The answer is to use the instance's default Compute Engine service account and assign the Pub/Sub Publisher role to it. This is the recommended approach because every Compute Engine instance is automatically attached to a default service account, which acts as its identity for Google Cloud API calls. By granting the roles/pubsub.publisher IAM role to that service account, you enable the instance to authenticate and send messages to a Pub/Sub topic without managing keys or credentials. On the Google Professional Cloud Developer exam, this scenario tests your understanding of IAM best practices for service-to-service authentication, often appearing as a trap where candidates might choose API keys or a custom service account. The key insight is that the default service account is purpose-built for this pattern, making it both secure and operationally simple. Memory tip: think "Default does the work" — the default service account is your go-to for Compute Engine to Pub/Sub auth.
PCD Integrating Google Cloud services Practice Question
This PCD practice question tests your understanding of integrating google cloud services. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 developer wants to allow a Compute Engine instance to send messages to a Pub/Sub topic. What is the recommended way to grant permissions?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use the instance's default Compute Engine service account and assign the Pub/Sub Publisher role to it.
Option B is correct because using the default Compute Engine service account and assigning the Pub/Sub Publisher role is the simplest and recommended approach. Option A works but is less efficient. Option C is wrong because API keys are not for service-to-service auth. Option D is insecure.
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.
✗
Generate an API key for the instance and include it in HTTP requests.
Why it's wrong here
API keys are not suitable for service account authentication.
✗
Create a service account and assign the Pub/Sub Publisher role; attach the service account to the instance.
Why it's wrong here
This works but creates an unnecessary custom service account.
✓
Use the instance's default Compute Engine service account and assign the Pub/Sub Publisher role to it.
Why this is correct
The default service account is convenient and secure.
Related concept
Read the scenario before looking for a memorised answer.
✗
Store the service account key file directly on the instance.
Why it's wrong here
Storing key files on disk is insecure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Integrating Google Cloud services — This question tests Integrating Google Cloud services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use the instance's default Compute Engine service account and assign the Pub/Sub Publisher role to it. — Option B is correct because using the default Compute Engine service account and assigning the Pub/Sub Publisher role is the simplest and recommended approach. Option A works but is less efficient. Option C is wrong because API keys are not for service-to-service auth. Option D is insecure.
What should I do if I get this PCD question wrong?
Identify which PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company wants to send events from a custom application to Cloud Pub/Sub, then process them with a Cloud Run service. The application runs on Compute Engine. What is the simplest way for the application to authenticate to Pub/Sub?
easy
A.Use an API key for the Pub/Sub API.
B.Embed a service account JSON key in the application code.
C.Set up Cloud Endpoints to proxy the Pub/Sub requests.
✓ D.Attach a service account to the Compute Engine instance with necessary Pub/Sub roles.
Why D: Option B is correct because using a service account attached to the Compute Engine instance allows automatic authentication via the instance metadata server, which is the simplest and most secure approach. Option A is wrong because storing a JSON key in the application code is not best practice. Option C is wrong because an API key does not provide identity-based access control for Pub/Sub. Option D is wrong because while Cloud Endpoints is an option, it adds unnecessary complexity.
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This PCD practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCD exam.
Question Discussion
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