hardMultiple Choice
PDE Practice Question: A financial services company uses Cloud Pub/Sub…
A financial services company uses Cloud Pub/Sub with ordering keys to process transactions in order. Some messages are failing processing and getting stuck. The team wants to ensure that if a message fails, it can be reprocessed later without blocking subsequent messages. What should they implement?
⚠ Common exam trap
Google Cloud often tests the misconception that increasing the acknowledgment deadline or adding flow control can resolve stuck messages with ordering keys, but the real solution is to use a dead letter topic to offload the failing message and unblock the ordered stream.
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
✓
Configure a dead letter topic and handle the failed message separately
A dead letter topic (DLT) allows failed messages to be moved aside after exhausting retry attempts, so they do not block the processing of subsequent ordered messages. In Cloud Pub/Sub, ordering keys require messages with the same key to be delivered in order; if a message fails and is not acknowledged, it blocks all later messages with the same key. By configuring a dead letter topic, the failed message is automatically forwarded to the DLT after a maximum of 5 delivery attempts (default), and the original subscription can continue processing the next messages in order. The team can then reprocess the failed message from the DLT separately, without affecting the order of other messages.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Create multiple subscriptions for the same topic
Why it's wrong here
Multiple subscriptions on one topic each receive every message, so a failing message still blocks its ordering key on whichever subscription processes it, and no redelivery-after-failure mechanism is added. Dead-letter topics with retry policy are what isolate failures. Multiple subscriptions suit fan-out to independent consumers, not ordered retry handling.
- ✗
Use a pull subscription with flow control settings
Why it's wrong here
Flow control limits outstanding messages and bytes per client; it does not remove a stuck message from its ordering key or schedule later reprocessing. Dead-letter topics with retry policy are required. Pull subscriptions with flow control suit protecting consumer memory during high throughput, not recovering messages that repeatedly fail.
- ✓
Configure a dead letter topic and handle the failed message separately
Why this is correct
A dead letter topic captures messages that exceed the maximum delivery attempts, removing them from the subscription so they stop blocking the ordered backlog. The failed transaction is then reprocessed separately, preserving ordering for subsequent messages.
- ✗
Increase the acknowledgment deadline to 600 seconds
Why it's wrong here
Extending the acknowledgment deadline only delays redelivery; the failing message stays outstanding and continues blocking later messages sharing its ordering key. A dead-letter topic with a maximum delivery attempt count moves poison messages aside so the key proceeds. Long deadlines suit slow-but-successful batch processing, not permanent failures.
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