A desktop app wants an undo history that survives a crash. An edge gateway buffers sensor readings while its uplink is down and has to replay them, in order, once the link is back. An AI agent writes down every action it takes so a bad run can be replayed later. All three are the same data structure: an append-only sequence of records, plus readers that remember where they stopped. Kafka is that structure, and also a cluster, partitions and the people who tune them.