Log compaction in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of the Kafka log compaction status check?

Given the following Kafka topic configuration and status check command, what will be the output?

Kafka

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

Topic: user-updates	PartitionCount: 3	ReplicationFactor: 2	Configs: cleanup.policy=compact
Topic: user-updates	Partition: 0	Leader: 1	Replicas: 1,2	Isr: 1,2
Topic: user-updates	Partition: 1	Leader: 2	Replicas: 2,3	Isr: 2,3
Topic: user-updates	Partition: 2	Leader: 3	Replicas: 3,1	Isr: 3,1

A The topic 'user-updates' has log compaction enabled with 3 partitions and replication factor 2.

B The topic 'user-updates' has log compaction disabled and 3 partitions.

C The topic 'user-updates' has 1 partition with log compaction enabled.

D The topic 'user-updates' has 3 partitions but replication factor is 1.

💡 Hint

Look for the cleanup.policy configuration in the topic description.

The output shows 'cleanup.policy=compact' which means log compaction is enabled. The topic has 3 partitions and replication factor 2 as shown.

🧠 Conceptual

intermediate

What does Kafka log compaction guarantee?

Choose the correct statement about Kafka log compaction guarantees.

A Log compaction guarantees message order across all partitions.

B Log compaction guarantees that the latest value for each key is retained in the topic.

C Log compaction compresses messages to reduce disk usage without deleting any data.

D Log compaction deletes all messages older than a configured time.

💡 Hint

Think about what happens to messages with the same key after compaction.

Log compaction keeps the latest message for each key, removing older duplicates, but does not delete messages based on time or compress data.

❓ Predict Output

advanced

What error occurs when enabling log compaction with invalid cleanup.policy?

What error will Kafka produce if you set cleanup.policy=compacted instead of compact for a topic?

Kafka

kafka-topics.sh --create --topic test-topic --bootstrap-server localhost:9092 --partitions 1 --replication-factor 1 --config cleanup.policy=compacted

A Error: Replication factor must be greater than 1 for compaction

B Topic created successfully with log compaction enabled

C Warning: Unknown cleanup.policy value, defaulting to delete

D Error: Invalid value 'compacted' for configuration 'cleanup.policy'

💡 Hint

Check the exact allowed values for cleanup.policy.

Kafka only accepts 'compact', 'delete', or both combined. 'compacted' is invalid and causes an error.

🚀 Application

advanced

How to configure a topic for both log compaction and time-based deletion?

Which configuration correctly enables both log compaction and time-based deletion on a Kafka topic?

A cleanup.policy=compact delete

B cleanup.policy=compact;delete

C cleanup.policy=compact,delete

D cleanup.policy=compact|delete

💡 Hint

Multiple policies are comma-separated in Kafka configs.

Kafka expects multiple cleanup policies separated by commas without spaces.

🧠 Conceptual

expert

Why might Kafka log compaction not immediately remove old records?

Select the best explanation why Kafka log compaction does not instantly delete old records after a new record with the same key is written.

A Compaction runs periodically and only removes older duplicates during background cleanup, not immediately.

B Kafka keeps all records forever regardless of compaction settings.

C Compaction requires manual trigger to delete old records.

D Old records are deleted immediately but only after consumer acknowledges.

💡 Hint

Think about how Kafka manages background tasks.

Kafka log compaction is a background process that runs periodically, so old duplicates remain until compaction runs.