Event vs Command vs Query in EDA (and Broker roles).

1. The Trinity of EDA:

• Command: A request for an action to happen in the future (CreateOrder). It can be rejected. Typically point-to-point.
• Event: A factual statement that something happened in the past (OrderCreated). It cannot be retracted. It is broadcasted (one-to-many).
• Query: A request for information (GetOrderDetails). It is idempotent and has no side effects.

2. Role of the Message Broker (Event Bus):

• Decoupling: The producer doesn't need to know who is listening.
• Buffering: Handles spikes in traffic by queuing messages.
• Durability: Ensures messages aren't lost if the consumer is temporarily down.
• Example: Kafka (High throughput, log-based), RabbitMQ (Complex routing, AMQP).