Probe failure and container restart behavior in Kubernetes - Time & Space Complexity

Time Complexity: Probe failure and container restart behavior

O(n)

Understanding Time Complexity

We want to understand how the system reacts over time when a probe fails in Kubernetes.

Specifically, how often the container restarts grow as probe failures increase.

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: example-container
    image: example-image
    livenessProbe:
      httpGet:
        path: /healthz
        port: 8080
      initialDelaySeconds: 5
      periodSeconds: 10
      failureThreshold: 3

This config checks the container health every 10 seconds and restarts it after 3 failures.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The liveness probe runs repeatedly every 10 seconds.
• How many times: It runs continuously until the container is restarted or stopped.

How Execution Grows With Input

As the number of probe failures increases, the container restart attempts grow linearly.

Input Size (failures)	Approx. Restarts
3	1 restart after 3 failures
6	2 restarts after 6 failures
30	10 restarts after 30 failures

Pattern observation: Restarts increase directly with the number of failures.

Final Time Complexity

Time Complexity: O(n)

This means the number of container restarts grows linearly as probe failures increase.

Common Mistake

[X] Wrong: "The container restarts immediately after one probe failure."

[OK] Correct: Kubernetes waits for the failureThreshold count before restarting, so restarts depend on multiple failures, not just one.

Interview Connect

Understanding how Kubernetes handles probe failures and restarts helps you explain system reliability and fault tolerance clearly.

Self-Check

What if we changed failureThreshold from 3 to 1? How would the time complexity of restarts change?

Practice

1. What happens when a Kubernetes liveness probe fails for a container?

easy

A. Traffic to the container is stopped but it is not restarted.

B. The container is restarted by Kubernetes.

C. The container continues running without changes.

D. The container is removed permanently.

Solution

1. Step 1: Understand liveness probe purpose

   Liveness probes check if a container is alive and healthy.

2. Step 2: Effect of liveness probe failure

   If the liveness probe fails, Kubernetes restarts the container to fix the issue.

3. Final Answer:

   The container is restarted by Kubernetes. -> Option B

4. Quick Check:

   Liveness probe failure = container restart [OK]

Hint: Liveness probe failure always triggers container restart [OK]

Common Mistakes:

• Confusing liveness with readiness probe effects
• Thinking container is deleted permanently
• Assuming no action on failure

2. Which of the following is the correct YAML snippet to define a liveness probe using HTTP GET on path /health at port 8080?

easy

A. livenessProbe:
httpGet:
path: health
port: 8080
initialDelaySeconds: 5
periodSeconds: 10

B. livenessProbe:
exec:
command: ["curl", "/health"]
initialDelaySeconds: 5
periodSeconds: 10

C. livenessProbe:
tcpSocket:
port: 8080
initialDelaySeconds: 5
periodSeconds: 10

D. livenessProbe:
httpGet:
path: /health
port: 8080
initialDelaySeconds: 5
periodSeconds: 10

Solution

1. Step 1: Check HTTP GET probe syntax

   The correct syntax uses httpGet with path starting with a slash and port as a number.

2. Step 2: Validate each option

   livenessProbe: httpGet: path: /health port: 8080 initialDelaySeconds: 5 periodSeconds: 10 correctly uses /health and port 8080. livenessProbe: exec: command: ["curl", "/health"] initialDelaySeconds: 5 periodSeconds: 10 uses exec with curl incorrectly. livenessProbe: tcpSocket: port: 8080 initialDelaySeconds: 5 periodSeconds: 10 uses tcpSocket which is different. livenessProbe: httpGet: path: health port: 8080 initialDelaySeconds: 5 periodSeconds: 10 misses the leading slash in path.

