Testing Streams Code

Importing the Test Utilities

To test a Kafka Streams application, Apache Kafka® provides a test-utils artifact that can be added as regular dependency to your test code base.

Here is an example pom.xml snippet when using Maven:


Testing a Streams Application

When you create a Streams application, you create a Topology, either using the StreamsBuilder DSL or using the low-level Processor API. Normally, you run the topology using the KafkaStreams class, which connects to your broker and begins processing when you call start(). For testing, though, running a broker and making sure to clean up state between tests adds a lot of complexity and time.

Streams provides the TopologyTestDriver in the kafka-streams-test-utils package as a drop-in replacement for the KafkaStreams class. It has no external system dependencies, and it also processes input synchronously, so you can verify the results immediately after providing input. There are hooks for verifying data sent to output topics, and you can also query state stores maintained by your application under test.

To set it up:

// Processor API
Topology topology = new Topology();
topology.addSource("sourceProcessor", "input-topic");
topology.addProcessor("processor", ..., "sourceProcessor");
topology.addSink("sinkProcessor", "output-topic", "processor");
// or
// using DSL
StreamsBuilder builder = new StreamsBuilder();
Topology topology = builder.build();

// setup test driver
Properties config = new Properties();
config.put(StreamsConfig.APPLICATION_ID_CONFIG, "test");
config.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "dummy:1234");
TopologyTestDriver testDriver = new TopologyTestDriver(topology, config);

To pipe records through the test-driver, you can create a TestInputTopic for each input topic of your topology:

TestInputTopic<String, Integer> inputTopic = testDriver.createInputTopic(
    new StringSerializer(),
    new IntegerSerializer());
inputTopic.pipeInput("key", 42);

For result verification, you first create a TestOutputTopic:

TestOutputTopic<String, Long> outputTopic = testDriver.createOutputTopic(
    new StringDeserializer(),
    new LongDeserializer());

You can either verify only the record value, the key and the values, or a full TestRecord that includes timestamp and header information. Consuming an output topic as a table is also possible using TestOutputTopic#readKeyValuesToMap().

// user assertion library of your choice to verify output
assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 21L)));

TopologyTestDriver supports punctuations, too. Event-time punctuations are triggered automatically based on the processed records’ timestamps. Wall-clock-time punctuations can also be triggered by advancing the test driver’s wall-clock time. The driver’s wall-clock time must be advanced manually (this is for test stability).

// supposing that a scheduled punctuator would emit this record...
assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("triggered-key", "triggered-value")));

Additionally, state stores are accessible via the test driver before or after a test. Accessing stores before a test is useful to pre-populate a store with some initial values. After data was processed, expected updates to the store can be verified.

KeyValueStore store = testDriver.getKeyValueStore("store-name");
assertEquals("some value", store.get("some key"));

Always close the test driver at the end to make sure all resources are released properly.



The following example demonstrates how to use the test driver and helper classes. The example creates a topology that computes the maximum value per key using a key-value-store. While processing, no output is generated, but only the store is updated. Output is only sent downstream based on event-time and wall-clock punctuations.

private TopologyTestDriver testDriver;
private TestInputTopic<String, Long> inputTopic;
private TestOutputTopic<String, Long> outputTopic;
private KeyValueStore<String, Long> store;

private Serde<String> stringSerde = new Serdes.StringSerde();
private Serde<Long> longSerde = new Serdes.LongSerde();

public void setup() {
    final Topology topology = new Topology();
    topology.addSource("sourceProcessor", "input-topic");
    topology.addProcessor("aggregator", new CustomMaxAggregatorSupplier(), "sourceProcessor");
            Serdes.Long()).withLoggingDisabled(), // need to disable logging to allow store pre-populating
    topology.addSink("sinkProcessor", "result-topic", "aggregator");

    // setup test driver
    final Properties props = new Properties();
    props.setProperty(StreamsConfig.APPLICATION_ID_CONFIG, "maxAggregation");
    props.setProperty(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "dummy:1234");
    props.setProperty(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
    props.setProperty(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.Long().getClass().getName());
    testDriver = new TopologyTestDriver(topology, props);

    // setup test topics
    inputTopic = testDriver.createInputTopic("input-topic", stringSerde.serializer(), longSerde.serializer());
    outputTopic = testDriver.createOutputTopic("result-topic", stringSerde.deserializer(), longSerde.deserializer());

