Consumers label themselves with a consumer group name, and each record
published to a topic is delivered to one consumer instance within each
subscribing consumer group. Consumer instances can be in separate
processes or on separate machines.If all the consumer instances have the same consumer group, then the
records will effectively be load balanced over the consumer instances.If all the consumer instances have different consumer groups, then
each record will be broadcast to all the consumer processes.
The group.id is a string that uniquely identifies the group of consumer processes to which this consumer belongs.
(Kafka intro)