How do multiple instances of the ACL service receive messages from Kafka?

Multiple instances of an ACL service consuming messages from Kafka are usually grouped into a single consumer group. Kafka guarantees that each message from a topic will be processed by exactly one instance within the group, distributing partitions among them. Thus, if you have, for example, 3 instances and 6 partitions, Kafka will distribute the partitions among the instances, ensuring parallel processing and load balancing.

Example in Java using KafkaConsumer:

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("group.id", "acl-service-group");
props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");

KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Collections.singletonList("acl-topic"));

while (true) {
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
    for (ConsumerRecord<String, String> record : records) {
        // Message processing
    }
}

Thus, each instance with the same group.id will receive its share of messages, ensuring scalability and fault tolerance.