Readiness probe concept in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of a Readiness Probe in Kubernetes

What is the main purpose of a readiness probe in a Kubernetes pod?

A To check if the container is alive and restart it if it is not

B To determine if the container is ready to accept traffic

C To monitor resource usage like CPU and memory inside the container

D To automatically scale the number of pods based on load

💡 Hint

Think about when Kubernetes should send traffic to a pod.

A readiness probe tells Kubernetes if a pod is ready to receive requests. If the probe fails, the pod is removed from service endpoints until it passes again.

💻 Command Output

intermediate

Readiness Probe Failure Effect

Given a pod with a readiness probe that fails continuously, what will be the state of the pod in the Kubernetes service endpoints?

A The pod will be removed from the service endpoints and not receive traffic

B The pod will be restarted automatically by Kubernetes

C The pod will be deleted from the cluster

D The pod will remain in the service endpoints and receive traffic

💡 Hint

Consider what happens when a pod is not ready but still running.

If the readiness probe fails, Kubernetes stops sending traffic to the pod by removing it from service endpoints, but it does not restart or delete the pod.

❓ Configuration

advanced

Correct Readiness Probe Configuration

Which of the following YAML snippets correctly configures a readiness probe that checks HTTP path /health on port 8080 every 10 seconds with a timeout of 5 seconds?

A

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

B

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

C

readinessProbe:
  exec:
    command: ["curl", "-f", "http://localhost:8080/health"]
  periodSeconds: 10
  timeoutSeconds: 5

D

readinessProbe:
  tcpSocket:
    port: 8080
  periodSeconds: 10
  timeoutSeconds: 5

💡 Hint

Look for the HTTP GET method with correct timing settings.

Option A correctly uses httpGet with the right path, port, and timing parameters. Other options either use wrong timing or different probe types.

❓ Troubleshoot

advanced

Troubleshooting Readiness Probe Failures

A pod's readiness probe is failing, but the container is running fine. Which of the following is the most likely cause?

A The pod has insufficient CPU resources

B The container is crashing and restarting frequently

C The readiness probe path or port is incorrect or the service inside the container is not responding

D The pod's liveness probe is misconfigured

💡 Hint

Think about what the readiness probe checks specifically.

Readiness probes check if the container is ready to serve traffic, often by hitting a specific path or port. If that path or port is wrong or the service is not responding, the probe fails even if the container is running.

🔀 Workflow

expert

Readiness Probe Impact on Rolling Updates

During a rolling update, how does a failing readiness probe affect the deployment rollout process?

A Kubernetes immediately rolls back the update if any pod fails its readiness probe

B Pods failing readiness probes are restarted repeatedly until they pass, blocking rollout progress

C Kubernetes pauses the rollout indefinitely until all pods pass readiness probes

D Pods failing readiness probes are not added to the service endpoints, so traffic is not sent to them, but rollout continues until maxUnavailable is reached

💡 Hint

Consider how Kubernetes balances availability and rollout progress.

Kubernetes removes pods failing readiness probes from service endpoints so they don't receive traffic. The rollout continues respecting maxUnavailable limits, allowing some pods to be unavailable temporarily.

Practice

1. What is the main purpose of a readiness probe in Kubernetes?

easy

A. To tell Kubernetes when a container is ready to receive traffic

B. To restart a container if it crashes

C. To monitor the CPU usage of a container

D. To update the container image automatically

Solution

1. Step 1: Understand readiness probe role

   A readiness probe checks if a container is ready to accept requests, preventing traffic to unready containers.

2. Step 2: Compare with other options

   Other options describe different Kubernetes features: restarts (liveness probe), monitoring (metrics), and updates (deployments).

3. Final Answer:

   To tell Kubernetes when a container is ready to receive traffic -> Option A

4. Quick Check:

   Readiness probe = container readiness [OK]

Hint: Readiness probe controls traffic flow to containers [OK]

Common Mistakes:

• Confusing readiness probe with liveness probe
• Thinking readiness probe restarts containers
• Assuming readiness probe monitors resource usage

2. Which of the following is a valid way to define a readiness probe in a Kubernetes pod spec?

easy

A. readinessProbe:
httpGet:
path: /health
port: 8080

B. readinessProbe:
restartPolicy: Always

C. readinessProbe:
exec:
command: ["curl", "-f", "http://localhost:8080"]

D. readinessProbe:
tcpSocket:
port: 8080

Solution

1. Step 1: Review readiness probe types

   Valid readiness probes include httpGet, tcpSocket, and exec commands.

