The answer is that the managed instance group fails to serve web traffic because the startup script does not configure a firewall rule to allow HTTP traffic. Even though the startup script installs and starts Nginx on port 80, Google Cloud’s default VPC firewall rules deny all inbound ingress traffic unless explicitly permitted. Without a corresponding firewall rule—created via `gcloud compute firewall-rules create` or `iptables`—external requests are blocked at the network layer, rendering the web server unreachable. On the Google Professional Cloud Developer exam, this scenario tests your understanding of the separation between application configuration and network security in managed instance groups. A common trap is assuming that installing a web server automatically makes it accessible; in reality, the startup script must also open the required port. Remember the mnemonic “Startup installs, firewall opens”—the instance template handles the app, but the network layer needs its own explicit rule for traffic to flow.
PCD Practice Question: Designing highly scalable, available, and reliable cloud-native applications
This PCD practice question tests your understanding of designing highly scalable, available, and reliable cloud-native applications. 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 team created the instance template above and used it in a managed instance group. However, instances fail to serve web traffic. What is the most likely cause?
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 startup script does not configure a firewall rule to allow HTTP traffic.
The instance template likely includes a startup script that installs and starts Nginx, but does not configure a firewall rule (e.g., via `gcloud compute firewall-rules create` or `iptables`) to allow inbound HTTP traffic on port 80. By default, GCP VPC firewall rules deny all ingress traffic unless explicitly allowed, so even if Nginx is running, external requests will be blocked. This is the most common reason why a managed instance group fails to serve web traffic despite the application being installed.
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 startup script does not configure a firewall rule to allow HTTP traffic.
Why this is correct
The default VPC firewall rules only allow SSH and ICMP. An ingress rule for HTTP (port 80) is needed for Nginx to serve traffic.
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 image family debian-11 does not have the necessary packages.
Why it's wrong here
Debian 11 includes apt-get, and the startup script installs Nginx successfully.
✗
The machine type e2-medium is too small for Nginx.
Why it's wrong here
e2-medium has sufficient resources for running Nginx and serving typical web traffic.
✗
The instance template is missing a service account.
Why it's wrong here
A default service account is automatically assigned, and Nginx does not require specific permissions to serve HTTP traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that installing and starting a web server (like Nginx) is sufficient to serve traffic, ignoring the separate requirement for network-level firewall rules to allow inbound connections.
Detailed technical explanation
How to think about this question
GCP VPC networks have an implied `deny-all` ingress rule, so any new instance must have an explicit firewall rule allowing HTTP (TCP port 80) or HTTPS (TCP port 443) traffic. The startup script in the instance template typically runs as root and can install Nginx, but it does not automatically open the required port. Even if the script uses `gcloud compute firewall-rules create`, it would need a service account with appropriate IAM permissions, but the core issue is the missing rule itself, not the identity. In practice, many candidates overlook that firewall rules are separate from instance configuration and must be created independently.
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.
Designing highly scalable, available, and reliable cloud-native applications — This question tests Designing highly scalable, available, and reliable cloud-native applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The startup script does not configure a firewall rule to allow HTTP traffic. — The instance template likely includes a startup script that installs and starts Nginx, but does not configure a firewall rule (e.g., via `gcloud compute firewall-rules create` or `iptables`) to allow inbound HTTP traffic on port 80. By default, GCP VPC firewall rules deny all ingress traffic unless explicitly allowed, so even if Nginx is running, external requests will be blocked. This is the most common reason why a managed instance group fails to serve web traffic despite the application being installed.
What should I do if I get this PCD 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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