Kafka Topic NotEnoughReplicasException

The number of in-sync replicas is less than the required minimum.

Understanding Kafka and Its Purpose

Apache Kafka is a distributed event streaming platform used by thousands of companies for high-performance data pipelines, streaming analytics, data integration, and mission-critical applications. It is designed to handle real-time data feeds with high throughput and low latency. Kafka's architecture is based on a distributed commit log, allowing it to scale horizontally and provide fault tolerance.

Identifying the Symptom: NotEnoughReplicasException

When working with Kafka, you might encounter the NotEnoughReplicasException. This exception is typically observed when the number of in-sync replicas (ISRs) for a partition falls below the configured minimum ISR count. This can lead to potential data loss if not addressed promptly.

What You Might Observe

Developers may notice that messages are not being produced to a Kafka topic, and the logs may show the NotEnoughReplicasException. This indicates that the Kafka cluster is unable to meet the replication requirements for the topic partitions.

Delving into the Issue: NotEnoughReplicasException

The NotEnoughReplicasException occurs when the number of replicas that are in sync with the leader falls below the minimum ISR threshold. This situation can arise due to broker failures, network issues, or insufficient broker resources.

Understanding In-Sync Replicas (ISR)

In Kafka, an ISR is a replica that is fully caught up with the leader. The minimum ISR configuration ensures that a certain number of replicas are in sync to guarantee data durability. If the ISR count drops below this threshold, Kafka will throw the NotEnoughReplicasException.

Steps to Resolve NotEnoughReplicasException

To resolve this issue, you can take several actions to ensure that the number of in-sync replicas meets the required minimum.

Step 1: Check Broker Availability

Ensure that all Kafka brokers are running and reachable. You can use the following command to check the status of Kafka brokers:

bin/kafka-broker-api-versions.sh --bootstrap-server <broker-host>:9092

If any brokers are down, restart them and monitor their status.

Step 2: Adjust the Replication Factor

If the cluster does not have enough brokers to meet the replication factor, consider reducing the replication factor for the affected topics. Use the following command to alter the replication factor:

bin/kafka-topics.sh --alter --topic <topic-name> --partitions <number> --replication-factor <new-factor> --zookeeper <zookeeper-host>:2181

Note: Reducing the replication factor may impact data durability.

Step 3: Increase Broker Resources

If brokers are running but unable to keep up with the load, consider increasing their resources. This may involve adding more brokers to the cluster or upgrading existing hardware.

Additional Resources

For more information on managing Kafka clusters and troubleshooting common issues, refer to the following resources:

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