PDE Designing data processing systems Practice Question
This PDE practice question tests your understanding of designing data processing systems. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 financial services company needs to process high-frequency trading data with strict ordering guarantees. They use Pub/Sub with ordering keys and Dataflow. The pipeline occasionally produces out-of-order results. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The window trigger allows late data to be included after the main output
Option D is correct because Dataflow's default window trigger behavior allows late data to arrive after the main pane is emitted. When using Pub/Sub with ordering keys, late-arriving events (e.g., due to network delays or retries) can be assigned to the correct window but emitted in a separate pane, causing the final output to appear out-of-order relative to the event time. This is a known behavior when combining event-time windows with late data handling.
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.
✗
Dataflow does not preserve order when using multiple workers
Why it's wrong here
Dataflow preserves order within a key even with multiple workers.
✗
Dataflow uses at-least-once processing, which can reorder events
Why it's wrong here
At-least-once does not cause reordering within a key; it may duplicate.
✗
Pub/Sub does not guarantee message ordering
Why it's wrong here
Pub/Sub with ordering keys guarantees order within a key.
✓
The window trigger allows late data to be included after the main output
Why this is correct
Late data can be emitted in a different pane, causing apparent out-of-order results.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that Pub/Sub's lack of ordering guarantees is the primary cause of out-of-order results in Dataflow, when in fact the issue is typically the window trigger and late data handling within Dataflow itself.
Detailed technical explanation
How to think about this question
Dataflow uses event-time processing with watermarks to track progress. When late data arrives after the watermark has passed, the default trigger (AfterWatermark) emits a late pane, which can interleave with earlier panes if not configured with .withAllowedLateness() and a single firing policy. In high-frequency trading, network jitter or retries can cause events to arrive out-of-order, and without setting allowed lateness or using a trigger that discards late data, the pipeline will produce multiple panes per window, leading to out-of-order results in the output PCollection.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: The window trigger allows late data to be included after the main output — Option D is correct because Dataflow's default window trigger behavior allows late data to arrive after the main pane is emitted. When using Pub/Sub with ordering keys, late-arriving events (e.g., due to network delays or retries) can be assigned to the correct window but emitted in a separate pane, causing the final output to appear out-of-order relative to the event time. This is a known behavior when combining event-time windows with late data handling.
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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Question Discussion
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