Rebalancing behavior in Kafka - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the consumer group ID for Kafka consumer.

Kafka

props.put("group.id", [1]);

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

A"localhost"

B"topic-name"

C"kafka-server"

D"my-group"

Attempts:

3 left

💡 Hint

Common Mistakes

Using server or topic names instead of a group ID.

Leaving the group.id property unset.

The group.id property sets the consumer group ID, which is required for rebalancing behavior.

2fill in blank

medium

Complete the code to subscribe the consumer to a topic named "orders".

Kafka

consumer.subscribe(Collections.[1]("orders"));

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

AemptyList

BsingletonList

Csingleton

DlistOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using emptyList which subscribes to no topics.

Using singleton which is not a method in Collections.

The subscribe method expects a collection of topics. singletonList creates a list with one element.

3fill in blank

hard

Fix the error in the rebalance listener method signature.

Kafka

consumer.subscribe(topics, new ConsumerRebalanceListener() {
    @Override
    public void [1](Collection<TopicPartition> partitions) {
        // handle partitions revoked
    }

    @Override
    public void onPartitionsAssigned(Collection<TopicPartition> partitions) {
        // handle partitions assigned
    }
});

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

AonPartitionsRevoked

BonRebalance

CpartitionsRevoked

DpartitionsAssigned

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names like partitionsRevoked or onRebalance.

Confusing assigned and revoked methods.

The correct method name is onPartitionsRevoked to handle revoked partitions during rebalance.

4fill in blank

hard

Fill both blanks to commit offsets synchronously after partitions are assigned.

Kafka

public void onPartitionsAssigned(Collection<TopicPartition> partitions) {
    consumer.[1]();
    consumer.[2](offsets);
}

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

AcommitSync

BcommitAsync

Cseek

Dpoll

Attempts:

3 left

💡 Hint

Common Mistakes

Using poll instead of commit methods.

Calling seek instead of commit.

commitSync() commits the current offsets synchronously; commitSync(offsets) commits specific offsets synchronously.

5fill in blank

hard

Fill all three blanks to implement a rebalance listener that commits offsets and seeks to correct positions.

Kafka

consumer.subscribe(topics, new ConsumerRebalanceListener() {
    @Override
    public void onPartitionsRevoked(Collection<TopicPartition> partitions) {
        consumer.[1]();
    }

    @Override
    public void onPartitionsAssigned(Collection<TopicPartition> partitions) {
        for (TopicPartition partition : partitions) {
            consumer.[2](partition, offsets.get(partition));
        }
        consumer.[3]();
    }
});

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

AcommitSync

Bseek

CcommitAsync

Dpoll

Attempts:

3 left

💡 Hint

Common Mistakes

Using poll instead of commit or seek.

Not committing offsets before rebalance.

On partitions revoked, commitSync is called to save offsets. On assigned, seek moves to saved offsets, then commitAsync commits asynchronously.

Practice

1. What triggers a rebalance in a Kafka consumer group?

easy

A. When a consumer commits an offset

B. When a message is consumed

C. When a topic is deleted

D. When a consumer joins or leaves the group

Solution

1. Step 1: Understand rebalance triggers

   Rebalancing happens when the group membership changes or partitions change.

2. Step 2: Identify correct trigger

   Joining or leaving a consumer causes the group to rebalance to share partitions evenly.

3. Final Answer:

   When a consumer joins or leaves the group -> Option D

4. Quick Check:

   Consumer join/leave triggers rebalance [OK]

Hint: Rebalance happens on group membership changes [OK]

Common Mistakes:

• Thinking rebalance happens on every message
• Confusing topic deletion with rebalance
• Assuming offset commit triggers rebalance

2. Which of the following is the correct way to listen for rebalance events in Kafka consumer code?

easy

A. consumer.subscribe(topics, new ConsumerRebalanceListener() {...})

B. consumer.on('rebalance', callback)

C. consumer.addRebalanceListener(callback)

D. consumer.listenRebalance(callback)

Solution

1. Step 1: Recall Kafka consumer API

   Kafka's Java consumer uses subscribe with a ConsumerRebalanceListener to handle rebalance events.

2. Step 2: Match correct syntax

   Only consumer.subscribe(topics, new ConsumerRebalanceListener() {...}) shows the correct method signature for rebalance listener.

3. Final Answer:

   consumer.subscribe(topics, new ConsumerRebalanceListener() {...}) -> Option A

4. Quick Check:

   Use subscribe with ConsumerRebalanceListener [OK]

Hint: Use subscribe with ConsumerRebalanceListener for rebalance [OK]

Common Mistakes:

• Using non-existent methods like on or addRebalanceListener
• Confusing event listeners with other APIs
• Missing the listener parameter in subscribe

3. Given a consumer group with 3 consumers and 6 partitions, what happens after one consumer leaves the group?

medium

A. The consumer group stops consuming messages

B. The remaining 2 consumers rebalance and each consumes 3 partitions

C. The partitions are left unassigned until the group is restarted

D. The remaining consumers continue consuming their original partitions

Solution

1. Step 1: Understand partition assignment

   Partitions are divided evenly among active consumers in the group.

2. Step 2: Calculate new assignment after consumer leaves

   With 6 partitions and 2 consumers left, each gets 3 partitions after rebalance.

3. Final Answer:

   The remaining 2 consumers rebalance and each consumes 3 partitions -> Option B

4. Quick Check:

   Partitions evenly split after consumer leaves [OK]

Hint: Partitions redistribute evenly after consumer leaves [OK]

Common Mistakes:

• Assuming partitions stay with old consumers
• Thinking partitions become unassigned
• Believing the group stops consuming

4. You wrote a rebalance listener but your consumer crashes with a NullPointerException during rebalance. What is the likely cause?

medium

A. Not subscribing to the topic before starting the consumer

B. Using the wrong Kafka version

C. Not handling partition assignment and revocation properly in the listener

D. Committing offsets manually instead of auto-commit

Solution

1. Step 1: Analyze rebalance listener role

   The listener must handle partition revocation and assignment to avoid null references.

2. Step 2: Identify cause of NullPointerException

   Failing to check or update state during rebalance causes null pointer errors.

3. Final Answer:

   Not handling partition assignment and revocation properly in the listener -> Option C

4. Quick Check:

   Proper rebalance handling prevents NullPointerException [OK]

Hint: Handle partition changes carefully in rebalance listener [OK]

Common Mistakes:

• Ignoring partition revocation steps
• Assuming auto-commit fixes rebalance issues
• Blaming Kafka version without checking code

5. You want to ensure no duplicate message processing during rebalance in a Kafka consumer group. Which approach is best?

hard

A. Implement a ConsumerRebalanceListener to commit offsets before partition revocation

B. Disable rebalancing by fixing the number of consumers

C. Use auto-commit offsets with a short interval

D. Restart consumers manually after each rebalance

Solution

1. Step 1: Understand duplicate processing risk

   During rebalance, uncommitted offsets can cause reprocessing of messages.

2. Step 2: Use rebalance listener to commit offsets

   Committing offsets in onPartitionsRevoked ensures processed messages are saved before rebalance.

3. Step 3: Evaluate other options

   Disabling rebalance or relying on auto-commit does not guarantee no duplicates; manual restarts are inefficient.

4. Final Answer:

   Implement a ConsumerRebalanceListener to commit offsets before partition revocation -> Option A

5. Quick Check:

   Commit offsets on revoke to avoid duplicates [OK]

Hint: Commit offsets in rebalance listener before revocation [OK]

Common Mistakes:

• Disabling rebalance which is not practical
• Relying only on auto-commit offsets
• Restarting consumers manually wastes resources