Stream topology in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Kafka Streams topology

What is the output printed by this Kafka Streams application when it processes the input record with key "key1" and value "hello"?

Kafka

StreamsBuilder builder = new StreamsBuilder();
KStream<String, String> source = builder.stream("input-topic");
KStream<String, String> uppercased = source.mapValues(value -> value.toUpperCase());
uppercased.to("output-topic");

// Assume the input record is ("key1", "hello")
// The application prints the output record key and value after processing.

A Output record: key=key1, value=hello

B Output record: key=key1, value=HELLO

C Output record: key=HELLO, value=key1

D No output record is produced

💡 Hint

Remember that mapValues changes only the value, not the key.

The mapValues function transforms the value to uppercase but keeps the key unchanged. So the output record has the same key "key1" and the value "HELLO".

🧠 Conceptual

intermediate

Understanding Kafka Streams topology nodes

In a Kafka Streams topology, which node type is responsible for consuming records from Kafka topics?

A State store node

B Processor node

C Source node

D Sink node

💡 Hint

Think about where the data enters the topology.

Source nodes read data from Kafka topics and feed it into the topology for processing.

❓ Predict Output

advanced

Output of a Kafka Streams topology with branching

Given this Kafka Streams topology code, what output records are sent to the "even-topic" and "odd-topic" when the input records have values 2 and 3?

Kafka

StreamsBuilder builder = new StreamsBuilder();
KStream<String, Integer> source = builder.stream("numbers-topic", Consumed.with(Serdes.String(), Serdes.Integer()));

Map<String, KStream<String, Integer>> branches = source.split(Named.as("branch-"))
    .branch((key, value) -> value % 2 == 0, Branched.withConsumer(ks -> ks.to("even-topic")))
    .branch((key, value) -> value % 2 != 0, Branched.withConsumer(ks -> ks.to("odd-topic")))
    .noDefaultBranch();

A even-topic receives (key, 2), odd-topic receives (key, 3)

B No records are sent to either topic

C even-topic receives both (key, 2) and (key, 3), odd-topic receives nothing

D even-topic receives (key, 3), odd-topic receives (key, 2)

💡 Hint

Check the condition for each branch carefully.

The first branch filters even numbers and sends them to "even-topic"; the second branch filters odd numbers and sends them to "odd-topic".

🔧 Debug

advanced

Identify the error in Kafka Streams topology definition

What error will this Kafka Streams topology code produce when building the topology?

Kafka

StreamsBuilder builder = new StreamsBuilder();
KStream<String, String> source = builder.stream("input-topic");
source.filter((key, value) -> value.length() > 3);
source.to("output-topic");

A Compilation error due to missing semicolon

B Runtime NullPointerException

C No records will be sent to output-topic

D The output topic will contain all records, including those with value length <= 3

💡 Hint

Consider what happens to the filtered stream.

The filter operation returns a new stream, but the original stream is sent to the output topic. So all records are sent, ignoring the filter.

❓ Predict Output

expert

Final output of a Kafka Streams topology with stateful aggregation

What is the output printed by this Kafka Streams topology after processing the input records ("key1", "apple"), ("key1", "banana"), and ("key1", "apple") on the "fruits-topic"?

Kafka

StreamsBuilder builder = new StreamsBuilder();
KStream<String, String> source = builder.stream("fruits-topic");

KTable<String, Long> counts = source.groupByKey().count();

counts.toStream().foreach((key, count) -> System.out.println(key + ": " + count));

A key1: 3

B key1: 2

C key1: 1

D No output is printed

💡 Hint

Count counts the number of records per key, regardless of value.

The count aggregates the number of records with the same key. Three records with key "key1" are processed, so the count is 3.