The secret to high availability is Asynchronous Communication. If Service B is down, Service A shouldn't care—it just drops a message in a queue and moves on.
Don't call RabbitMQ or Azure Service Bus APIs directly. Use **MassTransit**. It's an abstraction layer that handles retries, poison pill queues (Dead Letter Queues), and the Saga pattern for you. It allows you to switch your broker from RabbitMQ to Kafka with just a config change.
Q: "What is 'Backpressure' in a messaging system?"
Architect Answer: "Backpressure is when the consumer is slower than the producer. If you send 1,000 messages a second but your DB can only handle 100, the queue will grow until the server crashes. We handle this through **Rate Limiting** on the consumer, or by automatically scaling out more consumer instances (KEDA) to handle the load."