What is the difference in how a consumer works with an explicit group.id and without it?

In Kafka consumer, the group.id parameter defines a group of consumers that jointly read messages from topics.

• With an explicit group.id:

  • Multiple consumers with the same group.id form a group, and Kafka distributes the topic partitions among them.
  • This ensures load balancing and guarantees that each message is processed by exactly one consumer in the group.
  • It allows scaling message processing.

• Without group.id:

  • Each consumer operates as an independent consumer.
  • All consumers receive all messages from the topic (subscription in "each for itself" mode).
  • No load balancing, messages are not distributed among consumers.

Thus, using group.id enables cooperative message processing with load balancing, while the absence of group.id is a mode where each consumer receives the full message stream.