3. Final Answer:

   livenessProbe: httpGet: path: /health port: 8080 initialDelaySeconds: 5 periodSeconds: 10 -> Option D

4. Quick Check:

   HTTP GET probe with correct path and port = livenessProbe: httpGet: path: /health port: 8080 initialDelaySeconds: 5 periodSeconds: 10 [OK]

Hint: HTTP path must start with '/' and port is a number [OK]

Common Mistakes:

• Missing leading slash in path
• Using exec command incorrectly
• Confusing tcpSocket with httpGet

3. Given this pod spec snippet, what happens if the readiness probe fails but the liveness probe passes?

readinessProbe:
  httpGet:
    path: /ready
    port: 80
  initialDelaySeconds: 5
  periodSeconds: 10
livenessProbe:
  httpGet:
    path: /live
    port: 80
  initialDelaySeconds: 5
  periodSeconds: 10

medium

A. The container is restarted immediately.

B. The container is deleted and recreated.

C. The container is marked not ready and removed from service endpoints.

D. Nothing happens; container continues serving traffic.

Solution

1. Step 1: Understand readiness probe role

   Readiness probe controls if the container receives traffic by marking it ready or not.

2. Step 2: Effect of readiness probe failure

   If readiness probe fails but liveness probe passes, container stays running but is removed from service endpoints, so no traffic is sent to it.

3. Final Answer:

   The container is marked not ready and removed from service endpoints. -> Option C

4. Quick Check:

   Readiness fail = no traffic, container runs [OK]

Hint: Readiness failure stops traffic, liveness failure restarts container [OK]

Common Mistakes:

• Confusing readiness failure with container restart
• Thinking container is deleted
• Assuming no effect on traffic

4. You notice your container keeps restarting frequently. The liveness probe is configured with initialDelaySeconds: 5 and periodSeconds: 10. What is a likely cause and fix?

medium

A. The initial delay is too short; increase initialDelaySeconds to allow startup time.

B. The period is too long; decrease periodSeconds to check faster.

C. The probe path is incorrect; change it to /healthz.

D. The container image is corrupted; rebuild the image.

Solution

1. Step 1: Analyze probe timing settings

   If initialDelaySeconds is too short, the probe runs before the app is ready, causing false failures.

2. Step 2: Adjust initial delay

   Increasing initialDelaySeconds gives the container time to start before probing, preventing unnecessary restarts.

3. Final Answer:

   The initial delay is too short; increase initialDelaySeconds to allow startup time. -> Option A

4. Quick Check:

   Short initialDelay causes early probe failure [OK]

Hint: Increase initialDelaySeconds if container restarts too soon [OK]

Common Mistakes:

• Changing periodSeconds instead of initialDelaySeconds
• Assuming probe path is always wrong
• Blaming container image without checking probes

5. You have a container with both readiness and liveness probes. The readiness probe fails intermittently, but the liveness probe always passes. How should Kubernetes behave and what is the best way to improve user experience?

hard

A. Kubernetes removes the pod from service during readiness failure but does not restart it; improve by tuning readiness probe thresholds.

B. Kubernetes restarts the container on readiness failure; improve by disabling readiness probe.

C. Kubernetes ignores readiness probe failures if liveness passes; no action needed.

D. Kubernetes deletes the pod on readiness failure; improve by increasing liveness probe timeout.

Solution

1. Step 1: Understand probe roles on intermittent readiness failure

   Readiness failure removes pod from service endpoints but does not restart container if liveness passes.

2. Step 2: Improve user experience by tuning readiness probe

   Tuning thresholds like failureThreshold or periodSeconds reduces false negatives and prevents unnecessary traffic disruption.

3. Final Answer:

   Kubernetes removes the pod from service during readiness failure but does not restart it; improve by tuning readiness probe thresholds. -> Option A

4. Quick Check:

   Readiness fail = no traffic; tune probe to reduce false failures [OK]

Hint: Tune readiness probe to avoid false failures and traffic loss [OK]

Common Mistakes:

• Thinking readiness failure restarts container
• Disabling readiness probe instead of tuning
• Ignoring intermittent failures