Kafka Stream producing custom list of messages based on certain conditions - apache-kafka

Question

We have the following stream processing requirement.
Source Stream ->
transform(condition check - If (true) then generate MULTIPLE ADDITIONAL messages else just transform the incoming message) ->
output kafka topic
Example:
If condition is true for message B(D,E,F are the additional messages produced)
A,B,C -> A,D,E,F,C -> Sink Kafka Topic
If condition is false
A,B,C -> A,B,C -> Sink Kafka Topic
Is there a way we can achieve this in Kafka streams?

Answer 1

You can use flatMap() or flatMapValues() methods. These methods take one record and produce zero, one or more records.
flatMap() can modify the key, values and their datatypes while flatMapValues() retains the original keys and change the value and value data type.
Here is an example pseudocode considering the new messages "C","D","E" will have a new key.
KStream<byte[], String> inputStream = builder.stream("inputTopic");
KStream<byte[], String> outStream = inputStream.flatMap(
(key,value)->{
List<KeyValue<byte[], String>> result = new LinkedList<>();
// If message value is "B". Otherwise place your condition based on data
if(value.equalsTo("B")){
result.add(KeyValue.pair("<new key for message C>","C"));
result.add(KeyValue.pair("<new key for message D>","D"));
result.add(KeyValue.pair("<new key for message E>","E"));
}else{
result.add(KeyValue.pair(key,value));
}
return result;
});
outStream.to("sinkTopic");
You can read more about this :
https://docs.confluent.io/current/streams/developer-guide/dsl-api.html#streams-developer-guide-dsl-transformations-stateless