PDE Designing Data Processing Systems Practice Question
A Dataflow pipeline using Apache Beam processes unbounded data from Pub/Sub. The pipeline uses fixed windows of 1 minute and a trigger that fires early every 30 seconds and at watermark. The team observes that the output pane for window [10:00:00, 10:01:00) contains events with timestamps from 10:00:15 and 10:00:45, but also an event with timestamp 10:02:00. What is the most likely cause?
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
✓
Allowed lateness is set to more than 1 minute, so late data is still included in its original window
Late data can arrive after the watermark has passed, and with allowed lateness, it can be included in the original window. The event with timestamp 10:02:00 is late data that arrived after the watermark for window [10:00:00, 10:01:00), but within the allowed lateness period. It is not a trigger issue because the trigger fires correctly; the event is simply late.
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 trigger is firing too early, causing the window to close prematurely
Why it's wrong here
The trigger fires early and at watermark, but that does not cause late data to appear in a different window. Late data is still assigned to its original window if allowed lateness is set.
- ✓
Allowed lateness is set to more than 1 minute, so late data is still included in its original window
Why this is correct
When allowed lateness > 0, late data (with timestamp after window end but within allowed lateness) is still included in the correct window. The event with timestamp 10:02:00 is 1 minute late for window [10:00, 10:01), so allowed lateness must be at least 1 minute.
- ✗
The watermark is incorrectly estimated, allowing late data to be included
Why it's wrong here
Watermark estimation can be imperfect, but the event's timestamp is after the window end, so it is late by definition. Allowed lateness is the mechanism that lets it be included, not watermark error.
- ✗
The window duration is actually 2 minutes due to a misconfiguration
Why it's wrong here
If the window were 2 minutes, the event would fall into [10:00, 10:02) or similar; but the output pane is labeled as [10:00,10:01), indicating the window is 1 minute fixed.
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