PDE Designing Data Processing Systems Practice Question
A company is using Pub/Sub to ingest clickstream events. They need to ensure that events are delivered to a subscriber at least once, but duplicates can be tolerated. They also need to filter events by type before processing. Which subscription configuration should be used?
⚠ Common exam trap
PDE often tests the confusion between at-least-once and exactly-once delivery semantics, tricking candidates into selecting exactly-once when the scenario explicitly says duplicates are acceptable.
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
✓
Pull subscription with a filter on event type attribute
A pull subscription with a filter on the event type attribute satisfies both requirements: Pub/Sub's at-least-once delivery is the default for pull subscriptions, so duplicates are tolerated, and message filtering via subscription filter expressions (on attributes like event type) prevents unwanted events from being delivered. This is the only option that combines at-least-once semantics with server-side filtering.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Pull subscription with exactly-once delivery enabled
Why it's wrong here
Exactly-once delivery suppresses redelivery, which contradicts the stated tolerance for duplicates, and the pull configuration specifies no filter, so events are not selected by type. It is tempting because exactly-once sounds strictly stronger, and would be correct where duplicate processing is unacceptable.
- ✗
Push subscription with no filter
Why it's wrong here
A push subscription with no filter delivers every published message to the endpoint, so clickstream events of unwanted types still arrive and are processed. It is tempting because push endpoints are simple to operate, and would be correct when all events are needed and no attribute-based selection is required.
- ✓
Pull subscription with a filter on event type attribute
Why this is correct
A pull subscription supports attribute filters, so Pub/Sub discards non-matching messages server-side before delivery, satisfying the filtering requirement. Pull delivery retains at-least-once semantics, meaning occasional duplicates may reach the subscriber, which the stem explicitly tolerates.
- ✗
Push subscription with a dead letter topic
Why it's wrong here
A dead letter topic only diverts messages that repeatedly fail acknowledgement; it performs no attribute-based selection, so unwanted event types still reach the subscriber. It is tempting because it improves delivery reliability, and would be correct for isolating poison messages after retry exhaustion.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Google Cloud exam blueprint
This PDE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PDE exam.