Why Your Pub/Sub Subscriber Can't Pull Messages: Missing Subscription
This PDE practice question tests your understanding of ensuring solution quality. 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.
Refer to the exhibit. A subscriber is unable to pull messages from the topic. 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 answer is that no subscription has been created for the topic. In Google Cloud Pub/Sub, a topic is simply a named channel for publishing messages, but subscribers cannot directly pull messages from it; they must pull from a dedicated subscription resource that is attached to that topic. This subscription manages message delivery, acknowledgment, and retention, so without one, the subscriber has no endpoint to receive messages from. On the Google Professional Data Engineer exam, this concept tests your understanding of the Pub/Sub resource hierarchy, and a common trap is confusing the subscriber’s IAM role on the topic with the need for a subscription object. Remember that a topic is like a broadcast antenna, but you need a subscription as your receiver to tune in and pull the signal.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
No subscription has been created for the topic.
In Google Cloud Pub/Sub, a topic is a named resource to which messages are sent by publishers. Subscribers must create a subscription (pull or push) to receive messages from that topic. If no subscription exists, the subscriber cannot pull any messages because there is no delivery endpoint or pull queue attached to the topic. Option C correctly identifies this missing subscription as the root cause.
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 service account has the subscriber role but the topic is not configured correctly.
Why it's wrong here
The subscriber role on the topic is necessary but not sufficient without a subscription.
✗
The service account needs roles/pubsub.viewer to list subscriptions.
Why it's wrong here
The viewer role is not required for pulling; the subscriber role is sufficient.
✓
No subscription has been created for the topic.
Why this is correct
A subscription is required to pull messages; the topic only provides the ability to publish.
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 service account lacks roles/pubsub.publisher.
Why it's wrong here
The subscriber does not need the publisher role.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between topics and subscriptions, trapping candidates who assume that having a topic and a subscriber role is sufficient to receive messages, when in fact a subscription must be explicitly created.
Detailed technical explanation
How to think about this question
Under the hood, a Pub/Sub subscription is a separate resource that defines the delivery method (pull or push) and holds the message backlog. The subscriber calls the subscriptions/pull method on the subscription resource, not on the topic. If no subscription exists, the pull request returns a NOT_FOUND error. In real-world scenarios, forgetting to create a subscription is a common oversight when automating topic creation without corresponding subscription provisioning.
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.
Ensuring solution quality — This question tests Ensuring solution quality — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: No subscription has been created for the topic. — In Google Cloud Pub/Sub, a topic is a named resource to which messages are sent by publishers. Subscribers must create a subscription (pull or push) to receive messages from that topic. If no subscription exists, the subscriber cannot pull any messages because there is no delivery endpoint or pull queue attached to the topic. Option C correctly identifies this missing subscription as the root cause.
What should I do if I get this PDE 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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