Consumer group concept in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kafka consumer group example?

Consider a Kafka topic with 3 partitions and a consumer group with 2 consumers. Each consumer reads messages from assigned partitions. If the topic has 9 messages evenly distributed, how many messages will each consumer receive?

Kafka

Topic partitions: 3
Messages per partition: 3
Consumers in group: 2

# Messages are evenly distributed across partitions.
# Consumers get assigned partitions evenly or as balanced as possible.

A Consumer 1: 4 messages, Consumer 2: 5 messages

B Consumer 1: 3 messages, Consumer 2: 6 messages

C Consumer 1: 5 messages, Consumer 2: 4 messages

D Consumer 1: 6 messages, Consumer 2: 3 messages

💡 Hint

Partitions are assigned to consumers. Each partition's messages go to one consumer only.

With 3 partitions and 2 consumers, one consumer gets 2 partitions (6 messages) and the other gets 1 partition (3 messages). So one consumer reads 6 messages, the other 3.

🧠 Conceptual

intermediate

What happens when a new consumer joins an existing consumer group?

In Kafka, when a new consumer joins a consumer group that is already consuming from a topic, what is the expected behavior?

A The new consumer is ignored until the current consumers disconnect.

B The new consumer starts consuming from the last committed offset without affecting others.

C The group triggers a rebalance and partitions are reassigned among all consumers.

D The new consumer consumes from all partitions alongside existing consumers.

💡 Hint

Think about how Kafka ensures each partition is consumed by only one consumer in the group.

When a new consumer joins, Kafka triggers a rebalance to redistribute partitions evenly among all consumers in the group.

🔧 Debug

advanced

Why does Consumer 3 not receive any messages?

Given a topic with 2 partitions and a consumer group with 3 consumers, one consumer is not receiving any messages. Here is the consumer group assignment log snippet:

Consumer 1 assigned partitions: 0, 1
Consumer 2 assigned partitions: 
Consumer 3 assigned partitions: 

Why is Consumer 3 not receiving messages?

A Because the number of consumers exceeds the number of partitions, one consumer gets no partitions.

B Because Consumer 3 has a network issue and cannot receive messages.

C Because Kafka assigns partitions randomly and Consumer 3 was skipped by mistake.

D Because Consumer 3 is not subscribed to the topic.

💡 Hint

Consider how Kafka assigns partitions when consumers outnumber partitions.

Kafka assigns partitions so that each partition is consumed by only one consumer. If there are more consumers than partitions, some consumers get no partitions.

📝 Syntax

advanced

What error does this Kafka consumer group code produce?

Consider this Kafka consumer group code snippet in Java:

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

KafkaConsumer consumer = new KafkaConsumer<>(props);
consumer.subscribe(Arrays.asList("my-topic"));

while(true) {
    ConsumerRecords records = consumer.poll(Duration.ofMillis(100));
    for (ConsumerRecord record : records) {
        System.out.println(record.value());
    }
}
consumer.close();

What error will this code produce?

A Compilation error because consumer.close() is unreachable after infinite loop.

B NullPointerException because props is missing required properties.

C Compilation error due to missing import statements.

D No error; code runs and consumes messages correctly.

💡 Hint

Look at the infinite loop and the position of consumer.close().

The infinite while(true) loop makes consumer.close() unreachable, causing a compilation error (unreachable statement).

🚀 Application

expert

How many partitions will each consumer get after rebalance?

A Kafka topic has 7 partitions. A consumer group has 4 consumers. After a rebalance, how many partitions will each consumer be assigned?

A Consumers 1-2 get 3 partitions each, Consumers 3-4 get 0 partitions.

B Consumers 1-3 get 2 partitions each, Consumer 4 gets 1 partition.

C Each consumer gets exactly 1 partition, and 3 partitions remain unassigned.

D Consumers 1-4 get 1 partition each, and 3 partitions are assigned to a new consumer.

💡 Hint

Partitions are distributed as evenly as possible among consumers.

7 partitions divided among 4 consumers means three consumers get 2 partitions each (2*3=6) and one consumer gets 1 partition (1*1=1), totaling 7.