PDE Designing Data Processing Systems Practice Question
You are designing a streaming pipeline that needs to handle sudden spikes in traffic without losing data. The pipeline uses Pub/Sub and Dataflow. Which configuration ensures data is not lost if Dataflow falls behind?
⚠ Common exam trap
The trap is assuming that exactly-once delivery or push subscriptions prevent data loss, when actually retention and pull-based backpressure are what protect against Dataflow lag.
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 Pub/Sub with a pull subscription and set the message retention duration to 7 days
A Pub/Sub pull subscription with a 7-day message retention duration ensures that if Dataflow falls behind, messages are retained in the subscription backlog for up to 7 days, preventing data loss. Dataflow pulls messages and acknowledges them only after processing, so unacknowledged messages remain available for redelivery. This configuration decouples ingestion from processing and provides a buffer for spikes.
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 Pub/Sub with a pull subscription and set the message retention duration to 7 days
Why this is correct
A pull subscription with seven-day retention keeps unacknowledged messages durably stored while Dataflow's backlog grows during spikes, so no data is dropped. This satisfies the stem's no-loss constraint when the pipeline falls behind, since push subscriptions cannot replay expired messages.
- ✗
Use Cloud Pub/Sub Lite with a smaller retention period
Why it's wrong here
Pub/Sub Lite caps retention at 7 days and its smaller quota cannot absorb sustained spikes, so Dataflow backlog beyond that window is discarded. It suits cost-sensitive, high-throughput workloads with predictable volume where short retention is acceptable, not burst buffering.
- ✗
Use Pub/Sub with a push subscription and increase the acknowledgment deadline
Why it's wrong here
Push subscriptions with extended ack deadlines still redeliver only until the message retention window expires; a sustained spike exhausts that window and messages are dropped. Push suits low-latency webhook delivery to endpoints that acknowledge promptly, not pipelines that fall behind.
- ✗
Use Pub/Sub with exactly-once delivery and Dataflow with at-least-once processing
Why it's wrong here
Exactly-once delivery applies to publish/ack semantics, not to Dataflow's processing guarantees; at-least-once processing still permits duplicate emission and does not extend message retention, so a lagging pipeline loses nothing only while the subscription backlog persists. It targets duplicate suppression, not spike buffering.
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Last reviewed September 2026 · checked against the official Google Cloud exam blueprint
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