Liveness probe concept in Kubernetes - Time & Space Complexity

Time Complexity: Liveness probe concept

O(n)

Understanding Time Complexity

We want to understand how the time cost of running a liveness probe changes as the number of containers grows.

How does the system handle checking many containers to keep them healthy?

Scenario Under Consideration

Analyze the time complexity of the following Kubernetes liveness probe configuration.

apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  containers:
  - name: app-container
    image: myapp:latest
    livenessProbe:
      httpGet:
        path: /healthz
        port: 8080
      initialDelaySeconds: 5
      periodSeconds: 10

This code sets a liveness probe that checks the /healthz path on port 8080 every 10 seconds after an initial delay.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The kubelet repeatedly sends HTTP GET requests to the container's /healthz endpoint.
• How many times: This happens every 10 seconds indefinitely for each container with a liveness probe.

How Execution Grows With Input

As the number of containers with liveness probes increases, the kubelet must perform more health checks.

Input Size (n)	Approx. Operations per period
10 containers	10 HTTP requests every 10 seconds
100 containers	100 HTTP requests every 10 seconds
1000 containers	1000 HTTP requests every 10 seconds

Pattern observation: The number of health check requests grows directly with the number of containers.

Final Time Complexity

Time Complexity: O(n)

This means the time spent checking container health grows linearly as you add more containers.

Common Mistake

[X] Wrong: "The liveness probe checks happen only once or very rarely, so adding containers doesn't affect performance much."

[OK] Correct: The kubelet runs these checks repeatedly at fixed intervals for every container, so more containers mean more checks and more work.

Interview Connect

Understanding how liveness probes scale helps you design systems that stay healthy without overloading the cluster.

Self-Check

"What if we changed the liveness probe to run every 5 seconds instead of 10? How would the time complexity change?"

Practice

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

easy

A. To check if a container is alive and restart it if it is not

B. To check if a container is ready to accept traffic

C. To monitor the CPU usage of a container

D. To scale the number of pods automatically

Solution

1. Step 1: Understand the role of liveness probes

   Liveness probes check if a container is still running properly. If not, Kubernetes restarts it to fix issues.

2. Step 2: Differentiate from readiness probes

   Readiness probes check if a container can receive traffic, but liveness probes focus on container health and restarting.

3. Final Answer:

   To check if a container is alive and restart it if it is not -> Option A

4. Quick Check:

   Liveness probe = container health check and restart [OK]

Hint: Liveness probes restart unhealthy containers automatically [OK]

Common Mistakes:

• Confusing liveness probe with readiness probe
• Thinking it scales pods instead of restarting
• Assuming it monitors resource usage

2. Which of the following is a correct way to define a liveness probe using an HTTP GET request in a pod spec?

easy

A. "livenessProbe:\n httpGet:\n path: /healthz\n port: 8080\n initialDelaySeconds: 5"

B. "livenessProbe:\n exec:\n command: [\"curl\", \"/healthz\"]\n initialDelaySeconds: 5"

C. "livenessProbe:\n tcpSocket:\n path: /healthz\n port: 8080\n initialDelaySeconds: 5"

D. "livenessProbe:\n httpGet:\n path: healthz\n port: 'eighty'\n initialDelaySeconds: 5"

Solution

1. Step 1: Check HTTP GET syntax for livenessProbe

   The correct syntax uses httpGet with path and numeric port. "livenessProbe:\n httpGet:\n path: /healthz\n port: 8080\n initialDelaySeconds: 5" matches this.

2. Step 2: Identify errors in other options

   "livenessProbe:\n exec:\n command: [\"curl\", \"/healthz\"]\n initialDelaySeconds: 5" uses exec with curl incorrectly. "livenessProbe:\n tcpSocket:\n path: /healthz\n port: 8080\n initialDelaySeconds: 5" wrongly uses tcpSocket with a path. "livenessProbe:\n httpGet:\n path: healthz\n port: 'eighty'\n initialDelaySeconds: 5" uses a string for port instead of a number.

