Consumer poll loop 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 Kafka consumer poll loop in Java. What will be printed to the console?

Kafka

KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Collections.singletonList("my-topic"));
int messageCount = 0;
while (messageCount < 3) {
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
    for (ConsumerRecord<String, String> record : records) {
        System.out.println("Received: " + record.value());
        messageCount++;
    }
}
consumer.close();

A Throws a NullPointerException because poll duration is invalid.

B Prints messages indefinitely without stopping.

C Prints exactly 3 messages from the topic and then stops.

D Prints zero messages and exits immediately.

💡 Hint

Look at the messageCount condition in the while loop.

The loop continues polling until messageCount reaches 3, so it prints exactly 3 messages and then stops.

🧠 Conceptual

intermediate

What happens if you forget to call consumer.poll() in a Kafka consumer loop?

In a Kafka consumer application, what is the effect of not calling consumer.poll() regularly inside the consumer loop?

A The consumer will automatically commit offsets without polling.

B The consumer will continue receiving messages normally without any issues.

C The consumer will throw a compile-time error.

D The consumer will stop receiving messages and may be considered dead by the broker, causing a rebalance.

💡 Hint

Think about how Kafka detects active consumers in a group.

Kafka requires consumers to poll regularly to show they are alive. If polling stops, the broker assumes the consumer is dead and triggers a rebalance.

🔧 Debug

advanced

Why does this Kafka consumer poll loop cause a commit failure?

Examine the following code snippet. Why might this consumer fail to commit offsets properly?

Kafka

KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Arrays.asList("topic1"));
while (true) {
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
    for (ConsumerRecord<String, String> record : records) {
        process(record.value());
    }
    // Missing commit call here
}

A Offsets are never committed because commitSync or commitAsync is not called after processing.

B The consumer throws a CommitFailedException immediately.

C Offsets are committed automatically after poll, so no issue occurs.

D The consumer commits offsets before processing messages, causing data loss.

💡 Hint

Check where offset commits happen in Kafka consumer code.

Kafka consumers must explicitly commit offsets after processing messages. Without commit calls, offsets are not saved, risking duplicate processing.

📝 Syntax

advanced

Which option fixes the syntax error in this Kafka consumer poll loop?

Identify the correct fix for the syntax error in this Kafka consumer poll loop snippet:

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

A Change consumer.poll(100) to consumer.poll(Duration.ofMillis(100))

B Change consumer.poll(100) to consumer.poll(100L)

C Add a semicolon after consumer.poll(100)

D Change for loop to for (record in records)

💡 Hint

Check the expected argument type for poll method in KafkaConsumer.

The poll method expects a Duration object, not an integer. Using Duration.ofMillis(100) fixes the syntax error.

🚀 Application

expert

How to implement a graceful shutdown in a Kafka consumer poll loop?

You want to stop a Kafka consumer poll loop gracefully on receiving a shutdown signal. Which approach correctly implements this?

Kafka

KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.subscribe(Collections.singletonList("topic"));
final AtomicBoolean running = new AtomicBoolean(true);
Runtime.getRuntime().addShutdownHook(new Thread(() -> {
    running.set(false);
    consumer.wakeup();
}));
try {
    while (running.get()) {
        ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
        for (ConsumerRecord<String, String> record : records) {
            process(record.value());
        }
        consumer.commitSync();
    }
} catch (WakeupException e) {
    if (running.get()) throw e;
} finally {
    consumer.close();
}

A Use Thread.stop() to kill the consumer thread immediately.

B Use an AtomicBoolean flag and call consumer.wakeup() in a shutdown hook to exit poll loop safely.

C Ignore shutdown signals and let the JVM terminate the consumer abruptly.

D Call consumer.close() directly inside the poll loop without any flag or wakeup.

💡 Hint

Consider how to interrupt a blocking poll call safely.

The wakeup() method interrupts the blocking poll() call, allowing the loop to exit gracefully when the flag is set.