MLS-C01 Data Engineering Practice Question
A company uses Amazon Kinesis Data Analytics for Apache Flink to process real-time clickstream data. The application uses event time and watermarks for windowed aggregations. The team notices that the output from tumbling windows is delayed, and many late records are being dropped. What is the MOST likely cause?
⚠ Common exam trap
Many exam-takers confuse watermark stall (which delays output) with late record dropping—both involve watermarks, but stalled watermarks prevent window closure (no output), whereas low `allowedLateness` closes windows on time but discards subsequent late arrivals.
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
✓
The allowed lateness is set too low, causing late records to be discarded
The described symptoms—delayed output and dropped late records—are classic indicators that the `allowedLateness` parameter is set too low. In Apache Flink, event-time processing relies on watermarks to determine when a window is complete; if `allowedLateness` is too short, any record arriving after the watermark passes the window's end time is discarded as late. The team's observation that many late records are being dropped directly points to this configuration issue.
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 checkpointing interval is too long, causing state to be lost
Why it's wrong here
Checkpointing interval affects failure recovery, not late record handling.
- ✗
The parallelism is too low, causing backpressure
Why it's wrong here
Backpressure affects throughput but does not cause late records to be dropped.
- ✗
The source is marking itself as idle, causing watermarks to stall
Why it's wrong here
Idle sources can cause watermarks to not advance, but the symptom is delayed windows, not dropping late records.
- ✓
The allowed lateness is set too low, causing late records to be discarded
Why this is correct
Low allowed lateness means records arriving after the watermark are dropped.
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