- A
Use the legacy streaming inserts with insertId for deduplication
Why wrong: Legacy streaming inserts do not guarantee exactly-once; insertId deduplication is best-effort for 1 minute.
- B
Use at-least-once delivery on Pub/Sub and idempotent writes to BigQuery
Why wrong: This can achieve effectively-once but not exactly-once; duplicates may still occur.
- C
Use the Storage Write API in buffered mode with deduplication logic
Why wrong: Buffered mode does not provide exactly-once guarantees; committed mode is required.
- D
Use the Storage Write API in committed mode and enable exactly-once semantic in Dataflow
Committed mode guarantees exactly-once writes, and Dataflow can coordinate with Pub/Sub to avoid duplicates.
Question 284 of 1,000
PDE Ingesting and Processing the Data Practice Question
This PDE practice question tests your understanding of ingesting and processing the data. 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.
You are designing a Dataflow pipeline that needs to exactly-once process events from Pub/Sub and write to BigQuery using the Storage Write API. The pipeline may restart and could reprocess some messages. What setting ensures exactly-once semantics for the output?
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
✓
Use the Storage Write API in committed mode and enable exactly-once semantic in Dataflow
The Storage Write API supports exactly-once semantics when used with the 'committed' mode, which ensures each record is written exactly once. The pipeline also needs to use Pub/Sub with message IDs and deduplication. The other options either do not provide exactly-once or are unreliable.
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.
- ✗
Use the legacy streaming inserts with insertId for deduplication
Why it's wrong here
Legacy streaming inserts do not guarantee exactly-once; insertId deduplication is best-effort for 1 minute.
- ✗
Use at-least-once delivery on Pub/Sub and idempotent writes to BigQuery
Why it's wrong here
This can achieve effectively-once but not exactly-once; duplicates may still occur.
- ✗
Use the Storage Write API in buffered mode with deduplication logic
Why it's wrong here
Buffered mode does not provide exactly-once guarantees; committed mode is required.
- ✓
Use the Storage Write API in committed mode and enable exactly-once semantic in Dataflow
Why this is correct
Committed mode guarantees exactly-once writes, and Dataflow can coordinate with Pub/Sub to avoid duplicates.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
- Use explanations to understand the rule behind the answer.
TExam Day Tips
- Underline the problem statement mentally.
- 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 PDE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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Ingesting and Processing the Data — study guide chapter
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FAQ
Questions learners often ask
What does this PDE question test?
Ingesting and Processing the Data — This question tests Ingesting and Processing the Data — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use the Storage Write API in committed mode and enable exactly-once semantic in Dataflow — The Storage Write API supports exactly-once semantics when used with the 'committed' mode, which ensures each record is written exactly once. The pipeline also needs to use Pub/Sub with message IDs and deduplication. The other options either do not provide exactly-once or are unreliable.
What should I do if I get this PDE question wrong?
Identify which PDE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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