The answer is that firewall rules blocking traffic from Google’s health check probe IP ranges are the most likely cause. This happens because TCP health checks originate from specific Google Cloud probe addresses, such as 35.191.0.0/16 and 130.211.0.0/22, not from within the instance’s own network. When you manually test from inside the VM, you bypass the firewall, so the app responds fine, but the external probe packets are dropped if those source ranges aren’t allowed on port 8080. On the Google Professional Cloud DevOps Engineer exam, this scenario tests your understanding of how health check traffic differs from local traffic—a common trap is assuming a running app means the health check should pass. A quick memory tip: think of the health check as a “guest from outside” that needs a specific invite, not just an open door.
PCDOE Implementing service monitoring strategies Practice Question
This PCDOE practice question tests your understanding of implementing service monitoring strategies. 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.
A company is running a stateful workload on Compute Engine and has configured a TCP health check on port 8080. The health check is failing, but the application is running and responding on port 8080 when tested manually from within the instance. What is the most likely cause of the health check failure?
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 firewall rules are not allowing traffic from the health check probe IP ranges.
The health check probes originate from Google's health check systems, which use specific IP ranges (e.g., 35.191.0.0/16, 130.211.0.0/22). If firewall rules on the instance or VPC do not explicitly allow inbound traffic from these probe IP ranges on port 8080, the health check will fail even though the application is running and responding to manual tests from within the instance. This is the most common cause of health check failures when the application itself is healthy.
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 health check is configured to use port 80 instead of port 8080.
Why it's wrong here
The question says port 8080 is configured.
✓
The firewall rules are not allowing traffic from the health check probe IP ranges.
Why this is correct
Health check probes use specific IP ranges that must be allowed.
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.
✗
The instance's DNS resolution is failing, causing the health check to use the wrong IP.
The health check response timeout is set too low (e.g., 1 second).
Why it's wrong here
Default timeout is 5 seconds, usually enough.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that health check failures are always due to application misconfiguration or port mismatches, but the trap here is that the health check probes come from external Google IP ranges that must be explicitly allowed in firewall rules, not from within the instance's own network.
Detailed technical explanation
How to think about this question
Google Cloud health check probes use source IP ranges documented in the Google Cloud documentation (e.g., 35.191.0.0/16, 130.211.0.0/22, and 209.85.152.0/22 for legacy checks). These probes are sent from Google's infrastructure, not from within the VPC, so they must be allowed by firewall rules (ingress allow on the health check port from these ranges). A common real-world scenario is when an administrator configures a firewall rule allowing traffic only from internal VPC ranges (e.g., 10.0.0.0/8) but forgets to include the health check probe ranges, causing the health check to fail while the application works fine for internal users.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Implementing service monitoring strategies — This question tests Implementing service monitoring strategies — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall rules are not allowing traffic from the health check probe IP ranges. — The health check probes originate from Google's health check systems, which use specific IP ranges (e.g., 35.191.0.0/16, 130.211.0.0/22). If firewall rules on the instance or VPC do not explicitly allow inbound traffic from these probe IP ranges on port 8080, the health check will fail even though the application is running and responding to manual tests from within the instance. This is the most common cause of health check failures when the application itself is healthy.
What should I do if I get this PCDOE 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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Question Discussion
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