问题
I have many microservices reading/writing Avro messages in Kafka.
Schemas are great. Avro is great. But is a schema registry really needed? It helps centralize Schemas, yes, but do the microservices really need to query the registry? I don't think so.
Each microservice has a copy of the schema, user.avsc, and an Avro-generated POJO: User extends SpecificRecord. I want a POJO of each Schema for easy manipulation in the code.
Write to Kafka:
byte [] value = user.toByteBuffer().array();
producer.send(new ProducerRecord<>(TOPIC, key, value));
Read from Kafka:
User user = User.fromByteBuffer(ByteBuffer.wrap(record.value()));
回答1:
Schema Registry gives you a way for broader set of applications and services to use the data, not just your Java-based microservices.
For example, your microservice streams data to a topic, and you want to send that data to Elasticsearch, or a database. If you've got the Schema Registry you literally hook up Kafka Connect to the topic and it now has the schema and can create the target mapping or table. Without a Schema Registry each consumer of the data has to find out some other way what the schema of the data is.
Taken the other way around too - your microservice wants to access data that's written into a Kafka topic from elsewhere (e.g. with Kafka Connect, or any other producer) - with the Schema Registry you can simply retrieve the schema. Without it you start coupling your microservice development to having to know about where the source data is being produced and its schema.
There's a good talk about this subject here: https://qconnewyork.com/system/files/presentation-slides/qcon_17_-_schemas_and_apis.pdf
回答2:
Do they need to? No, not really.
Should you save yourself some space on your topic and not send the schema as part of the message or require the consumers to have the schema to read anything? Yes, and that is what the AvroSerializer is doing for you - externalizing that data elsewhere that is consumable as simply a REST API.
The deserializer then must know how that schema is gotten, and you can configure it with specific.avro.reader=true property rather than manually invoking the fromByteBuffer yourself, letting the AvroDeserializer handle it.
Also, in larger orgs, shuffling around a single user.avsc file (even if version controlled) doesn't control that copy becoming stale over time or handle evolution in a clean way.
回答3:
One of the most important features of the Schema Registry is to manage the evolution of schemas. It provides the layer of compatibility checking. By setting an appropriate Compatibility Type you determine the allowed schema changes.
You can find all the available Compatibility Types here.
来源：https://stackoverflow.com/questions/55730663/what-is-the-value-of-an-avro-schema-registry