Testing stream topologies in Kafka - Commands & Configuration

Introduction

When building applications that process data streams, you need to test how your stream processing logic works before running it live. Testing stream topologies helps you check if your data flows and transformations behave correctly.

When you want to verify that your Kafka Streams application processes input data correctly before deployment

When you need to simulate input and output topics to check your stream transformations

When you want to catch bugs early by running automated tests on your stream processing logic

When you want to test complex data flows involving joins, filters, and aggregations in your stream

When you want to ensure your stream topology handles edge cases and error scenarios properly

Commands

This command runs the unit tests in your Kafka Streams project, including tests for your stream topology using the TopologyTestDriver.

Terminal

mvn clean test

Expected Output

[INFO] --- maven-surefire-plugin:3.0.0-M5:test (default-test) @ kafka-streams-app ---

[INFO] Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 2.345 s - in com.example.StreamsTopologyTest

[INFO] 

[INFO] Results:

[INFO] 

[INFO] Tests run: 5, Failures: 0, Errors: 0, Skipped: 0

[INFO] 

[INFO] BUILD SUCCESS

This command runs your Kafka Streams application in production mode after you have tested the topology.

Terminal

java -jar kafka-streams-app.jar

Expected Output

INFO  KafkaStreams: Started Kafka Streams application

INFO  KafkaStreams: Stream thread started

INFO  KafkaStreams: Processing records from input topic

Key Concept

If you remember nothing else from this pattern, remember: use the TopologyTestDriver to simulate input and output topics and verify your stream processing logic without needing a live Kafka cluster.

Common Mistakes

Trying to test Kafka Streams logic by running the full application without mocks

This requires a live Kafka cluster and makes tests slow and flaky.

Use the TopologyTestDriver to run fast, isolated unit tests of your stream topology.

Not closing the TopologyTestDriver after tests

Leads to resource leaks and can cause tests to interfere with each other.

Always call close() on the TopologyTestDriver in a finally block or test teardown.

Not providing correct serializers and deserializers in tests

Causes serialization errors and test failures that do not reflect real logic issues.

Use the same Serdes as your application when creating the test driver and input/output records.

Summary

Use the TopologyTestDriver to simulate Kafka topics and test your stream processing logic.

Run 'mvn clean test' to execute your stream topology unit tests quickly without a live Kafka cluster.

After testing, run your Kafka Streams application normally with 'java -jar' to process real data.

Practice

1. What is the main purpose of using TopologyTestDriver in Kafka Streams testing?

easy

A. To simulate the stream processing logic without a real Kafka cluster

B. To deploy the Kafka Streams application to production

C. To monitor Kafka cluster health

D. To create new Kafka topics automatically

Solution

1. Step 1: Understand the role of TopologyTestDriver

   It runs the Kafka Streams topology in memory for testing purposes.

2. Step 2: Compare with other options

   Options A, B, and C relate to topic creation, deployment, or monitoring, which are not testing functions.

3. Final Answer:

   To simulate the stream processing logic without a real Kafka cluster -> Option A

4. Quick Check:

   TopologyTestDriver = simulate streams [OK]

Hint: Remember: TopologyTestDriver runs streams in memory for tests [OK]

Common Mistakes:

• Confusing testing with deployment
• Thinking it manages Kafka topics
• Assuming it monitors Kafka cluster health

2. Which of the following is the correct way to create a TopologyTestDriver instance in Java?

easy

A. TopologyTestDriver.create(topology, properties);

B. new TopologyTestDriver(topology, properties);

C. TopologyTestDriver.start(topology, properties);

D. new TopologyTestDriver(properties, topology);

Solution

1. Step 1: Recall the constructor signature

   The constructor takes the topology first, then the properties.

2. Step 2: Check each option

   new TopologyTestDriver(topology, properties); matches the correct constructor call. Options B and C use non-existent static methods. new TopologyTestDriver(properties, topology); reverses the argument order.

3. Final Answer:

   new TopologyTestDriver(topology, properties); -> Option B

4. Quick Check:

   Constructor = topology, properties [OK]

Hint: Constructor order: topology first, then properties [OK]

Common Mistakes:

• Using static methods instead of constructor
• Swapping argument order
• Calling non-existent start or create methods

3. Given the following code snippet using TopologyTestDriver:

var driver = new TopologyTestDriver(topology, props);
var inputTopic = driver.createInputTopic("input", Serdes.String().serializer(), Serdes.String().serializer());
var outputTopic = driver.createOutputTopic("output", Serdes.String().deserializer(), Serdes.String().deserializer());

inputTopic.pipeInput("key1", "value1");
var result = outputTopic.readKeyValue();
System.out.println(result.key + ":" + result.value);

What will be printed if the topology simply forwards input to output unchanged?

medium

A. key1:value1

B. null:null

C. value1:key1

D. Exception thrown

Solution

1. Step 1: Understand the topology behavior

   The topology forwards input key-value pairs unchanged to the output topic.

2. Step 2: Trace the test driver code

   Input "key1" and "value1" is piped in; outputTopic reads the same key-value pair.

3. Final Answer:

   key1:value1 -> Option A

4. Quick Check:

   Forwarded key-value = key1:value1 [OK]

Hint: If topology forwards, output matches input exactly [OK]

Common Mistakes:

• Mixing key and value order
• Expecting null output without input
• Assuming exceptions without errors

4. Identify the error in this test code snippet using TopologyTestDriver:

var driver = new TopologyTestDriver(topology, props);
var inputTopic = driver.createInputTopic("input", Serdes.String().deserializer(), Serdes.String().serializer());
inputTopic.pipeInput("key", "value");

medium

A. pipeInput method is called with wrong argument types

B. TopologyTestDriver constructor arguments are reversed

C. Input topic serializers and deserializers are swapped

D. Missing call to driver.close()

Solution

1. Step 1: Check serializer/deserializer usage

   Input topic requires serializers for key and value, not deserializers.

2. Step 2: Identify the swapped serializers

   Code uses deserializer for key and serializer for value, which is incorrect.

3. Final Answer:

   Input topic serializers and deserializers are swapped -> Option C

4. Quick Check:

   Input topic needs serializers [OK]

Hint: Input topics need serializers, output topics need deserializers [OK]

Common Mistakes:

• Using deserializers for input topic
• Ignoring proper resource closing
• Confusing constructor argument order

5. You want to test a Kafka Streams topology that filters out records with null values before forwarding. Which approach correctly verifies this behavior using TopologyTestDriver?

hard

A. Send records with null values and expect an exception from TopologyTestDriver

B. Send only null values to input topic and expect output topic to have the same null values

C. Send records with null values and check that output topic is empty without reading

D. Send records with null and non-null values to input topic, then assert output topic only has non-null values

Solution

1. Step 1: Prepare test input with null and non-null values

   This tests the filter logic by including both cases.

2. Step 2: Verify output topic contents

   Assert that only records with non-null values appear in the output topic.

3. Final Answer:

   Send records with null and non-null values to input topic, then assert output topic only has non-null values -> Option D

4. Quick Check:

   Filter test = input mixed, output filtered [OK]

Hint: Test filters by sending mixed input, check filtered output [OK]

Common Mistakes:

• Expecting null values to appear in output
• Not reading output topic to verify results
• Assuming exceptions occur on null values