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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.

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Kafka Streams: A Serializer Is Not Compatible to the Actual Type

Kafka Streams throws 'A serializer is not compatible to the actual key or value type' or 'ClassCastException invoking Processor'. Here's the SerDe fix.

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Kafka Streams RocksDBException: Too Many Open Files Fix

Kafka Streams dying with ProcessorStateException and RocksDBException 'Too many open files'? Here's the file-descriptor math and the config setter that fixes it.

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Kafka Streams MissingSourceTopicException: Fix It Fast

Kafka Streams dies with MissingSourceTopicException on startup or rebalance. Here's why the assignor flags a missing source topic and how to fix it.

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Kafka Streams LockException: Failed to Lock the State Directory

Kafka Streams throwing LockException: Failed to lock the state directory for task? Here's the exact cause, why it happens, and the fix.

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Kafka Streams Deserialization Exception Handler: Fix the Fail

Kafka Streams dies with 'Deserialization exception handler is set to fail upon a deserialization error.' Here's why one bad record kills the thread and how to fix it.

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Kafka SSL Handshake Failed: PKIX and Hostname Fixes

Fix org.apache.kafka.common.errors.SslAuthenticationException: SSL handshake failed — PKIX path errors, SAN mismatches, and listener protocol mixups fast.

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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.

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Kafka SerializationException: Schema Not Found (40403) Fix

Kafka consumer dies with SerializationException: Error retrieving Avro schema for id N, Schema not found; error code: 40403. Why the ID lookup fails and how to fix it.

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Schema Being Registered Is Incompatible: Kafka 409 Fix

Fix "Schema being registered is incompatible with an earlier schema; error code: 409" — why Schema Registry rejects your schema and how to evolve safely.

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Kafka SaslAuthenticationException: Authentication Failed — Fix

Getting SaslAuthenticationException: Authentication failed in Kafka? Fix invalid SASL credentials, JAAS config, and missing SCRAM users with working examples.