The answer is that the Compute Engine instance has no external IP address and cannot reach the internet to download packages. This is the most likely cause because startup scripts that install software—such as nginx—rely on outbound internet connectivity to fetch packages from public repositories. Without an external IP, the instance is isolated from the internet, so the script fails silently when it cannot complete the installation, leaving the service stopped. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how network configuration directly impacts instance initialization and automation. A common trap is assuming the startup script itself has a syntax error or that the instance lacks sufficient permissions, but the core issue is network egress. Remember the memory tip: “No IP, no repo” — if an instance lacks an external IP, it cannot reach external package repositories, causing any installation-based startup script to fail.
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.
An administrator runs the above command to create a Compute Engine instance. However, the nginx service does not start. 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.
The instance has no external IP address and cannot reach the internet to download packages.
By default, instances are created without an external IP unless --no-address is not specified. The command does not specify --no-address, but if the project's default is to not assign external IPs, the instance may lack internet access. However, in newer GCP projects, the default is to assign an ephemeral external IP. Actually, the default behavior depends on the project's VPC configuration. Without an external IP and without Cloud NAT, the instance cannot access the internet, causing apt-get to fail.
B
The metadata key is misspelled; it should be 'startup-script-url'.
Why wrong: The correct key is 'startup-script', not 'startup-script-url'. The exhibit uses the correct key, so this is not the cause.
C
The instance does not have the compute.instance.update permission.
Why wrong: The instance creation itself uses the caller's permissions, not the instance's service account. The service account is used for API calls from within the instance, but apt-get does not require that permission.
D
The startup script runs before the boot disk is fully mounted.
Why wrong: The startup script runs after the boot disk is mounted and the instance is booted, so this is not an issue.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instance has no external IP address and cannot reach the internet to download packages.
The command likely creates a Compute Engine instance without specifying an external IP address (e.g., using `--no-address` or omitting `--address`). Without an external IP, the instance cannot reach the internet to download the nginx package from repositories, causing the startup script that installs and starts nginx to fail. This is the most direct cause of the nginx service not starting.
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 instance has no external IP address and cannot reach the internet to download packages.
Why this is correct
By default, instances are created without an external IP unless --no-address is not specified. The command does not specify --no-address, but if the project's default is to not assign external IPs, the instance may lack internet access. However, in newer GCP projects, the default is to assign an ephemeral external IP. Actually, the default behavior depends on the project's VPC configuration. Without an external IP and without Cloud NAT, the instance cannot access the internet, causing apt-get to fail.
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 metadata key is misspelled; it should be 'startup-script-url'.
Why it's wrong here
The correct key is 'startup-script', not 'startup-script-url'. The exhibit uses the correct key, so this is not the cause.
✗
The instance does not have the compute.instance.update permission.
Why it's wrong here
The instance creation itself uses the caller's permissions, not the instance's service account. The service account is used for API calls from within the instance, but apt-get does not require that permission.
✗
The startup script runs before the boot disk is fully mounted.
Why it's wrong here
The startup script runs after the boot disk is mounted and the instance is booted, so this is not an issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the nuance that startup scripts execute after the boot disk is mounted and that missing external IP prevents internet-dependent operations, leading candidates to incorrectly blame script syntax or permissions.
Detailed technical explanation
How to think about this question
When a Compute Engine instance has no external IP, it cannot reach public repositories like Debian/Ubuntu apt mirrors or Google's package repositories unless it uses Cloud NAT or a proxy. The startup script typically runs `apt-get install nginx` or similar, which fails silently if network access is unavailable. Additionally, the instance can still use internal Google APIs (e.g., metadata server) without an external IP, but package downloads require internet connectivity.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 instance has no external IP address and cannot reach the internet to download packages. — The command likely creates a Compute Engine instance without specifying an external IP address (e.g., using `--no-address` or omitting `--address`). Without an external IP, the instance cannot reach the internet to download the nginx package from repositories, causing the startup script that installs and starts nginx to fail. This is the most direct cause of the nginx service not starting.
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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