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.
Network Topology
What is the most likely cause of data duplication after this command?
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 BigQuery sink is not idempotent.
Option D is correct because BigQuery sinks in Dataflow are not idempotent by default; if the pipeline retries writes (e.g., due to worker failures or checkpoint issues), duplicate rows can be inserted into the BigQuery table. This is a known limitation: BigQuery does not support deduplication at the sink level unless you implement custom deduplication logic or use a staging table with merge operations. The command likely triggered a retry scenario, and the non-idempotent sink caused the duplication.
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 Pub/Sub source is not exactly-once.
Why it's wrong here
Pub/Sub exactly-once would prevent duplicates from source, but duplication is likely from sink side.
✗
The pipeline uses at-least-once semantics.
Why it's wrong here
At-least-once can cause duplicates but the pipeline was working before, so the change likely caused it.
✗
The snapshot was taken before scaling.
Why it's wrong here
Snapshots are used for recovery, not for causing duplicates.
✓
The BigQuery sink is not idempotent.
Why this is correct
If the sink is not idempotent, duplicate data can be written when workers are re-added or when job state is replayed.
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 at-least-once semantics alone cause duplication, but the real trap is that the sink's idempotency (or lack thereof) is the decisive factor when retries occur.
Detailed technical explanation
How to think about this question
Under the hood, Dataflow's BigQuery sink uses streaming inserts (tabledata.insertAll) which are not idempotent; each insert request creates a new row even if the data is identical. In contrast, sinks like Cloud Storage (via TextIO) are idempotent because they write to unique file names. A real-world scenario: if a Dataflow pipeline fails after writing to BigQuery but before acknowledging the source, the retry will re-insert the same rows, causing duplicates unless you use a deduplication key or a staging table with a MERGE statement.
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: The BigQuery sink is not idempotent. — Option D is correct because BigQuery sinks in Dataflow are not idempotent by default; if the pipeline retries writes (e.g., due to worker failures or checkpoint issues), duplicate rows can be inserted into the BigQuery table. This is a known limitation: BigQuery does not support deduplication at the sink level unless you implement custom deduplication logic or use a staging table with merge operations. The command likely triggered a retry scenario, and the non-idempotent sink caused the duplication.
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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