PDE Designing Data Processing Systems Practice Question
You need to choose a messaging service for a real-time streaming application that requires low cost and can tolerate occasional message loss. Which service is MOST suitable?
⚠ Common exam trap
PDE often tests the distinction between Pub/Sub and Pub/Sub Lite by emphasizing cost tolerance for message loss — candidates incorrectly default to standard Pub/Sub for 'streaming' without weighing the durability/cost trade-off.
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
✓
Pub/Sub Lite
Pub/Sub Lite is designed for high-volume, low-cost streaming where occasional message loss is acceptable. Unlike standard Pub/Sub, it uses zonal or regional storage with pre-provisioned capacity, which dramatically lowers cost but does not guarantee the same durability or at-least-once delivery semantics under all failure conditions. This makes it the right fit when cost is prioritized over guaranteed delivery.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Cloud Scheduler
Why it's wrong here
Cloud Scheduler is a cron-based job trigger that invokes HTTP endpoints or Pub/Sub at scheduled times; it does not carry application message payloads. It is tempting as a low-cost serverless service, and would be correct for triggering recurring batch jobs rather than streaming events.
- ✓
Pub/Sub Lite
Why this is correct
Pub/Sub Lite provides zonal, low-cost messaging with no replication across zones, so it tolerates occasional message loss while meeting the low-cost constraint. Standard Pub/Sub replicates messages, incurring higher cost for durability the scenario does not require.
- ✗
Pub/Sub
Why it's wrong here
Pub/Sub delivers at-least-once, so duplicate delivery occurs; the stem tolerates loss, not duplication, and message ordering is not guaranteed per subscriber. It tempts as the canonical low-cost streaming ingestion service, and would be correct when durability and fan-out matter more than strict exactly-once semantics.
- ✗
Cloud Tasks
Why it's wrong here
Cloud Tasks queues discrete asynchronous tasks for targeted HTTP or App Engine handlers, not continuous high-throughput streams; it cannot publish to multiple subscribers. It tempts because it is cheap and tolerates retries, but it would be correct for decoupling background jobs or rate-limiting outbound calls, not real-time streaming.
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Last reviewed September 2026 · checked against the official Google Cloud exam blueprint
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