While looking through the Kafka Tutorials to see how I could setup a Spring Boot API project with Kafka Streams, I found it strange that there wasn't a complete or more informative example on how this could be achieved. Most use cases demonstrated how to compute aggregations and how to build simple topologies, but it was difficult to find a concrete example on how to build an API service that could query into these materialized name stores. Anyways, I thought I’d create my own using a more recent version of Spring Boot with Java 14.

• Java 14
• Maven 3.6.0+
• Docker 19+

We need to first launch the Confluent services (i.e. Schema Registry, Broker, ZooKeeper) locally by running the docker-compose up -d CLI command where the docker-compose.yml file is. Typically, you can create a stack file (in the form of a YAML file) to define your applications. You can also run docker-compose ps to check the status of the stack. Notice, the endpoints from within the containers on your host machine.

Note: you can run docker-compose down to stop all services and containers.

As part of this sample, I've retrofitted the average aggregate example from Confluent's Kafka Tutorials into this project. The API will calculate and return a running average rating for a given movie identifier. This should demonstrate how to build a basic API service on top of an aggregation result.

Notice in the ~/src/main/avro directory, we have all our Avro schema files for the stream of ratings and countsum. For your convenience, the classes were already generated under the ~/src/main/java/io/confluent/demo directory, but feel free to tinker with them and recompile the schemas if needed. The Avro classes can be programmatically generated using Maven or by manually invoking the schema compiler.

So before building and running the project, open a new terminal and run the following commands to generate your input and output topics.

$  docker-compose exec broker kafka-topics --create --bootstrap-server \
   localhost:9092 --replication-factor 1 --partitions 1 --topic ratings

$  docker-compose exec broker kafka-topics --create --bootstrap-server \
   localhost:9092 --replication-factor 1 --partitions 1 --topic rating-averages

Next, we will need to produce some data onto the input topic.

$  docker exec -i schema-registry /usr/bin/kafka-avro-console-producer --topic ratings --broker-list broker:9092\
    --property "parse.key=false"\
    --property "key.separator=:"\
    --property value.schema="$(< src/main/avro/rating.avsc)"

Paste in the following json data when prompted and be sure to press enter twice to actually submit it.

{"movie_id":362,"rating":10}
{"movie_id":362,"rating":8}

Optionally, you can also see the consumer results on the output topic by running this command on a new terminal window:

$  docker exec -it broker /usr/bin/kafka-console-consumer --topic rating-averages --bootstrap-server broker:9092 \
    --property "print.key=true"\
    --property "key.deserializer=org.apache.kafka.common.serialization.LongDeserializer" \
    --property "value.deserializer=org.apache.kafka.common.serialization.DoubleDeserializer" \
    --from-beginning

You can import the code straight into your preferred IDE or run the sample using the following command (in the root project folder).

$  mvn spring-boot:run

After the application runs, navigate to http://localhost:7001/swagger-ui/index.html?configUrl=/api-docs/swagger-config in your web browser to access the Swagger UI. If you used the same sample data from above, you can enter 362 as the movieId and it should return something similar like this below:

{
  "movieId": 362,
  "rating": 9
}

Note: keep in mind the various states of the client. When a Kafka Streams instance is in RUNNING state, it allows for inspection of the stream's metadata using methods like queryMetadataForKey(). While it is in REBALANCING state, the REST service cannot immediately answer requests until the state stores are fully rebuilt.

• In certain conditions, you may need to do a complete application reset. You can delete the application’s local state directory where the application instance was run. In this project, Kafka Streams persists local states under the ~/data folder.