PDE Ingesting and Processing the Data Practice Question
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.
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.
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