Testing stream topologies in Kafka - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a test driver for a Kafka Streams topology.

Kafka

TopologyTestDriver driver = new TopologyTestDriver(topology, [1]);

Drag options to blanks, or click blank then click option'

Aprops

Bparameters

Csettings

Dconfig

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an undefined variable instead of the configuration properties.

Using incorrect variable names like 'config' or 'settings'.

The TopologyTestDriver requires a Properties object named 'props' that contains the configuration for the test.

2fill in blank

medium

Complete the code to create an input topic for testing.

Kafka

TestInputTopic<String, String> inputTopic = driver.createInputTopic([1], stringSerializer, stringSerializer);

Drag options to blanks, or click blank then click option'

AinputTopicName

BinputTopic

CtopicName

Dinput

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable that is not defined or does not hold the topic name.

Passing the wrong variable like 'inputTopic' which is the output variable.

The method createInputTopic requires the name of the topic as a string variable, commonly named 'topicName'.

3fill in blank

hard

Fix the error in the code to read output records from the output topic.

Kafka

TestOutputTopic<String, String> outputTopic = driver.createOutputTopic(outputTopicName, stringDeserializer, [1]);

Drag options to blanks, or click blank then click option'

AstringEncoder

BstringSerializer

CstringDecoder

DstringDeserializer

Attempts:

3 left

💡 Hint

Common Mistakes

Using a serializer instead of a deserializer for output topic.

Using incorrect classes like 'stringEncoder' or 'stringDecoder' which are not Kafka classes.

To read output records, you need to use a deserializer for the value, which is 'stringDeserializer'.

4fill in blank

hard

Fill both blanks to configure the properties for the test driver.

Kafka

Properties props = new Properties();
props.put(StreamsConfig.APPLICATION_ID_CONFIG, [1]);
props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, [2]);

Drag options to blanks, or click blank then click option'

A"test-app"

B"localhost:9092"

C"kafka:9092"

D"app-test"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect bootstrap server addresses.

Using application IDs that are not strings.

The application ID is usually a test name like 'test-app', and the bootstrap servers for local testing is 'localhost:9092'.

5fill in blank

hard

Fill all three blanks to send a record to the input topic and read from the output topic.

Kafka

inputTopic.pipeInput([1], [2]);
KeyValue<String, String> outputRecord = outputTopic.readKeyValue();
assertEquals([3], outputRecord.value);

Drag options to blanks, or click blank then click option'

A"key1"

B"inputValue"

C"expectedValue"

D"outputValue"

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up input and expected output values.

Using keys or values not matching the test scenario.

You send the key 'key1' and value 'inputValue' to the input topic, then expect 'expectedValue' as the output value.

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