Event

An event is the fundamental unit of data in Kafka: an immutable record, published to a topic, representing a business fact that already occurred: "payment approved," "PIX received," "account blocked." Unlike a command (which instructs a future action), an event only reports something that already happened; whoever consumes it independently decides what to do with that information. It's this nature (a past fact, not an instruction) that lets multiple consumers react to the same event in completely different ways, without the producer needing to know any of them.