Kafka consumer receives message if set group_id to None, but it doesn’t receive any message if not None?

One partition of a topic can only be consumed by one consumer within the same ConsumerGroup.

If you do not set the group.id, the KafkaConsumer will generate a new, random group.id for you. As this group.id is unique you will see data is being consumed.

If you have multiple consumers running with the identical group.id, only one consumer will read the data whereas the other one stays idle not consuming anything.