How do you perform interactive queries in Kafka Streams using Java?

Kafka Streams provides support for interactive queries, which allow you to query the state stores of a Kafka Streams application in real-time. Interactive queries can be used to retrieve information about the state of the application, such as aggregate statistics, counts, and other metrics. Here’s how you can perform interactive queries in Kafka Streams using Java:

1. Define state stores: To enable interactive queries, you need to define the state stores that will be used to store and manage the state of the application. You can define a state store by calling the “builder.addStateStore()” method and specifying the name, type, and configuration of the store.

2. Enable interactive queries: To enable interactive queries, you need to set the “application.server” property to the hostname and port number of the application server. This property tells Kafka Streams where to listen for client requests.

3. Query state stores: Once interactive queries are enabled, you can query the state stores of the Kafka Streams application using the “store()” method. This method takes the name of the store as a parameter and returns a handle to the store.

4. Handle exceptions: It’s important to handle exceptions that may occur during interactive queries, such as “InvalidStateStoreException” or “TimeoutException”, and implement appropriate error handling and retry strategies as needed.

Here’s an example of how to perform interactive queries in Kafka Streams using Java:

Properties props = new Properties();
props.put(StreamsConfig.APPLICATION_ID_CONFIG, "my_app");
props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
props.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass());
props.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass());
props.put(StreamsConfig.APPLICATION_SERVER_CONFIG, "localhost:8080");

StreamsBuilder builder = new StreamsBuilder();
KStream stream = builder.stream("my_topic");

KTable counts = stream
    .groupByKey()
    .count()
    .mapValues(value -> value + 10L);

counts.toStream().to("my_output_topic");

KafkaStreams streams = new KafkaStreams(builder.build(), props);
streams.start();

KeyValueStore store = streams.store("counts-store", QueryableStoreTypes.keyValueStore());
Long count = store.get("my_key");

System.out.println("Count for my_key: " + count);

streams.close();

In this example, we have defined the properties for the Kafka Streams application and created a StreamsBuilder object to build the topology of the application. We have defined an input stream, grouped the stream by key, and counted the number of occurrences of each key. We have then added 10 to each count and written the results to an output topic.

We have enabled interactive queries by setting the “application.server” property to “localhost:8080”. We have then queried the state store of the application using the “store()” method and retrieved the count for a specific key.

Finally, we have closed the Kafka Streams application using the “close()” method.

By performing interactive queries in Kafka Streams using Java, you can retrieve real-time information about the state of the application and use this information to make data-driven decisions and optimizations.