Azure Service Bus is an enterprise-grade cloud messaging service. It decouples applications so they do not need to be online at the same time to communicate. Instead of calling each other directly, services send messages to Service Bus and other services pick them up when ready. Service Bus was used extensively at Blue Yonder to connect Logic Apps, Function Apps, and external systems like Salesforce and ServiceNow - it was the backbone of every reliable integration built there.
Why Messaging Instead of Direct API Calls
A direct API call creates tight coupling: System A calls System B over HTTP, and if B is down, A fails immediately; if B is slow, A waits and blocks. Service Bus replaces that direct call with loose coupling instead - System A sends a message to Service Bus, and System B reads it whenever it's ready. This means B can be temporarily down and the message simply waits safely, A never blocks since it's fire-and-forget, multiple systems can read the same message independently, and every message leaves a full audit trail.
Queues vs Topics
A queue is one sender, one receiver - a message goes in one end, exactly one consumer reads it, and once read the message is gone. This fits scenarios where one system should process each message exactly once, like an Order Service sending work to a Fulfillment Service through a queue, where each order gets processed by exactly one worker.
