Queue vs event vs worker?

Definition:

A queue holds work waiting to be processed, a worker executes that work, and an event describes a fact that already happened. Commands request an action, while events can notify multiple interested consumers. Delivery and acknowledgement rules determine what happens when workers crash or messages are repeated.

Key points:

• A queue holds work; a worker processes it.

• An event describes something that happened, such as OrderCreated.

• Delivery may repeat, so consumers need safe duplicate handling.

• Persist processing state before acknowledging when loss would be unacceptable, and use operation IDs to recognize repeated delivery.

Interview point:

• Separate delivery guarantees from business-effect guarantees; duplicate delivery can be normal even in a healthy system.