Kafka - not all consumers receive subscribed message

Question

To generically publish messages using Kafka, I'm using the class name as the topic:

kafkaProducer.send(new ProducerRecord(object.getClass().getName(), new DomainObjectAdapter(object).toJsonString()));

And the consumers subscribe to the classes they are interested in:

    for(Object sub:_subscriptions)
        topics.add(sub.getClass().getName());
    _kafkaConsumer.subscribe(topics);

The problem is, only one of the consumers ever receive the subscribed to message. My understanding is that kafka will assign a unique partition to each subscriber (if available). I currently only have 2 subscribers and my kafka server.properties specified 4 partitions. It appears that all consumers are reading from the same partition. Perhaps Kafka is a poor choice for a service bus due to this apparent limitation. Any help would be much appreciated!

Kafka consumer properties:

    properties.put("bootstrap.servers", _settings.getEndpoint());
    properties.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
    properties.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
    properties.put("enable.auto.commit", "false");
    properties.put("group.id", "TestGroup");
    properties.put("auto.offset.reset","earliest");

Kafka producer properties:

    properties.put("bootstrap.servers",_settings.getEndpoint());
    properties.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
    properties.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

Server properties (the only thing I changed from the default properties):

num.partitions=4

Note: I've also tried consumer settings as:

    properties.put("bootstrap.servers", _settings.getEndpoint());
    properties.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
    properties.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
    properties.put("auto.commit.interval.ms","1000");
    properties.put("enable.auto.commit", "true");
    properties.put("group.id", "testGroup");
    properties.put("auto.offset.reset","latest");
    properties.put("partition.assignment.strategy", "org.apache.kafka.clients.consumer.RoundRobinAssignor");

Answer 1

If all your consumers have the same consumer group (group.id property) then only one consumer from the group will receive the message. If you want all the consumers to receive the message, they need to have different group.id.

To check which consumers are bound to the partitions of the topic, you can use the following command

./bin/kafka-consumer-groups.sh --bootstrap-server yourhost:9092 --group testGroup --describe