Any Kafka streams operation that writes to disk requires a Serde for each type that needs to be serialized.

For each aggregate, a set of Serdes is required, and it the responsibility of the user to create these, by providing an implementation of the AggregateSerdes interface. For the Simple Sourcing client, a simpler CommandSerdes implementation must be provided.

Simple Sourcing provides some utilities to assist with this for both Avro and JSON formats. We recommend the Avro implementation due to its inbuilt support for schema evolution via the Confluent Schema Registry.

Unfortunately, there is still some unavoidable boilerplate code required to set up the Serdes.

The user example project includes several different configurations you can use as a template when setting up your own project.

Avro serialization recipe

These are the steps required to create a single Avro Serde, as a summary:

  1. Create your domain class definition. This is the type you work within your Simple Sourcing or KStream code.
  2. Create the Avro schema file (.avdl files) to represent the Avro schema for the class.
  3. Run the Maven goal generate-sources to generate Java files for the .avdl. Note that these generated classes will be subclasses of Avro GenericObject.

    This step happens automatically when you run mvn install.

  4. Create a GenericMapper that maps your domain object to the auto-generated Avro Java object.
  5. Create an instance of Serde<GenericRecord>. The underlying implementation for this Serde is provided by Confluent. We can create it with AvroGenericUtils.genericAvroSerde, passing in the necessary configuration for the Schema Registry.
  6. Use the GenericSerde.of method to create the desired Serde for your domain class.

    It works by combining the Serde<GenericRecord> with the GenericMapper. It uses the Serde<GenericRecord> to convert between byte[] and GenericRecord, and your GenericMapper to convert between GenericRecord and your domain class.

  7. Repeat the above process for the other domain entities that are required for the aggregate.
  8. Create a AggregateSerdes instance with the Serdes for all the required types for all the Simple Sourcing processing steps.

Create domain classes in Avro

When using Avro as your serialization format, you need to generate Avro classes for all your domain objects. We recommend using the Avro IDL to define your objects as it provides a more concise readable format than the .avsc JSON format. To get started, check out the Avro reference documentation and take a look at user.avdl from the included example projects.

You can have as many types as you like for events and commands. You don’t need to use a union type to represent your individual events and commands. We’re using the recent extension to the schema registry for multiple schemas registered under the same topic to support this pattern.

record PostCreated {
  string title;
  string body;
  long authorUserId;

record PostPublished {}

record PostLiked {
  long userId;

record PostUnliked {
  long userId;

record PostDeleted {}

Add your .avdl file to the src/main/avro directory in your project and configure the Avro plugin for whichever build tool you use (Maven, Gradle and sbt all have plugins) to generate code for your domain classes.

You can also explicitly run mvn generate-sources to generate the Java serialization classes.

Schema naming strategy

The schema for every Avro serialized type is saved in the Confluent Schema registry. The Schema Registry supports two naming strategies:

  • Topic Record name Strategy - This creates a schema from the topic name and the fully qualified name of the Java class generated from the .avdl file. This schema naming strategy must be used for Events and Commands, where different types of events and commands appear in the same topic.
  • Topic Name Strategy - This schema strategy uses only the topic name for the schema name. It will not work for commands and events, but is fine for aggregates and other KTable based projections, which are typically monomorphic. It is also the only schema naming strategy supported by some of the plugins for Kafka Connect.

This behavior is controlled by the SchemaNameStrategy in the AvroGenericUtils.genericAvroSerde helper method.

Json serialization

Generating Serdes for Json serialization involves very little additional boilerplate on the part of the user.

The /examples/user example app demonstrates how to create Serdes for your domain objects using Json serialization, using the Gson Json library.

Please take a look at the example for more details.