When does the need for organizing a Consumer Group in Kafka arise?

The need for organizing a Consumer Group in Apache Kafka arises when there is a requirement to ensure scalable and parallel message consumption from a topic by multiple consumers.

Main reasons:

• Load distribution: A Consumer Group allows multiple consumers to read from a single topic collectively, with each partition of the topic being processed by only one consumer in the group.
• Fault tolerance: If one consumer fails, another consumer in the group can take over its partitions.
• Guarantee of processing order: Since partitions are read by a single consumer, the order of messages within a partition is preserved.

Example: if you have a topic with 4 partitions and 2 consumers in a single Consumer Group, each consumer will read from two 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 is necessary for scaling and reliable message processing in Kafka.