PDE Ingesting and Processing the Data Practice Question
A data engineer is designing a Dataflow pipeline that reads from Pub/Sub and writes to BigQuery using the Storage Write API in exactly-once mode. The pipeline must handle late-arriving data (up to 1 hour) and maintain correct aggregation results. Which trigger configuration should they use?
⚠ Common exam trap
Google often tests the misconception that any trigger with a time-based interval (like fixed 5-minute triggers) can handle late data, but only watermark-based triggers with allowed lateness correctly align with event-time windows and late-arriving records.
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
✓
After watermark trigger with allowed lateness of 1 hour
The After watermark trigger with allowed lateness of 1 hour is the correct choice because it emits results once the watermark passes the end of the window, while still accepting late-arriving events up to 1 hour past the watermark and including them in the correct event-time window. This aligns with the requirement to handle up to 1 hour of late data and maintain correct aggregation results. Note that exactly-once delivery to BigQuery is provided by the Storage Write API sink itself, not by the trigger; the trigger choice affects completeness and correctness of the aggregation, not the exactly-once guarantee.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Default trigger without allowed lateness
Why it's wrong here
Default trigger discards late data; not suitable for late arrivals.
- ✗
After count trigger with count of 1000
Why it's wrong here
After count trigger does not consider event time; not suitable for late data handling.
- ✗
Fixed time trigger every 5 minutes
Why it's wrong here
Fixed time triggers ignore event time and may produce inaccurate aggregations for late data.
- ✓
After watermark trigger with allowed lateness of 1 hour
Why this is correct
This configuration ensures late data within 1 hour is included in aggregations.
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