if I don’t set any group id in the Consumer Properties, what group id will the Kafka Consumer be given?
The kafka consumer will not have any consumer group. Instead you will get this error : The configured groupId is invalid
Is there a single default value?
Yes, you can see the consumer.properties file of kafka for reference. The default consumer group id is: group.id=test-consumer-group
Does the client create a random value each time?
No, groupId seems to be mandatory for Java client starting Kafka 0.9.0.x consumers. You can refer to this JIRA: https://issues.apache.org/jira/browse/KAFKA-2648
Do I need to create a different id for each consumer to be sure that each one receives all messages?
Yes, if all consumers use the same group id, messages in a topic are distributed among those consumers. In other words, each consumer will get a non-overlapping subset of the messages. Having more consumers in the same group increases the degree of parallelism and the overall throughput of consumption. On the other hand, if each consumer is in its own group, each consumer will get a full copy of all messages.