Partitioner behavior in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the partition chosen for a given key?

Given a Kafka producer configured with 3 partitions and the default partitioner, what partition will the message with key "user123" be sent to?

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
KafkaProducer<String, String> producer = new KafkaProducer<>(props);

String topic = "test-topic";
String key = "user123";
int numPartitions = 3;

// Assume default partitioner uses key.hashCode() % numPartitions
int partition = Math.abs(key.hashCode()) % numPartitions;
System.out.println(partition);

A 1

B 2

C 0

D 3

💡 Hint

Remember the default partitioner uses the hash code of the key modulo the number of partitions.

The default Kafka partitioner calculates the partition by taking the absolute value of the key's hash code modulo the number of partitions. For key "user123" and 3 partitions, the calculation results in partition 1.

❓ Predict Output

intermediate

What happens if the key is null?

In Kafka's default partitioner, what partition will a message be sent to if the key is null and the topic has 4 partitions?

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
KafkaProducer<String, String> producer = new KafkaProducer<>(props);

String topic = "test-topic";
String key = null;
int numPartitions = 4;

// Default partitioner behavior for null key
int partition = new Random().nextInt(numPartitions);
System.out.println(partition);

A Partition 4 (out of range)

B Always partition 0

C Random partition between 0 and 3

D Throws NullPointerException

💡 Hint

Check how the default partitioner handles null keys.

When the key is null, Kafka's default partitioner assigns the message to a random partition among the available partitions.

🧠 Conceptual

advanced

Why use a custom partitioner?

Which of the following is the main reason to implement a custom partitioner in Kafka?

A To automatically retry failed messages

B To encrypt messages before sending

C To improve message serialization speed

D To control how messages are distributed across partitions based on custom logic

💡 Hint

Think about what partitioning controls in Kafka.

A custom partitioner lets you decide which partition a message goes to based on your own rules, helping with load balancing or message ordering.

❓ Predict Output

advanced

What error occurs with invalid partition number?

What error will Kafka producer throw if you try to send a message to partition 5 on a topic with only 4 partitions (0 to 3)?

Kafka

ProducerRecord<String, String> record = new ProducerRecord<>("test-topic", 5, "key", "value");
producer.send(record);

A IllegalArgumentException: Partition out of range

B NullPointerException

C No error, message sent to partition 0

D TimeoutException

💡 Hint

Check Kafka's validation for partition numbers.

Kafka producer validates the partition number and throws IllegalArgumentException if it is outside the valid range.

🧠 Conceptual

expert

How does Kafka ensure message order within a partition?

Which statement best explains how Kafka guarantees message order within a partition?

A Messages are ordered by timestamp regardless of partition

B Messages with the same key always go to the same partition, preserving order for that key

C Kafka uses a global sequence number for all messages

D Messages are randomly assigned to partitions to balance load

💡 Hint

Think about how keys and partitions relate to ordering.

Kafka guarantees order only within a partition. By sending messages with the same key to the same partition, order is preserved for that key.