You are designing an order-processing workflow on Azure Service Bus. All messages for the same OrderID must be processed in strict FIFO order, and you need to maintain per-order state across messages to track the workflow stage. What should you do?

To ensure strict FIFO ordering for messages belonging to the same OrderID and maintain per-order state, Azure Service Bus sessions are the correct approach. Enabling RequiresSession on the queue or subscription is fundamental, as it instructs Service Bus to only deliver messages that belong to a session and allows session-aware receivers to connect. Setting the SessionId of all messages for a given order to the OrderID groups these messages into a single session. This ensures that all messages with the same SessionId are processed sequentially by a single receiver. Finally, using a SessionReceiver allows you to interact with the session state using GetState and SetState, which is crucial for persisting the per-order workflow stage directly within the Service Bus session, ensuring state is maintained across messages for that specific order. Setting MessageId to OrderID does not guarantee FIFO processing or state management. Message deferral is for reordering messages within a single session, not for establishing sessions or managing state across them.