Message key and value in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kafka producer code snippet?

Consider this Kafka producer code in Java that sends a message with a key and value. What will be printed by the callback?

Kafka

ProducerRecord<String, String> record = new ProducerRecord<>("topic1", "key1", "value1");
producer.send(record, (metadata, exception) -> {
    if (exception == null) {
        System.out.println("Sent to partition: " + metadata.partition() + ", offset: " + metadata.offset());
    } else {
        System.out.println("Error sending message");
    }
});

A Compilation error due to missing key

B Sent to partition: 0, offset: 0

C Sent to partition: null, offset: null

D Error sending message

💡 Hint

The key determines the partition. If the topic has one partition, the message goes there with offset 0.

The producer sends the message with key "key1" to partition 0 (assuming one partition). The callback prints the partition and offset, which start at 0.

🧠 Conceptual

intermediate

What is the role of the message key in Kafka?

In Kafka, what does the message key control?

A It determines the partition to which the message is sent

B It is used as the message payload

C It controls the message timestamp

D It encrypts the message value

💡 Hint

Think about how Kafka distributes messages across partitions.

The message key is used by Kafka's partitioner to decide which partition the message goes to, ensuring messages with the same key go to the same partition.

❓ Predict Output

advanced

What happens if you send a Kafka message with a null key?

Given this Kafka producer code snippet, what will be the effect on partitioning?

Kafka

ProducerRecord<String, String> record = new ProducerRecord<>("topic1", null, "value1");
producer.send(record);

A Message is sent to a random partition

B Message is rejected with NullPointerException

C Message is sent to partition 0 always

D Message is sent to all partitions

💡 Hint

What does Kafka do when the key is missing?

If the key is null, Kafka uses a round-robin or random partitioner to distribute messages evenly across partitions.

🔧 Debug

advanced

Why does this Kafka consumer fail to read messages by key?

A Kafka consumer tries to filter messages by key but gets no results. What is the likely cause?

Kafka

ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(1000));
for (ConsumerRecord<String, String> record : records) {
    if (record.key() != null && record.key().equals("key1")) {
        System.out.println(record.value());
    }
}

A Message value is empty string

B Consumer is subscribed to wrong topic

C record.key() is null causing NullPointerException

D Consumer group id is missing

💡 Hint

Check if all messages have keys before calling equals.

If some messages have null keys, calling equals on null causes NullPointerException and no messages are printed.

🚀 Application

expert

How to ensure messages with the same key are processed in order?

You want to guarantee that all messages with the same key are processed in the order they were sent. Which Kafka feature helps achieve this?

A Setting message timestamp to current time

B Encrypting messages with the same key

C Using multiple consumer groups for parallel processing

D Using the same partition for messages with the same key

💡 Hint

Think about how Kafka guarantees order within partitions.

Kafka guarantees message order only within a partition. Using the same key ensures messages go to the same partition and are processed in order.