AVRO schema for generic type - Java

Question

I want to include traceId, and metadata for my Kafka message payload. so I have defined the below type for Kafka message payload.

public class KafkaMessage<T> {

    private String traceId;
    private Map<String, String> metaData = new HashMap<>();
    private T content;
}

then I have two type Dog and Cat. also I have created two Kafka message payload types two.

public class Dog {
    private int name;
}

public class DogKafkaMessage extends KafkaMessage<Dog>{

}

public class Car {
    private int numberOfWheels;
}

public class CarKafkaMessage extends KafkaMessage<Car> {

}

I want to represent these two types in Avro schema. is it possible to this way? if so please help me with this.

I can use JSON serializer/deserializer for Kafka message producer and consumer. but I look forward to using Avro.

Thanks.

Answer 1

Natively, Avro doesn't do generics or inheritance. Jackson Avro library might help with this, but you may run into strange edge cases.

Instead, if you'd like to support more than only JVM implementations of Avro, you could define a schema that wraps bytes, and you'd have to deserialize events twice (outer and inner messages) where you can even mix and match data formats

record Event {
    string traceId;
    map<string> metadata;
    bytes content;
}

This is very similar to how CloudEvents spec works

• https://github.com/cloudevents/spec/blob/v1.0.1/avro-format.md
• https://github.com/cloudevents/spec/blob/v1.0.1/kafka-protocol-binding.md