How does KafkaStream.KTable write data to kafka topic in (compacted) KV style

Question

In Kafka(0.11.0.1) Streams, A demo app Play with a Streams Application

// Serializers/deserializers (serde) for String and Long types
final Serde<String> stringSerde = Serdes.String();
final Serde<Long> longSerde = Serdes.Long();

// Construct a `KStream` from the input topic "streams-plaintext-input", where message values
// represent lines of text (for the sake of this example, we ignore whatever may be stored
// in the message keys).
KStream<String, String> textLines = builder.stream(stringSerde, stringSerde, "streams-plaintext-input");

KTable<String, Long> wordCounts = textLines
    // Split each text line, by whitespace, into words.
    .flatMapValues(value -> Arrays.asList(value.toLowerCase().split("\\W+")))

    // Group the text words as message keys
    .groupBy((key, value) -> value)

    // Count the occurrences of each word (message key).
    .count("Counts")

    // Store the running counts as a changelog stream to the output topic.
    wordCounts.to(stringSerde, longSerde, "streams-wordcount-output");

And step 5, after process some datas, we could see compacted KV pairs(e.g. streams 2) in sink topic streams-wordcount-output,

> bin/kafka-console-consumer.sh --bootstrap-server localhost:9092
    --topic streams-wordcount-output \
    --from-beginning \
    --formatter kafka.tools.DefaultMessageFormatter \
    --property print.key=true \
    --property print.value=true \
    --property key.deserializer=org.apache.kafka.common.serialization.StringDeserializer \
    --property value.deserializer=org.apache.kafka.common.serialization.LongDeserializer 

all     1 
streams 1 
lead    1 
to      1 
kafka   1 
hello   1 
kafka   2 
streams 2

The question is how does KTable wordCounts in the above data write data to topic streams-wordcount-output in Key-Value style?

The option cleanup.policy of topic streams-wordcount-output seems to be the default value, delete, not compact(via bin/kafka-configs.sh)

Matthias J. Sax Matthias J. Sax · Accepted Answer · 2017-09-21T16:25:38

All input and output topics are "out of scope" of Kafka Streams. It's the users responsibility to create and configure those topics.

Thus, your topic "streams-wordcount-output" will have the config you specified when creating the topic.

cf. https://docs.confluent.io/current/streams/developer-guide.html#managing-topics-of-a-kafka-streams-application

How does KafkaStream.KTable write data to kafka topic in (compacted) KV style

1 Answers