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Gopi Gorantala

I'm Gopi — 15+ years in Java, building Kafka and Flink platforms for banks, where one lost event is a financial discrepancy. I write javahandbook.com because the guides I needed didn't exist. Everything here is tested against a real cluster first.

Kafka: Consumer Is Not Subscribed to Any Topics — Fix

IllegalStateException: Consumer is not subscribed to any topics or assigned any partitions — why poll() throws it, the subscribe/assign contract, and the fix.

Kafka BufferExhaustedException: Fix Failed to Allocate Memory

Kafka producer throws BufferExhaustedException: failed to allocate memory within max blocking time? Here's why buffer.memory fills up and the exact fix.

Kafka "Connection to node -1 could not be established" — Fix

Fix Kafka's "Connection to node -1 (localhost/127.0.0.1:9092) could not be established. Broker may not be available" — the Docker advertised.listeners fix.

Kafka SerializationException: Fixing Poison Pill Records

Kafka SerializationException: "Error deserializing key/value" crashes your consumer in a loop. Fix it with ErrorHandlingDeserializer and a dead letter topic.

Kafka NotControllerException: Why It Happens and How to Fix It

Kafka throws NotControllerException (code 41) when an admin request hits a broker that isn't the active controller. Here's the mechanism and the real fix.

Kafka FencedInstanceIdException: Fix Static Membership Collisions

FencedInstanceIdException means two live consumers share one group.instance.id. Here's why the broker fences the old one, and how to fix it for good.

Kafka ProducerFencedException: Why It Happens and How to Fix It

ProducerFencedException: there is a newer producer with the same transactionalId. What fences a Kafka transactional producer and the exact fix, explained.

Kafka NetworkException: The Server Disconnected — How to Fix It

Fix org.apache.kafka.common.errors.NetworkException: the server disconnected before a response was received. Root causes, idempotent retries, LB timeouts.

Kafka RecordTooLargeException: The 1MB Limit and How to Fix It

Hitting RecordTooLargeException in Kafka? Learn why the 1MB default bites on both producer and broker, and the exact configs to fix it properly.

Kafka InvalidReplicationFactorException: Larger Than Available Brokers

Fix Kafka's InvalidReplicationFactorException — "Replication factor: 3 larger than available brokers: 1" — in Docker and single-broker dev setups fast.