What is the role and purpose of a consumer group in Apache Kafka?

In Apache Kafka, a consumer group is a group of consumers that jointly read data from topics. The main role of a consumer group is to ensure parallel message processing and scalability.

Each message from a topic partition will be read by only one consumer in the group, which helps distribute the load and avoid duplicate processing. If there are multiple consumers in the group, the topic partitions are distributed among them.

Example: if you have a topic with 4 partitions and 2 consumers in the group, each consumer will read data from 2 partitions. If a third consumer is added, one of them will be left without partitions, as there are fewer partitions than consumers.

Thus, a consumer group provides:

• Scalability of data reading
• Guarantee that each message is processed exactly once within the group
• The ability to horizontally scale the consumer application.