KStream: Error Reading and Writing Avro records - apache-kafka

Question

I’m trying to write avro record that I read from a topic into another topic, intentions it to augment it with transformation after I get this routing working. I have used the KStream with avro code from one of the example with some modifications to connect to Schema Registry for retrieving the avro schema.
streamsConfiguration.put(StreamsConfig.APPLICATION_ID_CONFIG, "mysql-stream-processing");
streamsConfiguration.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
streamsConfiguration.put(AbstractKafkaAvroSerDeConfig.SCHEMA_REGISTRY_URL_CONFIG, schemaRegistryUrl);
final Serde<GenericRecord> keySerde = new GenericAvroSerde(
new CachedSchemaRegistryClient(schemaRegistryUrl, 100),
Collections.singletonMap(AbstractKafkaAvroSerDeConfig.SCHEMA_REGISTRY_URL_CONFIG,
schemaRegistryUrl));
final Serde<GenericRecord> valueSerde = new GenericAvroSerde(
new CachedSchemaRegistryClient(schemaRegistryUrl, 100),
Collections.singletonMap(AbstractKafkaAvroSerDeConfig.SCHEMA_REGISTRY_URL_CONFIG,
schemaRegistryUrl));
streamsConfiguration.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
streamsConfiguration.put(StreamsConfig.COMMIT_INTERVAL_MS_CONFIG, 10 * 1000);
final KStreamBuilder builder = new KStreamBuilder();
final KStream<GenericRecord, GenericRecord> record = builder.stream("dbserver1.employees.employees");
record.print(keySerde, valueSerde);
record.to(keySerde, valueSerde, "newtopic");
record.foreach((key, val) -> System.out.println(key.toString()+" "+val.toString()));
final KafkaStreams streams = new KafkaStreams(builder, streamsConfiguration);
streams.cleanUp();
streams.start();
When run print() works as I can see the record in the console, but Im unable to get the record written to the “newtopic”, failing with the below error
Exception in thread "StreamThread-1" org.apache.kafka.streams.errors.StreamsException: Exception caught in process. taskId=0_0, processor=KSTREAM-SOURCE-0000000000, topic=dbserver1.employees.employees, partition=0, offset=0
at org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:217)
at org.apache.kafka.streams.processor.internals.StreamThread.runLoop(StreamThread.java:627)
at org.apache.kafka.streams.processor.internals.StreamThread.run(StreamThread.java:361)
Caused by: org.apache.kafka.streams.errors.StreamsException: A serializer (key: io.confluent.examples.streams.utils.GenericAvroSerializer / value: io.confluent.examples.streams.utils.GenericAvroSerializer) is not compatible to the actual key or value type (key type: [B / value type: [B). Change the default Serdes in StreamConfig or provide correct Serdes via method parameters.
at org.apache.kafka.streams.processor.internals.SinkNode.process(SinkNode.java:81)
at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:83)
at org.apache.kafka.streams.processor.internals.SourceNode.process(SourceNode.java:70)
at org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:198)
... 2 more
Caused by: java.lang.ClassCastException: [B cannot be cast to org.apache.avro.generic.GenericRecord
at io.confluent.examples.streams.utils.GenericAvroSerializer.serialize(GenericAvroSerializer.java:25)
at org.apache.kafka.streams.processor.internals.RecordCollectorImpl.send(RecordCollectorImpl.java:77)
at org.apache.kafka.streams.processor.internals.SinkNode.process(SinkNode.java:79)
... 5 more

Answer 1

I guess you need to configure the correct Serdes:
https://docs.confluent.io/current/streams/developer-guide/config-streams.html#default-key-serde
https://docs.confluent.io/current/streams/developer-guide/dsl-api.html
Either set correct global Serdes, or specify Serdes for each operator. If an operator needs a Serde, than it has a corresponding overload taking Serdes are parameters.