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

Process Flow - Readiness probe concept

Pod starts

↓

Kubelet runs readiness probe

↓

Probe sends request to container

↓

Container responds

↓

Success?

No→Pod marked NOT ready

↓

Retry probe after delay

+--> Yes↓

Pod marked READY

↓

Traffic routed to Pod

The readiness probe checks if a container is ready to receive traffic. If it succeeds, the pod is marked ready and receives traffic. If it fails, the pod is marked not ready and traffic is stopped.

Execution Sample

Kubernetes

readinessProbe:
  httpGet:
    path: /health
    port: 8080
  initialDelaySeconds: 5
  periodSeconds: 10

This readiness probe sends an HTTP GET request to /health on port 8080 every 10 seconds, starting 5 seconds after the container starts.

Process Table

Step	Action	Probe Result	Pod Readiness State	Effect
1	Pod starts, wait 5 seconds	No probe yet	Not Ready	No traffic sent
2	Send HTTP GET /health	Success	Ready	Pod receives traffic
3	Wait 10 seconds	No probe	Ready	Traffic continues
4	Send HTTP GET /health	Failure	Not Ready	Traffic stopped
5	Wait 10 seconds	No probe	Not Ready	No traffic
6	Send HTTP GET /health	Success	Ready	Traffic resumes

💡 Probe continues periodically; pod readiness state updates based on probe success or failure.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 6
Pod Readiness	Not Ready	Ready	Not Ready	Ready
Probe Result	None	Success	Failure	Success

Key Moments - 3 Insights

Why is the pod not ready immediately after starting?

Because the readiness probe has an initial delay (5 seconds) before it starts checking, as shown in step 1 of the execution_table.

What happens to traffic when the readiness probe fails?

The pod is marked Not Ready and stops receiving traffic, as shown in step 4 where the probe fails and traffic is stopped.

Does the pod stay not ready forever after a failure?

No, the probe runs periodically and if it succeeds later (step 6), the pod becomes ready again and traffic resumes.

Visual Quiz - 3 Questions

Look at the execution_table, what is the pod readiness state after the first successful probe?

A Not Ready

B Ready

C Unknown

D Terminated

💡 Hint

Check Step 2 in the execution_table where the probe result is Success.

At which step does the pod stop receiving traffic?

A Step 4

B Step 6

C Step 2

D Step 1

💡 Hint

Look at the Effect column in the execution_table for when traffic is stopped.

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

A Pod would skip readiness checks

B Pod would never become ready

C Probe would run immediately at Step 1

D Pod would be ready before starting

💡 Hint

Refer to Step 1 where currently no probe runs due to initial delay.

Concept Snapshot

Readiness probe checks if a container is ready to serve traffic.
It runs periodically after an initial delay.
If probe succeeds, pod is marked Ready and receives traffic.
If probe fails, pod is Not Ready and traffic stops.
Common probe types: HTTP GET, TCP socket, command execution.
Configured in pod spec under readinessProbe.

Full Transcript

A readiness probe in Kubernetes is a check that tells if a container inside a pod is ready to accept traffic. When a pod starts, Kubernetes waits for a configured initial delay before running the probe. The probe sends a request, like an HTTP GET, to the container. If the container responds successfully, the pod is marked ready and traffic is sent to it. If the probe fails, the pod is marked not ready and traffic is stopped. The probe runs repeatedly at set intervals to update the pod's readiness state. This helps Kubernetes only send traffic to pods that are fully ready to handle it.