2. Step 2: Check syntax correctness

   readinessProbe: httpGet: path: /health port: 8080 is now correctly indented. readinessProbe: exec: command: ["curl", "-f", "http://localhost:8080"] uses exec with correct command syntax as a list of strings. readinessProbe: tcpSocket: port: 8080 correctly defines tcpSocket with port. readinessProbe: restartPolicy: Always is invalid because restartPolicy is not part of readiness probe.

3. Final Answer:

   readinessProbe with tcpSocket and port -> Option D

4. Quick Check:

   Valid readiness probe syntax = tcpSocket [OK]

Hint: Check indentation and valid probe keys for readiness probe [OK]

Common Mistakes:

• Misplacing restartPolicy inside readinessProbe
• Incorrect command format in exec probe
• Wrong indentation causing YAML errors

3. Given this readiness probe configuration in a pod spec:

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

What happens if the HTTP GET request to /ready returns status 503?

medium

A. Kubernetes ignores the failure and continues sending traffic

B. Kubernetes restarts the container immediately

C. Kubernetes marks the container as not ready and stops sending traffic

D. Kubernetes deletes the pod

Solution

1. Step 1: Understand readiness probe failure effect

   If the readiness probe fails (like HTTP 503), Kubernetes marks the container as not ready and stops routing traffic to it.

2. Step 2: Differentiate from liveness probe

   Liveness probe failure causes restarts, but readiness probe failure only affects traffic routing, not container lifecycle.

3. Final Answer:

   Kubernetes marks the container as not ready and stops sending traffic -> Option C

4. Quick Check:

   Readiness failure = no traffic [OK]

Hint: Readiness failure stops traffic, does not restart container [OK]

Common Mistakes:

• Confusing readiness probe failure with liveness probe failure
• Assuming container restarts on readiness failure
• Thinking pod gets deleted on readiness failure

4. You defined this readiness probe in your pod YAML:

readinessProbe:
  exec:
    command: ["cat", "/tmp/ready"]
  initialDelaySeconds: 5
  periodSeconds: 10

But Kubernetes always marks the pod as not ready. What is the most likely cause?

medium

A. The exec command syntax is invalid

B. The file /tmp/ready does not exist or is empty

C. initialDelaySeconds is too short

D. periodSeconds is too long

Solution

1. Step 1: Understand exec readiness probe behavior

   The exec probe runs the command; it expects a zero exit code for success. If the file /tmp/ready does not exist, cat returns non-zero, causing failure.

2. Step 2: Check other options

   Command syntax is valid. Timing values affect delay but not constant failure. So the missing or unreadable file is the cause.

3. Final Answer:

   The file /tmp/ready does not exist or is empty -> Option B

4. Quick Check:

   Exec probe success = command exit code 0 [OK]

Hint: Exec probe needs command to succeed (exit code 0) [OK]

Common Mistakes:

• Assuming timing values cause readiness failure
• Misunderstanding exec command success criteria
• Ignoring file existence for exec probes

5. You want to ensure your web server container only receives traffic after it finishes loading configuration and opens port 8080. Which readiness probe setup best achieves this?

hard

A. Use an exec readiness probe that runs a script checking config file and port status

B. Use an HTTP GET readiness probe on path /health with periodSeconds set to 1

C. Use a TCP socket readiness probe on port 8080 with initialDelaySeconds set to 10

D. Use a liveness probe instead of readiness probe to check port 8080

Solution

1. Step 1: Analyze readiness probe requirements

   The container must be ready only after config is loaded and port is open. TCP socket or HTTP GET alone cannot check config loading.

2. Step 2: Evaluate probe options

   Use a TCP socket readiness probe on port 8080 with initialDelaySeconds set to 10 checks only port open after delay, Use an HTTP GET readiness probe on path /health with periodSeconds set to 1 checks HTTP path frequently but not config. Use an exec readiness probe that runs a script checking config file and port status runs a script that can verify both config and port, making it the best choice. Use a liveness probe instead of readiness probe to check port 8080 is wrong because liveness probes do not control traffic routing.

3. Final Answer:

   Use an exec readiness probe that runs a script checking config file and port status -> Option A

4. Quick Check:

   Complex readiness checks need exec probe [OK]

Hint: Use exec probe for multi-condition readiness checks [OK]

Common Mistakes:

• Using liveness probe instead of readiness probe
• Relying only on TCP or HTTP probes for config readiness
• Ignoring initialDelaySeconds impact on readiness