The answer is to use exactly one subscription for both the Cloud Function and Dataflow job. This is correct because Pub/Sub message ordering guarantees FIFO delivery only within a single subscription; when you enable message ordering on the topic and set an ordering key like device_id, all messages with that key are delivered to subscribers in publish order, but this guarantee breaks across multiple subscriptions since each subscription maintains its own independent delivery queue. On the Google Professional Data Engineer exam, this scenario tests your understanding that ordering is per subscription, not per topic—a common trap is assuming you can split messages across subscriptions while preserving order. The key insight is that if you need ordered processing for a specific device_id, both consumers must read from the same subscription to maintain sequence. Memory tip: think of a single subscription as a single checkout line—everyone in line gets served in order, but if you split into two lines, the order gets scrambled.
PDE Designing data processing systems Practice Question
This PDE practice question tests your understanding of designing data processing systems. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 company uses Pub/Sub to decouple services. They have a topic with two subscriptions: Subscription A is a push subscription that sends messages to a Cloud Function; Subscription B is a pull subscription used by a Dataflow job. They need to ensure that messages are processed in order for a specific device_id. Which TWO configurations should they apply?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable message ordering on the topic and set an ordering key for each message.
Option A is correct because enabling message ordering on the topic and setting an ordering key (e.g., device_id) ensures that messages with the same key are delivered to subscribers in the order they were published. This is a fundamental Pub/Sub feature that guarantees FIFO (first-in, first-out) delivery per ordering key, which directly addresses the requirement for processing messages in order for a specific device_id.
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.
✓
Enable message ordering on the topic and set an ordering key for each message.
Why this is correct
Ordering key is required for ordered delivery.
Related concept
Read the scenario before looking for a memorised answer.
✗
Disable duplicate filtering on the topic.
Why it's wrong here
Duplicate filtering doesn't relate to ordering.
✗
Configure the Cloud Function to retry on failure with exponential backoff.
Why it's wrong here
Retries don't preserve order.
✓
Use exactly one subscription for both the Cloud Function and Dataflow job.
Why this is correct
Ordering is guaranteed per subscription; having two subscriptions can lead to out-of-order processing across consumers of the same ordering key.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a single subscription with multiple concurrent consumers.
Why it's wrong here
Multiple consumers break ordering.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that multiple subscriptions or multiple consumers can maintain ordering independently, but in Pub/Sub, ordering is per subscription and per ordering key, and only a single subscriber per subscription can guarantee FIFO delivery.
Detailed technical explanation
How to think about this question
Under the hood, Pub/Sub assigns a sequence number to each message within an ordering key group, and the server ensures that messages with the same ordering key are delivered to subscribers in that sequence. However, if a subscriber fails to acknowledge a message, the server will block delivery of subsequent messages for that key until the earlier message is acknowledged or the deadline expires, which can cause head-of-line blocking. In real-world scenarios, this means that if one device_id message fails repeatedly, all later messages for that device_id will be delayed, so robust error handling is critical.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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: Enable message ordering on the topic and set an ordering key for each message. — Option A is correct because enabling message ordering on the topic and setting an ordering key (e.g., device_id) ensures that messages with the same key are delivered to subscribers in the order they were published. This is a fundamental Pub/Sub feature that guarantees FIFO (first-in, first-out) delivery per ordering key, which directly addresses the requirement for processing messages in order for a specific device_id.
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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