    // pre-populate store
    store = testDriver.getKeyValueStore("aggStore");
    store.put("a", 21L);

public void tearDown() {

public void shouldFlushStoreForFirstInput() {
    inputTopic.pipeInput("a", 1L);
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 21L)));
    assertThat(outputTopic.isEmpty(), is(true));

public void shouldNotUpdateStoreForSmallerValue() {
    inputTopic.pipeInput("a", 1L);
    assertThat(store.get("a"), equalTo(21L));
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 21L)));
    assertThat(outputTopic.isEmpty(), is(true));

public void shouldNotUpdateStoreForLargerValue() {
    inputTopic.pipeInput("a", 42L);
    assertThat(store.get("a"), equalTo(42L));
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 42L)));
    assertThat(outputTopic.isEmpty(), is(true));

public void shouldUpdateStoreForNewKey() {
    inputTopic.pipeInput("b", 21L);
    assertThat(store.get("b"), equalTo(21L));
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 21L)));
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("b", 21L)));
    assertThat(outputTopic.isEmpty(), is(true));

public void shouldPunctuateIfEvenTimeAdvances() {
    final Instant recordTime = Instant.now();
    inputTopic.pipeInput("a", 1L, recordTime);
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 21L)));

    inputTopic.pipeInput("a", 1L, recordTime);
    assertThat(outputTopic.isEmpty(), is(true));

    inputTopic.pipeInput("a", 1L, recordTime.plusSeconds(10L));
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 21L)));
    assertThat(outputTopic.isEmpty(), is(true));

public void shouldPunctuateIfWallClockTimeAdvances() {
    assertThat(outputTopic.readKeyValue(), equalTo(new KeyValue<>("a", 21L)));
    assertThat(outputTopic.isEmpty(), is(true));

public static class CustomMaxAggregatorSupplier implements ProcessorSupplier<String, Long> {
    public Processor<String, Long> get() {
        return new CustomMaxAggregator();

public static class CustomMaxAggregator implements Processor<String, Long> {
    ProcessorContext context;
    private KeyValueStore<String, Long> store;

    public void init(final ProcessorContext context) {
        this.context = context;
        context.schedule(Duration.ofSeconds(60), PunctuationType.WALL_CLOCK_TIME, time -> flushStore());
        context.schedule(Duration.ofSeconds(10), PunctuationType.STREAM_TIME, time -> flushStore());
        store = (KeyValueStore<String, Long>) context.getStateStore("aggStore");

    public void process(final String key, final Long value) {
        final Long oldValue = store.get(key);
        if (oldValue == null || value > oldValue) {
            store.put(key, value);

    private void flushStore() {
        final KeyValueIterator<String, Long> it = store.all();
        while (it.hasNext()) {
            final KeyValue<String, Long> next = it.next();
            context.forward(next.key, next.value);

    public void close() {}

Unit Testing for Processors

Using the Processor API, you can define custom Processor, Transformer, or ValueTransformer implementations.

Because these classes forward their results to the ProcessorContext rather than returning them, unit testing requires a mocked context capable of capturing forwarded data for inspection.

Streams provides MockProcessorContext in kafka-streams-test-utils for this purpose.

To begin with, instantiate your processor and initialize it with the mock context:

final Processor processorUnderTest = ...;
final MockProcessorContext context = new MockProcessorContext();

If you need to pass configuration to your processor or set the default serdes, you can create the mock with config:

final Properties config = new Properties();
config.put(StreamsConfig.APPLICATION_ID_CONFIG, "unit-test");
config.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "");
config.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass());
config.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.Long().getClass());
config.put("some.other.config", "some config value");
final MockProcessorContext context = new MockProcessorContext(config);

The mock captures any values that your processor forwards. You can make assertions on them:

processorUnderTest.process("key", "value");

final Iterator<CapturedForward> forwarded = context.forwarded().iterator();
assertEquals(forwarded.next().keyValue(), new KeyValue<>(..., ...));

// you can reset forwards to clear the captured data. This may be helpful in constructing longer scenarios.

assertEquals(context.forwarded().size(), 0);

If your processor forwards to specific child processors, you can query the context for captured data by child name:

final List<CapturedForward> captures = context.forwarded("childProcessorName");

The mock also captures whether your processor has called commit() on the context:


// commit captures can also be reset.


In case your processor logic depends on the record metadata (topic, partition, offset, or timestamp), you can set them on the context, either all together or individually:

context.setRecordMetadata("topicName", /*partition*/ 0, /*offset*/ 0L, /*timestamp*/ 0L);

Once these are set, the context continues returning the same values, until you set new ones.

In case your punctuator is stateful, the mock context allows you to register state stores. You are encouraged to use a simple in-memory store of the appropriate type (KeyValue, Windowed, or Session), since the mock context does not manage changelogs, state directories, etc.

final KeyValueStore<String, Integer> store =
        .withLoggingDisabled() // Changelog is not supported by MockProcessorContext.
store.init(context, store);
context.register(store, /*parameter unused in mock*/ null);

Processors can schedule punctuators to handle periodic tasks. The mock context does not automatically execute punctuators, but it does capture schedule calls so that you can unit test the punctuator scheduling behavior yourself:

final MockProcessorContext.CapturedPunctuator capturedPunctuator = context.scheduledPunctuators().get(0);
final long interval = capturedPunctuator.getIntervalMs();
final PunctuationType type = capturedPunctuator.getType();
final boolean cancelled = capturedPunctuator.cancelled();
final Punctuator punctuator = capturedPunctuator.getPunctuator();
punctuator.punctuate(/*timestamp*/ 0L);

If you need to write tests involving automatic firing of scheduled punctuators, you should use the TopologyTestDriver on a simple topology containing your processor.