3. Final Answer:

   "livenessProbe:\n httpGet:\n path: /healthz\n port: 8080\n initialDelaySeconds: 5" -> Option A

4. Quick Check:

   HTTP GET probe syntax = "livenessProbe:\n httpGet:\n path: /healthz\n port: 8080\n initialDelaySeconds: 5" [OK]

Hint: HTTP GET needs numeric port and valid path [OK]

Common Mistakes:

• Using string instead of number for port
• Adding path to tcpSocket probe
• Using exec command incorrectly

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

livenessProbe:
  exec:
    command:
    - cat
    - /tmp/healthy
  initialDelaySeconds: 3
  periodSeconds: 3

What happens if the file /tmp/healthy is deleted inside the container?

medium

A. The container will continue running without restart

B. The container will be restarted by Kubernetes

C. The pod will be deleted permanently

D. The liveness probe will be ignored

Solution

1. Step 1: Understand exec liveness probe behavior

   The exec probe runs the command. If the command fails (non-zero exit), Kubernetes restarts the container.

2. Step 2: Effect of deleting /tmp/healthy file

   Without the file, cat /tmp/healthy fails, so the probe fails and triggers a restart.

3. Final Answer:

   The container will be restarted by Kubernetes -> Option B

4. Quick Check:

   Exec probe fails = container restart [OK]

Hint: Exec probe failure restarts container [OK]

Common Mistakes:

• Thinking pod is deleted permanently
• Assuming probe failure is ignored
• Confusing liveness with readiness probe

4. You configured a liveness probe with this snippet:

livenessProbe:
  httpGet:
    path: /health
    port: 80
  initialDelaySeconds: 10
  periodSeconds: 5
  failureThreshold: 3

But your container keeps restarting even though the app is healthy. What is the most likely cause?

medium

A. The port number 80 is invalid

B. The initialDelaySeconds is too high

C. The probe path /health is incorrect or not responding

D. The failureThreshold is set too high

Solution

1. Step 1: Analyze probe failure reasons

   If the probe path is wrong or the app does not respond at /health, the probe fails and restarts the container.

2. Step 2: Check other parameters

   Initial delay of 10 seconds is usually enough. Failure threshold of 3 means 3 failures before restart, which is normal. Port 80 is valid.

3. Final Answer:

   The probe path /health is incorrect or not responding -> Option C

4. Quick Check:

   Wrong probe path causes restarts [OK]

Hint: Check probe URL path correctness first [OK]

Common Mistakes:

• Changing initialDelaySeconds unnecessarily
• Misunderstanding failureThreshold effect
• Assuming port 80 is invalid

5. You want to create a liveness probe that checks a TCP port 3306 on your container every 10 seconds, but only start checking after 30 seconds from container start. Which is the correct configuration snippet?

hard

A. "livenessProbe:\n tcpSocket:\n port: '3306'\n initialDelaySeconds: 10\n periodSeconds: 30"

B. "livenessProbe:\n httpGet:\n path: /\n port: 3306\n initialDelaySeconds: 10\n periodSeconds: 30"

C. "livenessProbe:\n exec:\n command: [\"nc\", \"-z\", \"localhost\", \"3306\"]\n initialDelaySeconds: 30\n periodSeconds: 10"

D. "livenessProbe:\n tcpSocket:\n port: 3306\n initialDelaySeconds: 30\n periodSeconds: 10"

Solution

1. Step 1: Identify TCP socket probe syntax

   The tcpSocket probe uses port as a number, with initialDelaySeconds and periodSeconds set as required. "livenessProbe:\n tcpSocket:\n port: 3306\n initialDelaySeconds: 30\n periodSeconds: 10" matches this.

2. Step 2: Check timing and port correctness

   "livenessProbe:\n tcpSocket:\n port: 3306\n initialDelaySeconds: 30\n periodSeconds: 10" sets initial delay to 30 seconds and period to 10 seconds, matching the requirement. Other options have wrong timing or use HTTP or exec probes incorrectly.

3. Final Answer:

   "livenessProbe:\n tcpSocket:\n port: 3306\n initialDelaySeconds: 30\n periodSeconds: 10" -> Option D

4. Quick Check:

   TCP probe with correct timing = "livenessProbe:\n tcpSocket:\n port: 3306\n initialDelaySeconds: 30\n periodSeconds: 10" [OK]

Hint: TCP probe uses numeric port and correct delays [OK]

Common Mistakes:

• Using string instead of number for port
• Mixing HTTP probe for TCP check
• Setting wrong initialDelaySeconds or periodSeconds