Java consumer client in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kafka consumer poll loop?

Consider the following Java Kafka consumer code snippet. What will be printed to the console when the consumer polls messages from the topic?

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("group.id", "test-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("my-topic"));

ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(1000));
for (ConsumerRecord<String, String> record : records) {
    System.out.println("Received message: " + record.value());
}
consumer.close();

A Compilation error due to missing imports

B

Received message: Hello
Received message: World

C No output because poll returns empty records

D Runtime exception due to missing group.id property

💡 Hint

Assume the topic 'my-topic' has two messages: "Hello" and "World".

The consumer subscribes to 'my-topic' and polls messages. Since the topic contains two messages, the loop prints each message's value. The group.id is set, so no runtime error occurs. Imports are assumed present.

🧠 Conceptual

intermediate

What does the 'auto.offset.reset' property control in a Kafka consumer?

In Kafka consumer configuration, what is the role of the 'auto.offset.reset' property?

A It controls what happens when there is no initial offset or the current offset is invalid

B It sets the maximum number of messages to fetch in one poll

C It defines the timeout for consumer session heartbeat

D It specifies the serializer class for message keys

💡 Hint

Think about what happens if the consumer group is new or the offset is lost.

The 'auto.offset.reset' property tells the consumer where to start reading if no committed offset exists or the offset is out of range. Common values are 'earliest' or 'latest'.

🔧 Debug

advanced

Why does this Kafka consumer code throw a SerializationException?

Examine the code below. Why does it throw a SerializationException at runtime?

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("group.id", "group1");
props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
// Missing value.deserializer property

KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Collections.singletonList("topic1"));
ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(1000));

A Because the value.deserializer property is missing, so Kafka cannot deserialize message values

B Because the group.id is invalid and causes consumer failure

C Because the topic name is incorrect and causes deserialization failure

D Because the poll timeout is too short and causes serialization error

💡 Hint

Check the deserializer properties carefully.

Kafka requires both key and value deserializers to convert bytes to objects. Missing value.deserializer causes SerializationException when reading message values.

📝 Syntax

advanced

Which option correctly creates a Kafka consumer with generic types for String keys and Integer values?

Select the correct Java code snippet that creates a KafkaConsumer instance with String keys and Integer values.

A KafkaConsumer<String, Integer> consumer = new KafkaConsumer<String, Integer>(props);

B KafkaConsumer<String, Integer> consumer = new KafkaConsumer(props);

C KafkaConsumer<String, Integer> consumer = new KafkaConsumer<>(props);

D KafkaConsumer<String, Integer> consumer = new KafkaConsumer<Integer, String>(props);

💡 Hint

Consider Java's diamond operator and generic type matching.

Option C uses the diamond operator <> to infer types correctly. Option C is valid Java but verbose. Option C lacks generics on constructor, causing raw type warning. Option C swaps key and value types incorrectly.

🚀 Application

expert

How many partitions will the consumer group consume concurrently with this configuration?

Given a Kafka topic with 4 partitions and a consumer group with 3 consumers, how many partitions will each consumer consume concurrently?

A Each consumer will consume 4 partitions concurrently

B Each consumer will consume exactly 1 partition

C One consumer will consume all 4 partitions, others none

D Two consumers will consume 1 partition each, and one consumer will consume 2 partitions

💡 Hint

Partitions are divided among consumers in a group, but not always evenly.

Kafka assigns partitions to consumers in a group as evenly as possible. With 4 partitions and 3 consumers, two consumers get 1 partition each, and one consumer gets 2 partitions.