next check. If the probe fails, Kubernetes kills the container and restarts it. This cycle repeats to keep the application running smoothly. The YAML example shows a simple HTTP GET probe on /healthz at port 8080. The execution table traces the container state and probe results step-by-step, showing how Kubernetes reacts to success and failure. Key points include the wait before the first probe, the restart on failure, and no action on success. This helps beginners understand how liveness probes maintain container health automatically.