The correct answer is to use Cloud Pub/Sub notifications for Cloud Storage and trigger the Cloud Function from the subscription. This resolves the race condition because Cloud Storage object finalize events are only sent after the file is fully written and committed, but in practice, the Cloud Function can be invoked before the upload is fully flushed to disk due to internal replication timing. By routing the event through Pub/Sub, you decouple the trigger and gain a reliable, asynchronous delivery guarantee that the file is completely available before the function executes. On the Google Professional Data Engineer exam, this scenario tests your understanding of event-driven architectures and the subtle latency between object finalize and actual file readiness—a common trap is assuming the finalize event is instantaneous. Remember the memory tip: “Finalize fires, but Pub/Sub ensures the file is finalized.”
PDE Designing data processing systems Practice Question
This PDE practice question tests your understanding of designing data processing systems. 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 media company ingests video files from partners via a REST API. Files are stored in Cloud Storage, and metadata is written to Firestore. A Cloud Function is triggered on object finalize to transcode video using Transcoder API. Sometimes, the function fails because the file is still being uploaded when triggered. How should this be fixed?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Cloud Pub/Sub notifications for Cloud Storage and trigger the function from the subscription.
Option D is correct because Cloud Storage object finalize notifications are sent only after the entire file has been written and committed. By using Pub/Sub notifications for Cloud Storage and triggering the Cloud Function from the subscription, you decouple the trigger from the upload process, ensuring the function only runs when the file is fully available. This eliminates the race condition where the function is triggered before the upload completes.
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.
✗
Implement a Cloud Composer workflow to poll for file existence.
Why it's wrong here
Adds complexity unnecessarily.
✗
Require partners to use resumable uploads.
Why it's wrong here
Does not prevent premature trigger.
✗
Increase the Cloud Functions timeout to allow time for the upload to finish.
Why it's wrong here
Function is triggered after finalize, not during.
✓
Use Cloud Pub/Sub notifications for Cloud Storage and trigger the function from the subscription.
Why this is correct
Pub/Sub notifications are sent after object finalization.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume 'object finalize' means the upload is complete, but in practice, the event can fire before the upload is fully committed, leading to the misconception that increasing timeouts or changing upload methods will fix the issue.
Detailed technical explanation
How to think about this question
Cloud Storage object finalize notifications are published to Pub/Sub only after the object's metadata is fully written and the object is in a consistent state. However, when using a Cloud Function directly triggered by the bucket event, there is a known race condition where the function can be invoked before the upload stream is fully closed, especially for large files or slow connections. Using Pub/Sub as an intermediary ensures the notification is delivered only after the object is finalized, and it also provides at-least-once delivery semantics, which can be combined with a dead-letter queue for reliability.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 data processing systems — This question tests Designing data processing systems — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use Cloud Pub/Sub notifications for Cloud Storage and trigger the function from the subscription. — Option D is correct because Cloud Storage object finalize notifications are sent only after the entire file has been written and committed. By using Pub/Sub notifications for Cloud Storage and triggering the Cloud Function from the subscription, you decouple the trigger from the upload process, ensuring the function only runs when the file is fully available. This eliminates the race condition where the function is triggered before the upload completes.
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.
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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