To fully understand Apache Kafka's role you should get a wider picture and know Kafka's use cases. Modern data processing systems try to break with the classic application architecture. You can start from the kappa architecture overview:
- http://milinda.pathirage.org/kappa-architecture.com
In this architecture you don't store the current state of the world in any SQL or key-value database. All data is processed and stored as one or more series of events in an append-only immutable log. Immutable events are easier to replicate and store in a distributed environment. Apache Kafka is a system that is used storing these events and for brokering them between other system components.
