The Publisher/Subscriber Pattern in .NET

Pub/Sub Mastery

Implementing Pub/Sub in .NET 8 is seamless thanks to MassTransit. It abstracts away the complexity of the underlying broker (RabbitMQ/Service Bus) and let's you focus on the messages.

1. Define the Message (POCO)

A message should be a simple record. **Architect Rule:** Messages should be immutable and contain only the minimal data needed for the consumer to do its work (or an ID to look up more data).

2. The Consumer Interface

Implement `IConsumer`. MassTransit will automatically handle the connection, deserialization, and error handling.

public class OrderPlacedConsumer : IConsumer {
    public async Task Consume(ConsumeContext context) {
        // Business logic here
    }
}

3. Retries and Redelivery

If the consumer fails (e.g., the DB is down), MassTransit can automatically retry with an **Exponential Backoff**. This ensures transient errors don't lead to lost data. If it fails 5 times, it moves the message to an **_error** queue for manual intervention.

4. Interview Mastery

Q: "How do you handle 'Message Ordering' in a distributed system?"

Architect Answer: "True global ordering is impossible at scale. Instead, we use **Competing Consumers with Partitioning**. In Kafka, we use a 'Partition Key' (like UserID). All messages for User A go to Partition 1, and Partition 1 only has one consumer. This ensures that for a specific user, events are processed in the order they were produced, while still allowing the system to scale across multiple partitions for other users."

Interview Q&A for this course