PDE Designing Data Processing Systems Practice Question

A company uses Pub/Sub to ingest events from multiple sources. They need to ensure that messages from a specific source are processed in order (per source partition). They also need to deduplicate messages. Which TWO features should they use?

⚠ Common exam trap

The trap is confusing schema enforcement or dead letter topics with ordering and deduplication — candidates pick schema (A) thinking it 'enforces order' or DLQ (B) thinking it 'handles out-of-order messages,' when only ordering keys and exactly-once delivery address the actual requirements.

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

✓

Enable exactly-once delivery on the subscription

Option E is correct because Pub/Sub message ordering is enabled by setting an ordering key on messages; messages sharing the same ordering key are delivered to subscribers in the order they were published, which satisfies the per-source-partition ordering requirement. Option C is correct because enabling exactly-once delivery on the subscription ensures that messages are not redelivered after successful acknowledgment, providing the required deduplication behavior. Option A is incorrect because schemas validate message format and structure, not ordering. Option B is incorrect because a dead letter topic only captures messages that cannot be successfully processed after retries, and does not reorder or deduplicate messages. Option D is incorrect because a large ack deadline only extends the processing window to reduce premature redelivery; it does not guarantee ordering or exactly-once deduplication.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Set a message schema to enforce ordering
Why it's wrong here
A message schema validates payload structure and encoding; it has no mechanism to sequence messages or remove duplicates. It is tempting because schemas are configured on topics alongside ordering, but ordering requires the message ordering property and deduplication requires exactly-once delivery, not schema enforcement.
✗
Use a dead letter topic to handle out-of-order messages
Why it's wrong here
A dead letter topic only diverts messages that repeatedly fail acknowledgement; it neither reorders nor deduplicates. It is tempting because dead letter topics handle problematic messages, but out-of-order delivery is prevented by enabling message ordering on the subscription, while duplicates are removed by exactly-once delivery.
✓
Enable exactly-once delivery on the subscription
Why this is correct
Exactly-once delivery on the subscription removes duplicate messages by tracking acknowledgement state, satisfying the deduplication requirement. However, it does not guarantee ordering; that needs the ordering key or message ordering property set on the publisher side, so this feature alone covers only one of the two stated constraints.
✗
Use a pull subscription with a large ack deadline
Why it's wrong here
Extending the ack deadline only delays redelivery of unacknowledged messages; it cannot enforce per-partition ordering or deduplicate. It is tempting because large ack deadlines reduce duplicate deliveries from retries, but true deduplication requires exactly-once delivery, and ordering requires the ordering property.
✓
Enable message ordering by setting an ordering key
Why this is correct
Setting an ordering key ensures Pub/Sub delivers messages sharing that key in publish order, satisfying the per-source sequencing constraint. Ordering keys are the specific mechanism for ordered delivery; deduplication is handled separately by the data plane's message ID.

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PDE Designing Data Processing Systems Practice Question

Why each option matters

Option-by-option breakdown

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