Short answer: choose AWS SQS when the shipment-update fanout needs the least complicated retry path. Choose Google Cloud Pub/Sub when its greater configuration flexibility justifies creating and operating an additional subscription for dead-letter handling. The right alert is the one that fires before the latency SLO is already lost.

A page fires at 02:17. The on-call sees shipment updates still being accepted, but one subscriber has stopped acknowledging messages. Retries are accumulating, and the fanout's tail latency is moving away from its SLO. Work backwards from that page: what signal should have fired earlier, and what action would have contained the failure?

For this fintech workflow, alert on message age and retry pressure at the subscriber boundary, then move poison messages to a DLQ after a deliberate delivery limit.…