Consume messages from Google Cloud Pub/Sub reliably with at-least-once delivery, correct ack deadlines, and a dead-letter topic

Steps

1. Create a subscription with a pull or push delivery type; set ackDeadlineSeconds to slightly longer than your worst-case processing time (maximum is 600 seconds)
2. When pulling, use the streaming pull client (StreamingPull RPC) rather than synchronous pull for throughput; acknowledge each message by message ID before the ack deadline expires
3. Extend the ack deadline mid-processing by calling ModifyAckDeadline if processing may exceed the initial deadline; most client libraries do this automatically when auto-ack extension is enabled
4. Configure a dead-letter topic on the subscription by setting deadLetterPolicy.deadLetterTopic to a separate topic and deadLetterPolicy.maxDeliveryAttempts (5–100); failed messages are forwarded after that many unacknowledged deliveries
5. Grant the Pub/Sub service account the roles/pubsub.publisher role on the dead-letter topic and roles/pubsub.subscriber on the original subscription so the service can forward and nack messages automatically
6. Monitor the subscription/oldest_unacked_message_age and subscription/dead_letter_message_count Cloud Monitoring metrics to detect processing lag and DLQ build-up