Java consumer client in Kafka - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Kafka consumer with the correct group ID.

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("group.id", "[1]");
props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");

Drag options to blanks, or click blank then click option'

Amy-group

Bgroup-123

Cdefault

Dconsumer1

Attempts:

3 left

💡 Hint

Common Mistakes

Using an empty string as group ID

Using an invalid property key

The group ID identifies the consumer group. Here, 'consumer1' is used as the group ID.

2fill in blank

medium

Complete the code to subscribe the consumer to a topic named 'test-topic'.

Kafka

KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer.[1](Collections.singletonList("test-topic"));

Drag options to blanks, or click blank then click option'

Asubscribe

Bconnect

Clisten

Dassign

Attempts:

3 left

💡 Hint

Common Mistakes

Using assign() instead of subscribe()

Using a non-existent method like listen()

The subscribe() method is used to subscribe the consumer to one or more topics.

3fill in blank

hard

Fix the error in the code to correctly poll messages from Kafka.

Kafka

ConsumerRecords<String, String> records = consumer.[1](100);

Drag options to blanks, or click blank then click option'

Apoll

Bfetch

Cread

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using fetch() or read() instead of poll()

Passing wrong argument types to poll()

The poll() method is used to fetch records from Kafka. The argument should be a Duration in newer versions, but here an integer is used for simplicity.

4fill in blank

hard

Fill both blanks to process each record and print its key and value.

Kafka

for (ConsumerRecord<String, String> record : records) {
    System.out.println("Key: " + record.[1]() + ", Value: " + record.[2]());
}

Drag options to blanks, or click blank then click option'

Akey

Bvalue

CgetKey

DgetValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using key() or value() methods which do not exist

Trying to access key or value as public fields

The ConsumerRecord class provides getKey() and getValue() methods to access the key and value.

5fill in blank

hard

Fill all three blanks to commit offsets synchronously after processing records.

Kafka

try {
    consumer.[1]();
} catch ([2] e) {
    e.[3]();
}

Drag options to blanks, or click blank then click option'

AcommitSync

BcommitAsync

CprintStackTrace

DInterruptedException

Attempts:

3 left

💡 Hint

Common Mistakes

Using commitAsync() instead of commitSync()

Catching wrong exception types

Not printing the stack trace in catch block

commitSync() commits offsets synchronously. InterruptedException is caught and printStackTrace() prints the error.

Practice

1.

What is the main purpose of a Java consumer client in Kafka?

easy

A. To create Kafka topics

B. To send messages to Kafka topics

C. To read messages from Kafka topics

D. To delete Kafka topics

Solution

1. Step 1: Understand Kafka client roles

   Kafka clients include producers (send messages) and consumers (read messages).

2. Step 2: Identify the consumer's role

   The Java consumer client reads messages from Kafka topics to process or display them.

3. Final Answer:

   To read messages from Kafka topics -> Option C

4. Quick Check:

   Consumer = reads messages [OK]

Hint: Consumer reads, producer writes messages in Kafka [OK]

Common Mistakes:

• Confusing consumer with producer
• Thinking consumer creates topics
• Assuming consumer deletes topics

2.

Which of the following is the correct way to subscribe a Kafka consumer to a topic named my-topic?

consumer._____ (Collections.singletonList("my-topic"));

easy

A. subscribe

B. assign

C. poll

D. commitSync

Solution

1. Step 1: Understand consumer methods

   The subscribe method registers the consumer to listen to topics.

2. Step 2: Match method to code

   Using consumer.subscribe(Collections.singletonList("my-topic")) correctly subscribes to the topic.

3. Final Answer:

   subscribe -> Option A

4. Quick Check:

   Subscribe = listen to topics [OK]

Hint: Use subscribe() to listen to topics, not assign() [OK]

Common Mistakes:

• Using assign() instead of subscribe()
• Calling poll() to subscribe
• Using commitSync() incorrectly

3.

What will the following Java Kafka consumer code print if the topic has one message with value "Hello Kafka"?

ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
for (ConsumerRecord<String, String> record : records) {
    System.out.println(record.value());
}

medium

A. Hello Kafka

B. null

C. ConsumerRecords object

D. No output

Solution

1. Step 1: Poll messages from topic

   The poll method fetches available messages; here it gets one message.

2. Step 2: Print each message value

   The loop prints the value() of each record, which is "Hello Kafka".

3. Final Answer:

   Hello Kafka -> Option A

4. Quick Check:

   Print record.value() = Hello Kafka [OK]

Hint: poll() fetches messages; print record.value() to see content [OK]

Common Mistakes:

• Printing the record object instead of value
• Expecting poll() to return null if no messages
• Confusing key and value fields

4.

Identify the error in this Kafka consumer code snippet:

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");
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(100);
for (ConsumerRecord<String, String> record : records) {
    System.out.println(record.value());
}

medium

A. group.id property is missing

B. subscribe() should use assign()

C. Deserializers are incorrectly set

D. poll() argument should be Duration, not int

Solution

1. Step 1: Check poll() method signature

   In modern Kafka clients, poll requires a Duration argument, not an int.

2. Step 2: Identify incorrect argument type

   The code uses poll(100) (int), should be poll(Duration.ofMillis(100)).

3. Final Answer:

   poll() argument should be Duration, not int -> Option D

4. Quick Check:

   poll() needs Duration argument [OK]

Hint: Use Duration.ofMillis() with poll(), not plain int [OK]

Common Mistakes:

• Passing int to poll() instead of Duration
• Missing group.id property
• Using assign() instead of subscribe()

5.

You want to consume messages from two topics topicA and topicB and print their keys and values. Which code snippet correctly subscribes and processes messages?

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");
props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
consumer._____ (Arrays.asList("topicA", "topicB"));
while (true) {
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
    for (ConsumerRecord<String, String> record : records) {
        System.out.println("Key: " + record.key() + ", Value: " + record.value());
    }
}

hard

A. assign

B. subscribe

C. poll

D. commitSync

Solution

1. Step 1: Choose method to listen to multiple topics

   subscribe accepts a list of topics to listen to, while assign assigns specific partitions.

2. Step 2: Confirm correct usage in code

   Using consumer.subscribe(Arrays.asList("topicA", "topicB")) subscribes to both topics properly.

3. Final Answer:

   subscribe -> Option B

4. Quick Check:

   subscribe() listens to multiple topics [OK]

Hint: Use subscribe() with list to consume multiple topics [OK]

Common Mistakes:

• Using assign() instead of subscribe() for topics
• Calling poll() to subscribe
• Forgetting to print keys