This CKAD practice question tests your understanding of application deployment. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
You are deploying a multi-tier application consisting of a frontend web service and a backend API. The frontend is deployed as a Deployment with 3 replicas, exposed via a ClusterIP Service named 'frontend-svc'. The backend is a StatefulSet with 3 replicas, each with its own PersistentVolumeClaim, and exposed via a headless Service named 'backend-svc'. The backend pods need to discover each other via DNS for clustering. After deployment, the backend pods cannot resolve the hostnames of other backend pods. The frontend can reach the backend via the Service name. You verify that the StatefulSet and its Service are correctly named. What is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The backend Service is not headless
For a StatefulSet to provide DNS-based pod discovery, its associated Service must be headless (clusterIP: None). A headless Service creates DNS A/AAAA records for each pod's hostname (e.g., backend-svc-0.backend-svc.default.svc.cluster.local), enabling direct pod-to-pod resolution. If the Service is not headless, DNS queries for individual pod hostnames will not resolve, breaking clustering discovery.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The StatefulSet's serviceName does not match the backend Service name
Why it's wrong here
If serviceName mismatched, pods might not be part of the Service, but frontend would also fail.
✗
The backend pods are not using the correct DNS domain
Why it's wrong here
DNS domain is typically 'cluster.local' and is correct by default.
✗
The frontend Service is interfering with the backend DNS
Why it's wrong here
Services are independent; frontend Service does not affect backend DNS.
✓
The backend Service is not headless
Why this is correct
A headless Service is needed for StatefulSet pod DNS.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CNCF often tests the subtle requirement that a StatefulSet's Service must be explicitly headless (clusterIP: None) for pod DNS resolution, and candidates mistakenly think any Service name match is sufficient.
Detailed technical explanation
How to think about this question
A headless Service (clusterIP: None) instructs the cluster DNS to return pod IPs directly via A/AAAA records for each pod's hostname (e.g., backend-svc-0.backend-svc). Without it, the DNS returns the Service's ClusterIP, not individual pod IPs, so pod hostnames like 'backend-svc-0' do not resolve. This is critical for StatefulSets requiring stable network identities, such as databases (Cassandra, Elasticsearch) or clustered applications that use DNS for peer discovery.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CKAD question in full detail.
Application Deployment — This question tests Application Deployment — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The backend Service is not headless — For a StatefulSet to provide DNS-based pod discovery, its associated Service must be headless (clusterIP: None). A headless Service creates DNS A/AAAA records for each pod's hostname (e.g., backend-svc-0.backend-svc.default.svc.cluster.local), enabling direct pod-to-pod resolution. If the Service is not headless, DNS queries for individual pod hostnames will not resolve, breaking clustering discovery.
What should I do if I get this CKAD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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