Serialization

1. "GenericRecord vs SpecificRecord?"

Both encode to the same Avro bytes under the same schema. GenericRecord is a schema-driven object that works like a map (get/put, untyped Object). SpecificRecord is a generated class with typed getters and setters and compile-time safety. It is the production choice. Use Generic for schema-agnostic plumbing (Connect, generic pipelines).

2. "Why Avro over Java serialization or Kryo for Kafka/at-rest data?"

Avro has an explicit, language-neutral, evolvable schema. It is compact, and Schema Registry supports it. Java serialization and Kryo work only on the JVM, have no schema (they are reflective), and break when classes change. They are fine as an internal fallback. They are wrong for durable or cross-language data.

3. "What is the Flink Kryo fallback and why do you care?"

Suppose a type is not an Avro or recognized type, and it is not a valid POJO. Then Flink serializes it with generic Kryo. This gives larger state, slower throughput, and fragile behaviour under schema evolution and savepoints. You care because it silently degrades production jobs.

4. "What makes a class a valid Flink POJO?"

It is a public class with a public no-argument constructor. Every field is either public or exposed through a public getter/setter (bean convention). The field types are serializable. If the class meets these rules, Flink uses the efficient POJO serializer instead of Kryo.

5. "Does Avro binary data include the schema per record?"

No. Binary encoding assumes that the reader knows the schema. The schema comes from a container-file header, or from Schema Registry by id. This is why the data is compact. It is also why schema compatibility (KFK-06) is a separate, managed concern.

6. "How would you prove, without deploying, that your event type serializes efficiently?"

Do two checks:

1. Check that the type really round-trips through Avro binary encode/decode to an equal object. This shows that it is a real Avro type, not Java/Kryo-generic.
2. Verify Flink's POJO rules with reflection (public class, public no-arg ctor, accessible fields).

If both pass, there is no Kryo fallback.