Azure Service Bus: How Topics, Subscriptions, and Dead Letter Queues Work

Azure Service Bus is a cloud messaging service that decouples applications by allowing them to exchange messages asynchronously without needing to be online simultaneously. Unlike direct API calls, it prevents system failures from cascading by letting messages wait safely until the receiving service is ready. While queues deliver each message to a single consumer, topics allow one published message to be independently received by multiple subscribers, each of which can apply filters to receive only relevant messages. When a message repeatedly fails processing, Service Bus automatically moves it to a Dead Letter Queue rather than discarding it, keeping the main queue unblocked. The Dead Letter Queue gives engineers a reliable way to inspect failures, fix underlying issues, and reprocess messages while maintaining a full audit trail for compliance.