Kafka - The Definitive Guide, ed. 2
Kafka: The Definitive Guide, Real-Time Data and Stream Processing at Scale
Chapter 1. Meet Kafka - Components and Terms
Chapter 2. Installing Kafka
Chapter 3. Kafka Producers - Writing Messages to Kafka
High level producer overview (p.94)
Chapter 4. Kafka Consumers - Reading Data from Kafka
If several applications subscribes to all the messages from the same topic, each application must have a consumer group.
Scaling of reading and processing of messages from the topics happen by adding consumers to an existing consumer group. Each additional consumer in a group will only get a subset of the messages.