In-sync replicas (ISR) in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of the ISR status check command?

Given a Kafka topic with 3 partitions and replication factor 3, you run the command to describe the topic and check the ISR for partition 0. What output will you see if all replicas are in sync?

Kafka

kafka-topics.sh --describe --topic my-topic --bootstrap-server localhost:9092

A Partition: 0 Leader: 1 Replicas: 1,2,3 Isr: 1

B Partition: 0 Leader: 1 Replicas: 1,2,3 Isr: 1,2,3

C Partition: 0 Leader: 1 Replicas: 1,2,3 Isr: 2,3

D Partition: 0 Leader: 1 Replicas: 1,2,3 Isr: 1,3

💡 Hint

ISR includes all replicas that are fully caught up with the leader.

When all replicas are caught up, the ISR list includes all replicas. The leader is also part of the ISR.

🧠 Conceptual

intermediate

What happens when a replica falls out of the ISR?

In Kafka, if a follower replica falls behind the leader and cannot keep up, what is the expected behavior regarding the ISR?

A The ISR size automatically increases to include more replicas.

B The replica remains in the ISR but stops receiving data.

C The leader shuts down to prevent data loss.

D The replica is removed from the ISR until it catches up again.

💡 Hint

ISR only contains replicas fully caught up with the leader.

Kafka removes replicas from the ISR if they lag behind, ensuring only in-sync replicas are counted.

🔧 Debug

advanced

Why does the ISR not include a replica after network partition?

You notice that after a network partition, one replica is no longer listed in the ISR for a partition. Which of the following is the most likely cause?

A The replica failed to fetch data and fell behind the leader's log.

B The leader replica crashed and was removed from ISR.

C The replica was promoted to leader and removed from ISR.

D The ISR list is fixed and does not update dynamically.

💡 Hint

ISR updates dynamically based on replica sync status.

A network partition can cause a replica to lag behind, so Kafka removes it from the ISR until it catches up.

📝 Syntax

advanced

Which command correctly lists the ISR for a Kafka topic?

Select the correct Kafka CLI command to describe a topic and show the ISR for all partitions.

A kafka-topics.sh --describe --topic my-topic --bootstrap-server localhost:9092

B kafka-consumer-groups.sh --describe --group my-group --topic my-topic

C kafka-configs.sh --describe --entity-type topics --entity-name my-topic

D kafka-producer-perf-test.sh --topic my-topic --describe

💡 Hint

The topic description command shows ISR info.

The kafka-topics.sh describe command lists partitions, leaders, replicas, and ISR.

🚀 Application

expert

How many replicas are in the ISR after this scenario?

A Kafka topic has replication factor 4. Initially, all replicas are in the ISR: [1,2,3,4]. Replica 3 falls behind due to slow disk and replica 2 loses network connectivity temporarily. After some time, replica 3 catches up but replica 2 is still disconnected. What is the current ISR?

A [1,4]

B [1,2,3,4]

C [1,3,4]

D [3,4]

💡 Hint

ISR only includes replicas currently caught up and connected.

Replica 2 is disconnected and out of sync, so it is removed. Replica 3 caught up and rejoins ISR.