Pub/Sub At-Least-Once Delivery

Quick Answer

The correct choice is to use push subscriptions with a dead-letter topic. This configuration ensures at-least-once delivery because Cloud Pub/Sub will automatically retry pushing a message if the subscriber endpoint returns an error, and only after exhausting the maximum delivery attempts—defaulting to five—does the system forward the undelivered message to a dead-letter topic for later analysis or reprocessing. This guarantees that no message is silently lost; it is either successfully processed or preserved for manual intervention, directly satisfying the at-least-once requirement. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how Pub/Sub’s retry mechanism and dead-lettering work together to handle processing failures without dropping data. A common trap is assuming a pull subscription with manual acknowledgment alone provides the same guarantee, but without a dead-letter topic, messages that repeatedly fail can be lost if the subscriber stops acknowledging them. Memory tip: think “push, retry, then dead-letter” as the three-step safety net for at-least-once delivery.