Kafka stream word count instance



Kafka is not a message queue, but a log storage system in essence, and stream processing is its recently promoted feature. This paper briefly introduces the example of word count.



Prepare topic

sh kafka-topics.sh --create --topic wc-input --replication-factor 1 --partitions 1 --zookeeper localhost:2181

One is WC input, and then output. For simplicity and convenience, console output is used here. Of course, it can also be output to another topic, etc.

configuration parameter

Properties props = new Properties();
        props.put(StreamsConfig.APPLICATION_ID_CONFIG, "word-count-demo");
        props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        props.put(StreamsConfig.KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
        props.put(StreamsConfig.VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());

        // setting offset reset to earliest so that we can re-run the demo code with the same pre-loaded data
        // Note: To re-run the demo, you need to use the offset reset tool:
        // https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Streams+Application+Reset+Tool
        props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");

Construct kstream

        KStreamBuilder builder = new KStreamBuilder();
        KStream<String, String> source = builder.stream("wc-input");
        KTable<String, Long> counts = source
                .flatMapValues(new ValueMapper<String, Iterable<String>>() {
                    public Iterable<String> apply(String value) {
                        return Arrays.asList(value.toLowerCase(Locale.getDefault()).split(" "));
                }).map(new KeyValueMapper<String, String, KeyValue<String, String>>() {
                    public KeyValue<String, String> apply(String key, String value) {
                        return new KeyValue<>(value, value);
        KafkaStreams streams = new KafkaStreams(builder, props);

Start / stop

        final CountDownLatch latch = new CountDownLatch(1);
        // attach shutdown handler to catch control-c
        Runtime.getRuntime().addShutdownHook(new Thread("streams-wordcount-shutdown-hook") {
            public void run() {

        try {
        } catch (Throwable e) {



sh kafka-console-producer.sh --broker-list localhost:9092 --topic wc-input

Kafka Streams is a client library for building applications and microservices, where the input and output data are stored in Kafka clusters. It combines the simplicity of writing and deploying standard Java and Scala applications on the client side with the benefits of Kafka’s server-side cluster technology.


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